GRB 130427A

A. Maselli (INAF-IASFPA), A. P. Beardmore (U Leicester),
A. Y. Lien (NASA/GSFC/ORAU), V. Mangano (INAF-IASFPA),
C. J. Mountford (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the
Swift Team:

At 07:47:57 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 130427A (trigger=554620).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 173.139, +27.692 which is 
   RA(J2000) = 11h 32m 33s
   Dec(J2000) = +27d 41' 29"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows an extremely bright
complex peak about 20 seconds long starting at T-50, while Swift was 
slewing from the previous pre-planned target, followed by a smaller 
peak during the slew to the burst with emission through at least T+200. 
The peak count rate was ~ 100,000 counts/sec (15-350 keV), 
at T-40 sec, before the trigger. 

The XRT began observing the field at 07:50:17.7 UT, 140.2 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 173.1362, 27.7129 which is equivalent to:
   RA(J2000)  = +11h 32m 32.69s
   Dec(J2000) = +27d 42' 46.4"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 75 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 147 seconds after the BAT trigger.  A blurred bright source
appears to be located near the XRT position in the initial 2.7'x2.7'
sub-image.  However, the lack of reference stars and lack of star tracker
lock prevents a definitive identification and the measurement of a position
or magnitude. 

This is an extremely bright burst in all three instruments,
and it was also seen by Fermi/GBM. 

After the slew to the burst, the star trackers had trouble
locking on to give a spacecraft attitude, so the XRT position
may be inaccurate at the arcminute level. 

The apparent optical counterpart appears extremely blurred, 
possibly due to the lack of star tracker lock, but more likely to be 
due to instrumental effects on a very bright optical counterpart. 


Further follow-up is warranted for all ground-based observatories. 

Burst Advocate for this burst is A. Maselli (maselli AT ifc.inaf.it). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)

D. A. Perley (Caltech) reports on behalf of a larger collaboration:

The Palomar 60-inch telescope automatically responded to GRB 130427A 
(Maselli et al., GCN 14448
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 ) and began taking observations at 07:52:22 UT 
(4.42 minutes after the BAT trigger).  A series of 60-second images were 
taken in r, i, and z filters; observations are still ongoing. Inspection 
of individual frames shows no detection of an optical transient 
consistent with the XRT position to approximately r > 20.6 mag, i > 20.7 
mag, z > 19.7 mag.

Nondetection of an afterglow in rapidly-triggered P60 observations is 
unusual (Cenko et al. 2009, ApJ 693, 1484), especially in the presence 
of an very bright GRB and early XRT afterglow.  This is suggestive of a 
highly extinguished (or possibly high-redshift) burst, and may indicate 
that the possible UVOT source reported by Maselli et al. was spurious. 
Further observations, especially in the NIR, are strongly encouraged.

L. Elenin (KIAM),  A. Volnova ( IKI),  V. Savanevych (KNURE),   A. 
Bryukhovetskiy (NSFCTC),  I. Molotov (KIAM), A. Pozanenko (IKI) report on
behalf of  larger GRB  follow-up collaboration:

We started observation of  the field of  the Swift GRB 130427A (Maselli et 
al., GCN 14448
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 )  with 0.45-m telescope of ISON-NM observatory on Apr. 27 
(UT)   07:50:17.
On the first images of 30 s exposure we identified very bright optical 
counterpart of GRB 130427A (Maselli et al., GCN 14448
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 ) at about 12m in the 
intitial images.

The coordinates of the counterpart are (J2000) 11 32 32.84 +27 41 56.2 with 
uncertainties of about 1".

Observations are continuing.

D. A. Perley (Caltech) reports on behalf of a larger collaboration:

Upon further inspection of the P60 images, we identify a very bright 
optical source separated by about 51 arcseconds from the XRT afterglow 
location but consistent with the optical afterglow location reported by 
Elenin et al. (GCN 14450
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 )   The source is saturated in the early images. 
  Further observations are ongoing.

We also note that there is an SDSS galaxy at this position, suggesting a 
low-redshift event.

A. Melandri (INAF-OAB), C. Guidorzi (U. Ferrara), A. Gomboc,
J. Japelj (U. Ljubljana), C.G. Mundell (LJMU) report on
behalf of a larger collaboration:

The 2-m Faulkes Telescope North robotically followed up GRB130427A
(SWIFT trigger 554620,  Maselli et al. GCN 14448
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  ) starting 4.3 min
after the GRB trigger time. We detect a bright fading source
at the position reported by Elenin et al. (GCN 14450
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 ), Perley
(GCN 14451
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 ) with R ~11.5 mag.

We note also a faint, 21-mag, underlying source visible in SDSS
images that may suggest this GRB is nearby with a bright host galaxy.

Observations are ongoing.

A. N. Morgan (UC Berkeley) reports:

We observed the field of GRB 130427A (Maselli et al., GCN 14448
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 ) with the
1.3m PAIRITEL located at Mt. Hopkins, Arizona. Observations began at
2013-Apr-27 07h59m24s UT, 11.45 minutes after the Swift Trigger.  In
preliminary mosaics (consisting of 75 ~8 second images for a total
effective exposure time of 9.75 minutes) taken simultaneously in the J, H,
and Ks filters, we detect the optical afterglow (Elenin et al., GCN 14450
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 ;
Perley, GCN 14451
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 ; Melandri et al., GCN 14452
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 ).

The preliminary photometry yields:

post burst
t_mid (hr) exp.(m)   filt  mag   m_err
0.36       9.75      J     12.13  0.03
0.36       9.75      H     11.75  0.04
0.36       9.75      Ks    11.02  0.08

Observations continued until the source passed beyond telescope limits. All
magnitudes are given in the Vega system, calibrated to 2MASS. No correction
for Galactic extinction has been made to the above reported values.

Y. Yatsu, Y. Yano,  R. Usui, Y. Tachibana, K. Ito, T. Yoshii,
S. Kurita, Y. Saito, and N. Kawai (Tokyo Tech)
report on behalf of the MITSuME collaboration:

We observed GRB 130427A (Maselli et al., GCNC 14448) with an
optical tri-color (g, Rc, and Ic)  camera attached to the MITSuME
50 cm telescope of Akeno Observatory, Yamanashi, Japan.

The observation started on 2013-04-27 10:06:09.60 UT (8293 sec after the
trigger).
And we found the optical counterpart reported in the twilight sky.
The measured magnitudes were listed below.

MID-MJD  T-EXP[sec]         g'                 Rc                Ic
-------------------------------------------------------------------------------------------
 56409.4231929  360  15.66+/-0.10  15.03+/-0.07  14.71+/-0.09
-------------------------------------------------------------------------------------------
T-EXP: Total Exposure time [sec]
(The photon flux were calibrated against GSC2.3 catalog.)

A.J. Levan (U. Warwick), S.B. Cenko (U.C. Berkeley), D.A. Perley
(Caltech) and N.R. Tanvir (U. Leicester) report for a larger
collaboration:

We obtained spectroscopy of the afterglow of GRB 130427A (Maselli
et al. GCN 14448
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 , Elenin et al. GCN 14450
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 , Perley et al. GCN 14451
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 )
with Gemini-North / GMOS, begininnig at 09:19 UT roughly 1.5 hours
after the burst. Two different central wavelengths were observed
giving a coverage from ~3100-6700 A. The resulting spectra are of
very high signal to noise given the brightness of the afterglow.
In these spectra we identify absorption lines due to Ca H and K,
Mg I as well as the Mg II doublet at a common redshift of z=0.34.
We suggest this to be the redshift of GRB 130427A.

We do not see evidence for emission lines from an underlying host,
although given the brightness of the afterglow this is not surprising.
The absolute magnitude of object in SDSS, if at z=0.34 is M_R ~
-19.7, relatively bright for a GRB host.

We thank the Gemini-staff for their help in rapidly obtaining these
observations.

D. A. Perley (Caltech) and S. B. Cenko (UC Berkeley) report:

We have continued observations of the afterglow of GRB130427A (Maselli et
al., GCN 14448
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 ) with the Palomar 60-inch telescope until the field set.
Relative to SDSS reference stars in the field, in some select recent
images we estimate afterglow magnitudes of:

r' = 14.86 mag  (t = 1.40 hours)
r' = 15.32 mag  (t = 2.31 hours)
r' = 15.55 mag  (t = 3.10 hours)

This is brighter (by about 1 mag) than any other Swift burst as measured
at similar epoch (as of 2010; Kann et al. ApJ 720:1513), though still
significantly fainter than GRB 030329 which reached approximately 13 mag
at 0.1 day.  The rate of fading corresponds to a relatively shallow decay
index of approximately alpha~-0.85, suggesting that this afterglow will
remain bright for an extended period.  Worldwide small-telescope follow-up
(including by amateurs) is strongly encouraged.

Veli-Pekka Hentunen, Markku Nissinen and Tuomo Salmi (Taurus Hill
Observatory, Varkaus, Finland) report:

We have detected GRB 130427A optical afterglow at iTelescope 
observatory T11 (Mayhill, New Mexico) 0.50-m/6.8 astrograph and FLI 
ProLine PL11002M CCD. Three unfiltered and three photometric V filter 
images with 120 sec exposure time were made.

The afterglow was detected at following position RA 11:32:32.83
and DEC +27:41:56.4

The following magnitudes were obtained from the observations using
NOMAD1 1176-0248446 (R=13.520, V=13.120) as the comparison:

Tmid(sec)+T0   Filter         Exp. time    Mag          Mag err. 
4597              unfiltered     120            15.02CR     0.04
4743              unfiltered     120            15.09CR     0.05
4887              unfiltered     120            15.11CR     0.05
5438              V                120            14.89V       0.04
5582              V                120            14.89V       0.04
5726              V                120            14.86V       0.04

D. Xu (DARK/NBI), C. Cao, S.-M. Hu, C.-M. Zhang (SDU) report:

We observed the field of GRB 130427A (Maselli et al., GCN 14448
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 ) with
the 1m telescope located in Weihai, Shandong Province, China. A series
of I-, R-, V-, and B-band frames were obtained, and observations are
still ongoing.

Although under quite much cloud, at the afterglow position (Elenin et
al., GCN 14450
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 ; Perley, GCN 14451
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 ) we detect the optical counterpart
with R=15.5 mag, with a median time of 4.178 hr after the BAT trigger
and calibrated with nearby USNO B1 stars.

Nat Butler (ASU), Alan M. Watson (UNAM), Alexander Kutyrev (GSFC), William
H. Lee (UNAM), Michael G. Richer (UNAM), Chris Klein (UCB), Ori Fox (UCB)
J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (UCSC),
Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz (UCSC),
José A. de Diego (UNAM), Leonid Georgiev (UNAM), Jesús González (UNAM),
Carlos Román-Zúñiga (UNAM), Neil Gehrels (GSFC), and Harvey Moseley (GSFC)
report:

We observed the field of GRB 130427A (Maselli, et al., GCN 14448
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 ) with the
Reionization and Transients Infrared Camera (RATIR; www.ratir.org) on the
1.5m Harold Johnson Telescope at the Observatorio Astronómico Nacional on
Sierra San Pedro Mártir from 2013/04 27.34 to 2013/04 27.39 UTC (0.25 to
1.67 hours after the BAT trigger), obtaining a total of 1.07 hours exposure
in the r' and i' bands and 0.45 hours exposure in the Z, Y, J, and H bands.

For a source at afterglow location reported by Elenin et al. (GCN 14450
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 ),
in comparison with SDSS DR8 and 2MASS, we obtain the following detections:

  i' 14.46 +/- 0.01
  Z  14.13 +/- 0.03
  Y  14.02 +/- 0.03
  J  14.05 +/- 0.02
  H  13.77 +/- 0.03

These magnitudes are not corrected for Galactic extinction in the direction
of the GRB.

We thank the staff of the Observatorio Astronómico Nacional in San Pedro
Mártir.

T. Kawamuro, M. Shidatsu  (Kyoto U.), S. Nakahira (JAXA), H. Negoro (Nihon U.),
S. Ueno, H. Tomida, M. Kimura, M. Ishikawa (JAXA),
T. Mihara, M. Sugizaki, M. Serino, M. Morii, T. Yamamoto, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN),
N. Kawai, R. Usui, K. Ishikawa, T. Yoshii (Tokyo Tech),
A. Yoshida, T. Sakamoto, Y. Nakano (AGU),
H. Tsunemi, M. Sasaki (Osaka U.),
M. Nakajima, T. Onodera, K. Fukushima, K. Suzuki (Nihon U.),
Y. Ueda, M. Shidatsu (Kyoto U.),
Y. Ueda (Kyoto U.),
Y. Tsuboi, M. Higa (Chuo U.),
M. Yamauchi, K. Yoshidome, Y. Ogawa, H. Yamada (Miyazaki U.),
K. Yamaoka (Nagoya U.) report on behalf of the MAXI team

MAXI/GSC detected the bright GRB 130427A in the scan transit
at 2013-04-27T08:42 UT, about 50 min after the trigger by Swift-BAT (Maselli et al. GCN 14448
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 ).
The one-scan averaged 2-10 keV fluxes were 71 +- 19 mCrab and 23 +- 13 mCrab in the scan transits
at 2013-04-27T08:42 and 2013-04-27T10:13, respectively.

There were no significant excess fluxes in the previous scan transit at 07:12 UT and
in the transit at 11:46 with an upper limit of 30 mCrab for each.

M. Im (CEOU/SNU) on behalf of a larger collaboration

We observed GRB 130427A (Maselli et al., GCN 14448
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 ) 
in BVRI filters using T30 telescope (0.5m) of the iTelescope
network (AAO, Australia), and also are conducting 
follow-up imaging observations using facilities in Korea.

The T30 observation started at about 4 hours after the BAT alert,
and other observations starting at a similar epoch. We clearly 
identify the fading afterglow in T30 images, at the location 
reported earlier (Elenin et al. GCN 14450
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 ).

Preliminary BRI magnitudes of the afterglow in one of the epochs, 
calibrated against a USNO B-1 star in the vicinity of the afterglow,
are given below:

   T(mid, UT)         Mag
04-27 11:42:56     I=14.86 +- 0.12
04-27 11:45:29     R=15.60 +- 0.10
04-27 11:50:29     B=16.10 +- 0.15

Follow-up observations are still ongoing.

D. Kuroda, K. Yanagisawa, Y. Shimizu, H. Toda (OAO, NAOJ),
S. Nagayama (NAOJ), M. Yoshida (Hiroshima), K. Ohta (Kyoto)
and N. Kawai(Tokyo Tech)
report on behalf of the MITSuME collaboration:

We observed the field of GRB 130427A (Masell et al., GCNC 14448)
with the optical three color (g', Rc and Ic) CCD camera attached
to the MITSuME 50cm telescope of Okayama Astrophysical Observatory.

The observation started on 013-04-27 10:57:02 UT (~3.2 h after the burst).
We detected the previously reported afterglow (Elenin et al., GCNC 14450)
in all the three bands.

Photometric results of the OT are listed below.
We used SDSS catalog for flux calibration.

#T0+[day]  MID-UT    T-EXP[sec]   g'   g'_err Rc    Rc_err Ic    Ic_err
-----------------------------------------------------------------------
0.13478    11:02:01     540.0    15.82  0.03  15.48  0.02  15.04  0.03
0.30675    15:09:39     540.0    16.45  0.06  16.27  0.03  15.87  0.04
------------------------------------------------------------------------
T0+ : Elapsed time after the burst [day]
T-EXP: Total Exposure time [sec]

X.-H. Zhao (YNAO and PSU), J. Mao (RIKEN and YNAO), Y. X. Xin(YNAO), J.-M. Bai 
(YNAO) report on behalf of a larger collaboration:

We observed the field of GRB 130427A (Maselli et al., GCN 14448
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 ) with 2.4m Gao-Mei-Gu (GMG) telescope. Observation
was started at 13:25:18 UT on 2013-04-27 (i.e., 5.6 hrs after the burst) under a bad weather condition. The sloan filters of g', r',i' and z' were used. The afterglow is very bright in every band with magnitudes of g'~16.36, r'~16.1, i'~15.9 and z'~15.8. The observations are ongoing. 
   
We thank the GMG staff, especially Hong-Yan Gao and Jian-Duo He for
performing these observations.

B. Gendre, A. Klotz (IRAP-CNRS-OMP), D. Macpherson (UWA/ICRAR),
D. Coward (UWA), M. Boer, K. Siellez, H. Dereli ,
O. Bardho (UNS-CNRS-ARTEMIS-OCA), A. Williams (PO-UWA),
R. Martin (PO-UWA) report:

We imaged the field of GRB 130427A detected by SWIFT
(trigger 554620) with the Zadko robotic telescope (D=100cm)
located at the observatory - Gingin, Australia.

The observations started 3.44h after the GRB trigger
(the event occured during the local afternoon).
The elevation of the field increased from
23 degrees above horizon and weather conditions
were good.

We detect the optical couterpart discovered by
Elenin et al. (GCNC 14450). The unfiltered images
are analyzed regarding the R magnitude of star
catalogues. The preliminary light curve is:

Tstart Tstop Rmag  1sig
(mins) (mins)
    207   219 15.50 0.1
    220   232 15.54 0.1
    238   250 15.37 0.1
    252   264 15.56 0.1
    265   277 15.51 0.1
    304   319 15.58 0.1
    447   450 16.27 0.2

Magnitudes are not corrected for galactic dust extinction.

This message may be cited.

S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (GSFC/UMBC),
E. E. Fenimore (LANL), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
A. Y. Lien (NASA/GSFC/ORAU), C. B. Markwardt (GSFC), A. Maselli (INAF-IASFPA),
D. M. Palmer (LANL), T. Sakamoto (AGU), G. Sato (ISAS),
M. Stamatikos (OSU), J. Tueller (GSFC), T. N. Ukwatta (MSU)
(i.e. the Swift-BAT team):
 
Using the data set from T-60 to T+243 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 130427A (trigger #554620)
(Maselli, et al., GCN Circ. 14448
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 ).  The BAT ground-calculated position is
RA, Dec = 173.150, 27.706 deg which is 
   RA(J2000)  =  11h 32m 36.1s 
   Dec(J2000) = +27d 42' 20.3" 
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 14%.
 
The mask-weighted light curve shows the main emission occuring during a
pre-planned slew.  The burst location came into the BAT FoV at ~T-52 sec.
The first recorded peak started at ~T-51 sec and ending at ~T-50 sec.
The main, large peak started at ~T-50 sec, peaked at ~T-42 sec (~310 cnts/cm2/sec),
and ended at ~T-10 sec.  The spacecraft settled at T+0, and then BAT triggered
(a 64-sec image trigger) at the beginning a long slow peak starting at ~T+0 sec,
peaking at ~T+90 sec, and returning to baseline ~T+2000 sec.
T90 (15-350 keV) is 162.83 +- 1.36 sec (estimated error including systematics).
We note that (a) there is a possibility that there was emission from this burst
before it came into the BAT FoV at ~T-52 sec, and (b) that Fermi-GBM triggered
on this burst at 07:47:06.42 (T_bat-51 sec) [GCN/FERMI_GBM_FLIGHT_POSITION Notice,
TrigNum=388741629].
 
The time-averaged spectrum from T-51.05 to T+223.50 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.21 +- 0.02.  The fluence in the 15-150 keV band is 3.1 +- 0.03 x 10^-4 erg/cm2.
The 1-sec peak photon flux measured from T-44.25 sec in the 15-150 keV band
is 331.0 +- 4.6 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level. 
 
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/554620/BA/

S. Zhu, J. Racusin, D. Kocevski, J. McEnery (NASA/GSFC), F. Longo (Univ of Trieste and INFN), J. Chiang (SLAC), G. Vianello (Stanford) report on behalf of the Fermi-LAT team:

At 07:47:06 UT on 27 April 2013, Fermi LAT detected high energy emission from GRB 130427A, which was also detected by Fermi-GBM (trigger 388741629/130427324) and by Swift (Maselli et al. GCN 14448
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 ).  The GBM detection triggered an autonomous repoint of the spacecraft.

The LAT on-ground location is consistent with the optical position reported in Elenin et al. (GCN 14450
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 ). The burst was about 47 deg from the LAT boresight at the time of the trigger and within the LAT field of view for the next 700 seconds.

The data from the Fermi LAT show a multi-peaked light curve consistent with the GBM trigger. More than 200 photons above 100 MeV are observed within 100 seconds with a TS of >1000.  Using the non-standard LAT Low Energy (LLE) data selection, thousands of counts above background were detected within a 100 s interval coinciding with the time of the GBM emission, with a significance of ~40 sigma. The highest energy LAT photon has an energy of 94 GeV.

A GBM circular is forthcoming.

The Fermi LAT point of contact for this burst is Sylvia Zhu (s.jc.zhu@gmail.com).

The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.

[GCN OPS NOTE(30apr13): Per author's request, time in the first sentence was changed from "07:47:15" to "07:47:06".]

T. Pritchard (PSU), A. Maselli (INAF-IASFPA), N. P. M. Kuin (MSSL),
S. R. Oates (MSSL-UCL), M. De Pasquale (MSSL-UCL),
and M. H. Siegel (PSU), report on behalf of the Swift/UVOT team:

Further to Maselli et al., GCN Circ. 14448
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  we have performed manual
stacking and aspect corrections on the initial UVOT image.  These
resulting images still retain some instrumental effects due to the
brightness of the object and loss of star tracking lock.

Preliminary 3-sigma detections using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the UVOT
observations are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag
wh          1522         1542           19.5         13.8�1.1
v         382          401           19.5        12.1�0.04
b            480          500           19.5        12.6�1.09
u            456          475           19.5        11.8�1.09
w1           430          450           19.5        11.2�0.04
m2           406          426           19.5        11.2�0.04
w2           358          377           19.5        11.2�0.04

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.24 in the direction of the burst
(Schlegel et al. 1998).

A. von Kienlin (MPE) reports on behalf of the Fermi GBM Team:

"At 07:47:06.42 UT on 27 April 2013, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 130427A (trigger 388741629/130427324) which
was also detected by the Swift/BAT and Fermi/LAT (Maselli et al. 2013, 
GCN 14448
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   and S. Zhu et al., GCN 14471
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 ) The GBM on-ground location is 
consistent with the Swift position.

The angle from the Fermi LAT boresight is 48 degrees.

Based on hard and intense emission in a GBM BGO detector, GBM 
initiated an Autonomous Report Request. This request caused Fermi 
to reorient towards this GRB (GBM flight location).  

The GBM light curve consists of a bright structured peak followed 
at ~T0+120 s by a FRED-like pulse. The overall duration (T90) 
is about 138s s (50-300 keV).

Owing to the brightness of the burst, systematic effects are very large 
and no single model gives an adequate fit in this preliminary analysis.  
A Band function fit in the interval  from T0+0.002 s to T0+18.432 s yields 
the following parameters Epeak = 830 +/- 5 keV, alpha = -0.789 +/- 0.003, 
and beta = -3.06 +/- 0.02. 

The event fluence (10-1000 keV) in this time interval is
(1.975 +/- 0.003)E-03 erg/cm^2. The 1.024 sec peak photon flux measured
starting from T0+7.48808 s in the 8-1000 keV band
is 1052 +/- 2 ph/s/cm^2, making this the most intense and fluent GRB 
detected by Fermi GBM.

Further analysis is being performed"

A. Volnova ( IKI),   D. Shakhovskoy (CrAO),   V. Rumyantsev (CrAO), A. 
Pozanenko (IKI) report on behalf of  larger GRB  follow-up collaboration:

We started observation of the field of  the Swift GRB 130427A (Maselli et 
al., GCN 14448
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 )  with AZT-11 telescope of CrAO observatory on Apr. 27 (UT) 
18:21:45 in VRI filters. We clearly detect  still bright optical counterpart 
of GRB 130427A (Maselli et al., GCN 14448
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 , Elenin et al. GCN 14450
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 , Perley 
GCN 14451
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 ).    Preliminary photometry is following

   filter   tstart (UT) exp,s   T-t0,d       OT +/- err
   R       18:21:45   120      0,76580   16,71 +/- 0,06
   R       18:27:52   120      0,77005   16,87 +/- 0,06
   R       18:34:00   120      0,77431   16,83 +/- 0,07
   R       18:40:07   120      0,77855   16,81 +/- 0,06

Observation is continuing.

E. Sonbas (Adiyaman Univ.), T. Guver (Sabanci Univ.), E. Gogus (Sabanci
Univ.), H. Kirbiyik (TUG) report on behalf of a larger collaboration

We observed the field of Swift  GRB 130427A (Maselli et al. GCN#14448
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 )
with  the  1.0 meter  T100  telescope  (TUBITAK National  Observatory,
Antalya - Turkey),  starting April, 27, 20:39:28 UT (~ 13 hours after
the trigger). Observations were carried out in the R filter under good
weather conditions. The afterglow is clearly  detected in 60 s R band
images at a position that is consistent with Elenin et al. GCN#14450
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Using USNO-B1 star USNO-B1 1177-0254824  (RA=  11:32:31.23, Dec=
+27:42:23.02 ) in the field, the magnitudes of the OT were estimated as
follows;

t-t0 (hr)         exp.(s)      filt            mag             err (+/-)
12.850        60               R            16.2              0.05
12.871        60               R            16.6              0.03
12.874        60               R            16.3              0.03
12.877        60               R             16.5             0.01

Further observations using the same filter are ongoing.

We are grateful to the TUBITAK National Observatory staff for promptly
scheduling the observations and their technical support.

J. Wren, W.T. Vestrand, P. Wozniak, and H. Davis,
of Los Alamos National Laboratory report:

Three RAPTOR full-sky persistent monitors at Maui, HI, and Los Alamos, NM,
independently detected  bright optical emission at the location of the
Swift trigger 554620 (Maselli, et al., GCN 14448
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 ).  Starting at 07:47:07.28
UT (50.2 seconds before the Swift trigger) we detect a brightening counterpart
which peaks at magnitude R~7.4.  After the peak, the source fades to below
10th magnitude at approximately the Swift trigger time.  The unfiltered images
from all three monitors are calibrated to the Tycho-2 V-band catalog.

D. Xu (DARK/NBI), A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), S.
Schulze (PUC and MCSS), J. Jessen-Hansen (AU), G. Leloudas (OKC,
Stockholm), T. Kruehler, J.P.U. Fynbo (DARK/NBI), P. Jakobsson (U.
Iceland) report on behalf of a larger collaboration:

We observed the field of GRB 130427A (Maselli et al., GCN 14448
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 ) with
the 2.5m Nordic Optical Telescope (NOT) equipped with the ALFOSC
camera. Observations started at 20:39:12 UT on 2013-04-27 (i.e., 12.85
hr after the BAT trigger).

We first obtained a series of frames in the sloan u-, g-, r-, i-, and
z-band filters. The optical afterglow (ref., Elenin et al., GCN 14450
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 )
is clearly detected in the stacked images in each of these filters.
Preliminary analysis gives the following magnitudes

u=17.70+/-0.03
g=17.34+/-0.03
r=17.07+/-0.03
i=16.94+/-0.03
z=16.88+/-0.03

at a mean time of 12.96 hr post-trigger, calibrated with nearby stars
in the SDSS field.

We also carried out a 1800s spectroscopic exposure. The spectrum
covers 3200 - 9100 AA at a resolution of ~700 and has a high S/N. We
detect prominent absorption lines of the MgII doublet, MgI and CaII K
& H as well as weak emission lines of the [OII] doublet and H-beta,
all at a common redshift of z=0.338+/-0.002, consistent with the
redshift reported by Levan et al. (GCN 14455
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 ).

B. A. Zauderer, E. Berger, S. Chakraborti, and A. Soderberg (Harvard)
report on behalf of a larger collaboration:

"We observed the position of GRB 130427A (Maselli et al.; GCN 14448
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 )
beginning 2013 Apr 27.99 (dt = 0.67 d) with the Very Large Array in its
D configuration (synthesized beam ~18'' x 11'').  At a mean frequency of 
~5.8 GHz, we detect a source with a flux of ~1 mJy +/- 20 uJy at a
position consistent with the optical afterglow (e.g. Elenin et al., 
Perley et al.; GCNs 14450
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 , 14451
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 ). Followup observations are planned for 
this nearby z~0.34 GRB (Levan et al., Xu et al; GCNs 14455
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 , 14478
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 ).

We thank the VLA observatory staff for their support of these observations."

M. Im, C. Choi (CEOU/SNU), H.-I. Sung, Y.-B. Jeon (KASI), 
and Y. Urata (NCU) on behalf of a larger collaboration

We continued our follow-up imaging observation of
GRB 130427A (Maselli et al., GCN 14448
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 ) in BVRI filters at 
Seoul National University Observatory (SNUO, 0.6m), Sobaeksan 
Optical Astoronomy Observatory (SOAO, 0.6m), and Bohyunsan 
Optical Astronomy Observatory (BOAO), all located in Korea.

The observations span the time period of about 4 hrs 
between 04-27 11:28 and 04-27 15:48 UT. The GRB afterglow 
(e.g., Elenin et al. GCN 14450
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 ; Im GCN 14464
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 ) is identified 
in the observed frames at magnitudes of ~16-17 mag

Further analysis of the data is ongoing, and additional follow-up 
observation with these and other facilities is ongoing and planned.

We thank the staffs of SOAO for performing the ToO observation.

B. A. Zauderer, E. Berger, S. Chakraborti, and A. M. Soderberg (Harvard) 
report on behalf of the CARMA Key Project "A Millimeter View of the 
Transient Universe":

"We began observing the field of GRB 130427A (Maselli et al.; GCN 14448
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 ) 
with the Combined Array for Research in Millimeter Astronomy on 2013 Apr 
28.13 UT (dt ~ 0.8 d).  In 45 minutes integration on source at a mean 
frequency of 85 GHz, we detect a radio counterpart with a preliminary 
flux of ~3 mJy (>10-sigma) at a position consistent with the reported 
optical afterglow (Elenin et al., Perley et al.; GCNs 14450
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 , 14451
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 ) and 
our VLA 5 GHz detection (Zauderer et al.; GCN 14480
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 ). Observations are 
ongoing.

We thank the CARMA observers and the observatory staff for their support."

Nat Butler (ASU), Alan M. Watson (UNAM), Alexander Kutyrev (GSFC), William
H. Lee (UNAM), Michael G. Richer (UNAM), Chris Klein (UCB), Ori Fox (UCB)
J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (UCSC),
Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz (UCSC),
José A. de Diego (UNAM), Leonid Georgiev (UNAM), Jesús González (UNAM),
Carlos Román-Zúñiga (UNAM), Neil Gehrels (GSFC), and Harvey Moseley (GSFC)
report:

We again observed the field of GRB 130427A (Maselli, et al., GCN 14448
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 )
with the Reionization and Transients Infrared Camera (RATIR; www.ratir.org)
on the 1.5m Harold Johnson Telescope at the Observatorio Astronómico
Nacional on Sierra San Pedro Mártir from 2013/04 28.14 to 2013/04 28.20 UTC
(19.62 to 20.99 hours after the BAT trigger), obtaining a total of 1.07
hours exposure in the r' and i' bands and 0.45 hours exposure in the Z, Y,
J, and H bands.

We continue to detect the optical/NIR afterglow (Elenin et al; GCN 14450
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 )
in all bands.  In comparison with SDSS DR8 and 2MASS, we find:

  r'    17.68 +/- 0.01
  i'    17.52 +/- 0.01
  Z     17.26 +/- 0.04
  Y     17.12 +/- 0.02
  J     17.21 +/- 0.02
  H     17.01 +/- 0.03

These magnitudes are in the AB system and not corrected for Galactic
extinction in the direction of the GRB.  Uncertaints are 1-sigma.  The
source has faded by about 3 magnitudes in all bands as compared to our
measurements last night (Butler et al. 2013; GCN 14459
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 ).

We thank the staff of the Observatorio Astronómico Nacional in San Pedro
Mártir.

A. Pozanenko (IKI), P. Minaev (IKI), A. Volnova (IKI) report:

The SPI-ACS detector (> 80 keV) of INTEGRAL observatory was triggered by the 
Swift GRB 130427A  (Maselli et al., GCN 14448
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 ) on (UT) T0=07:47:06.36. Light 
curve of the GRB 130427A 
(http://www.isdc.unige.ch/integral/ibas/cgi-bin/ibas_acs_web.cgi) consists 
of the first peak of ~ 1 s duration  which coincides with bright optical 
emission detected by RAPTOR (Wren et al., GCN 14476
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 ). This peak is also 
clearly visible in the BAT light curve at ~T0_BAT-51 s ( 
http://gcn.gsfc.nasa.gov/notices_s/554620/BA/ ). Following the first peak 
extremely intense emission was detected coinciding with BAT/Swift and 
Fermi-LAT detection (Zhu et al., GCN 14471
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 ). The emission in SPI-ACS 
detector is significantly detectable at least up to 100 s after T0.

The SPI-ACS light curve can be also found at
http://grb.rssi.ru/GRB130427A/GRB130427A_SPI-ACS.png

J.A. Kennea (PSU), M.C. Stroh (PSU), D.N. Burrows (PSU), J.P. Osborne
(U. Leicester), K.L. Page (U. Leicester), C. Pagani (U. Leicester), A.
Melandri (INAF-OAB), B. Sbarufatti (INAF-OAB/PSU), G. Stratta (ASDC)
and A. Maselli report on behalf of the Swift-XRT team:

We have analysed 2.4 ks of XRT data for GRB 130427A (Maselli  et al.
GCN Circ. 14448
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 ),  from 130 s to 48.1 ks after the  BAT trigger. The
data comprise 84 s in Windowed Timing (WT) mode (the first 9 s were
taken while Swift was slewing) with the remainder in Photon Counting
(PC) mode. The refined XRT position is RA, Dec = 173.13594, +27.69769
which is equivalent to:

RA (J2000): 11 32 32.63
Dec(J2000): +27 41 51.7

with an uncertainty of 3.5 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=7.9980 (+0.0021, -1.0800), followed by a break at T+252
s to an alpha of 0.61 (+/-0.10).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.79 (+0.17, -0.16). The
best-fitting absorption column is  9.9 (+6.4, -6.0) x 10^20 cm^-2, at a
redshift of 0.34, in addition to the Galactic value of 1.8 x 10^20
cm^-2 (Kalberla et al. 2005). The counts to observed (unabsorbed)
0.3-10 keV flux conversion factor deduced from this spectrum  is 4.0 x
10^-11 (4.7 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.8 x 10^20 cm^-2
Intrinsic column:    9.9 (+6.4, -6.0) x 10^20 cm^-2 at z=0.34
Photon index:	     1.79 (+0.17, -0.16)

If the light curve continues to decay with a power-law decay index of
0.61, the count rate at T+24 hours will be 2.3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.4 x
10^-11 (1.1 x 10^-10) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00554620.

This circular is an official product of the Swift-XRT team.

R. Itoh, K. Kawaguchi, Y. Moritani, K. Takaki, K. S. Kawabata, M. Ohno, 
H. Takahashi, Y. Tanaka, and M. Yoshida (Hiroshima Univ.) report on behalf 
of Kanata team:

We performed a series of optical imaging polarimetry for the optical 
afterglow of GRB 130427A (Elenin et al., GCN 14450
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 ) from 3.87 hr through
10.04 hr after the Swift/BAT trigger (Maselli et al., GCN 14448
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 ) with HOWPol 
attached to the 1.5-m Kanata telescope at Higashi-Hiroshima Observatory, Japan. 

Our quick-look analysis indicates that the R-band magnitude of the optical 
afterglow was 15.9 at the beginning of our observation, and then smoothly 
declined. Its decline rate between 3.87 hr and 10.04 hr can be approximated by 
a single power-low decay with an index -1.0. The polarization seems not so 
large (<~3%) in that period. Further analysis is ongoing.

S. Golenetskii, R.Aptekar, D. Frederiks, E. Mazets, V. Pal'shin,
P. Oleynik, M. Ulanov, D. Svinkin, and T. Cline on behalf
of the Konus-Wind team, report:

The long extremely intense GRB 130427A (Swift-BAT trigger #554620:
Maselli et al., GCN 14448
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 ; Barthelmy et al., GCN 14470
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 ;
Fermi-LAT detection: Zhu et al., GCN 14471
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 ;
Fermi-GBM observation: von Kienlin, GCN 14473
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 ;
SPI-ACS/INTEGRAL observations: Pozanenko et al., GCN 14484
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 )
triggered Konus-Wind (K-W) at T0=28029.501s UT (07:47:09.501)
~50 s before the BAT trigger.

The K-W light curve shows a huge mult-peaked emission complex
started at ~T0, peaked at ~T0+8 s, and having a duration of ~20 s.
The emission is seen up to ~12 Mev.
This extremely bright phase of the event passes into a weaker decaying
tail out to ~T0+120s, when the second emission episode started.
It shows a FRED-like pulse, ~100 times weaker in a peak count
rate than the initial complex.
The decaying emission is detectable by K-W in the 20-1200 keV band
out to ~T0+250s, when the instrument switched into the data readout mode.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB130427_T28029/

As observed by Konus-Wind, the main phase of the burst had
a fluence of (2.68 � 0.01)x10^-3 erg/cm2 and a 16-ms peak flux,
measured from T0+7.774 s, of (6.9 � 0.1)x10^-4 erg/cm2/s
(both in the 20 - 1200 keV energy range).
The fluence of GRB 130427A is the highest observed by K-W
for ~18 years of GRB observations and its peak flux
is only ~30% lower than measured by K-W for the
ultra-luminous GRB 110918A.

The time-averaged spectrum of the main phase of the burst
(measured from T0 to T0+18.688 s) is best fit in the 20 keV-15 MeV range
by the GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -0.958 � 0.006,
the high energy photon index beta = -4.17 � 0.16,
the peak energy Ep = 1028 � 8 keV,
chi2 = 124/96 dof.

The spectrum at the maximum count rate (measured from T0+7.680 to T0+7.936 s)
is best fit in the 20 keV - 15 MeV range
by a power law with exponential cutoff model, for which:
the photon index alpha = -0.57 � 0.02, and
the peak energy Ep = 1213 � 31 keV,
chi2 = 63/84 dof.

Modelling the 3-channel time-averaged spectrum
of the second emission episode (from T0+120 s to T0+250 s)
by a power law with exponential cutoff model
yiels the photon index alpha~-1.6 and
the peak energy Ep ~ 240 keV.
The 20-1200 keV energy fluence measured at this
phase of the event is ~9x10^-5 erg/cm2.

Assuming z=0.34 (Levan et al., GCN 14455
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 ; Xu et al. GCN 14478
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 )
and a standard cosmology model with H_0 = 70 km/s/Mpc,
Omega_M = 0.27, and Omega_Lambda = 0.73,
the isotropic energy release E_iso is ~8.5x10^53 erg,
and the peak luminosity (L_iso)_max is ~2.7x10^53 erg/s.

All the quoted errors are estimated at the 1 sigma confidence level.

Veli-Pekka Hentunen, Markku Nissinen and Tuomo Salmi (Taurus Hill
Observatory, Varkaus, Finland) report:

We have continued observing GRB 130427A optical afterglow at iTelescope
observatory T21 (Mayhill, New Mexico) 0.43-m/6.8 astrograph and
FLI-PL6303E CCD. Two unfiltered, two photometric R filter and two
photometric V filter images with 600 sec exposure time were made.

The following magnitudes were obtained from the observations using
NOMAD1 1176-0248446 (R=13.520, V=13.120) as the comparison:

Tmid(h)+T0      Filter     Exp. time         Mag          Mag err. 
22.67              V              600                17.73V         0.11
22.85              V              600                17.72V         0.11
23.10              unfiltered   600                18.01CR       0.07
23.27              unfiltered   600                18.22CR       0.09
23.56              R              600                18.18R          0.12
23.74              R              600                18.36R          0.14

S. B. Pandey and Brajesh Kumar (ARIES Nainital India, on behalf of larger
Indian GRB collaboration)

We observed Swift GRB 130427A field (Maselli et al., GCN 14448
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 ) using 1.04m
telescope at ARIES Nainital on 2013-04-27 starting at 14:25:16 UT in
UBV(RI)_c filters. Several frames were obtained in average sky conditions.

We clearly detect the bright optical counterpart of GRB 130427A (Maselli et
al. GCN 14448
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 , Elenin et al. GCN 14450
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 ) in each individual exposures.

The preliminary photometry is as following:

 (UT)Start    filter    exp(sec)      mag
 14:25:16       B        300        16.61 +/- 0.06
 14:53:36       I         200       14.83 +/- 0.05

The photometry was performed in comparison to nearby USNO- B1 stars. Our
observations seem to follow the power-law decay exponent ~ -0.9 as noticed 
by Perley and Cenko GCN 14456
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 .

Further observations are going on. This massage may be cited.

Patrick Wiggins (Stansbury Park, Utah, United States) reports the following 
optical observations of the error box of GRB 130427A (GCN Circ #14448
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 ,
Maselli et al.) to the AAVSO International High Energy Network:

P. Wiggins reports three hours of time-series photometry of the bright GRB 
130427A (Maselli et al., GCN Circ. #14448
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 ; Elenin et al., GCN Circ. # 
14450). Wiggins used a 0.35-m C-14 f/5.5 telescope with an SBIG ST-10XME 
camera and clear filter located in Stansbury Park, Utah.  Observations 
commenced 2013 April 27 08:05:12 UT -- less than 20 minutes post trigger 
-- and continued through 11:10:06 UT.  The first four observations were 
made with 60-second exposure times, which were found to be close to 
saturation; subsequent exposure times were 30 seconds.  The transient was 
detected by visual inspection of the first frame, and found to be very 
bright.  Time-series observations were begun immediately.

Thirty-three images of the field were taken, and the resulting light curve 
is as follows.  All magnitudes are taken with respect to the comparison 
star GSC 01984-00021, which has an r-magnitude of 12.553 (APASS DR7, 
http://www.aavso.org/apass ); the resulting magnitudes should be 
interpreted as through a clear filter with r'-band zero point.  Note that 
the first four magnitudes with integration times of 60 seconds may be 
saturated.  All times are given as the mid-point of the exposure, UT on 
2013 April 27, and T(0)+h is hours since trigger (07:47:57 UT):

     Obs. UT	T(0)+h	Exp. time	magn.	mag.err.
08:05:41.529	0.296	   60.00	13.108	 0.006
08:06:57.779	0.317	   60.00	13.194	 0.007
08:14:16.467	0.439	   60.00	13.557	 0.009
08:27:07.951	0.653	   60.00	13.995	 0.013
08:40:35.701	0.877	   30.00	14.304	 0.024
08:46:36.857	0.978	   30.00	14.413	 0.027
08:50:51.998	1.049	   30.00	14.47	 0.029
08:55:17.311	1.122	   30.00	14.546	 0.030
09:00:52.545	1.215	   30.00	14.623	 0.033
09:06:26.014	1.308	   30.00	14.684	 0.035
09:11:59.514	1.401	   30.00	14.731	 0.038
09:17:33.139	1.493	   30.00	14.792	 0.041
09:23:06.748	1.586	   30.00	14.829	 0.043
09:28:40.357	1.679	   30.00	14.969	 0.050
09:34:18.373	1.773	   30.00	14.957	 0.050
09:39:51.873	1.865	   30.00	14.911	 0.050
09:45:25.545	1.958	   30.00	15.025	 0.055
09:50:59.404	2.051	   30.00	15.158	 0.064
09:56:32.982	2.143	   30.00	15.104	 0.064
10:02:06.529	2.236	   30.00	15.22	 0.073
10:07:40.154	2.329	   30.00	15.138	 0.071
10:13:13.732	2.421	   30.00	15.249	 0.080
10:15:01.029	2.451	   30.00	15.219	 0.078
10:20:47.498	2.547	   30.00	15.302	 0.088
10:26:21.123	2.64	   30.00	15.424	 0.099
10:31:54.732	2.733	   30.00	15.387	 0.100
10:37:28.451	2.825	   30.00	15.247	 0.091
10:43:02.248	2.918	   30.00	15.512	 0.123
10:48:35.811	3.011	   30.00	15.496	 0.124
10:54:09.561	3.103	   30.00	15.517	 0.128
10:59:43.279	3.196	   30.00	15.633	 0.150
11:05:19.092	3.289	   30.00	15.555	 0.150
11:10:20.982	3.373	   30.00	15.48	 0.153

The AAVSO International High Energy Network is supported through the
AAVSO Endowment Fund.  We thank the Charles Curry Foundation and
NASA for past support.

GRB 130427A: VLT/X-shooter redshift confirmation

H. Flores (Obs. de Paris), S. Covino (INAF), D. Xu, T. Kruehler,
J. Fynbo, B. Milvang-Jensen(DARK/NBI), A. de Ugarte Postigo (IAA-CSIC,
DARK/NBI), L. Kaper (UVA) and  K. Wiersema (U. Leicester) report on behalf
of the X-shooter GRB collaboration:

VLT/X-shooter observed the afterglow of GRB 130427A (Maselli et al.,
GCN 14448
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 ; Elenin et al., GCN 14450
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 ), starting on 2013-04-28 at UT
00:23. The observation consisted of 2x600s exposures.

In the spectra we detect the continuum in the complete range from 3000
to 24800 A. We find several absorption features including FeII, MnII,
MgII, MgI, TiII, CaII, NaI, and emission lines such as H-alpha,
H-beta,  [OIII], and [OII], all at a common redshift of z=0.3399 +/- 0.0002,
consistent with the measurement in Levan et al. (GCN 14455
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 ) and
Xu et al. (GCN 14478
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 ).

We thank the Paranal staff for excellent support, in particular Andrea
Mehner and Christophe Martayan.

L.P. Xin, J. Y. Wei, Y. L. Qiu,  J. Wang, 
J. S. Deng, C. Wu, X.H. Han  report on behalf of EAFON team:

We began to observe the field of GRB 130427A  (Maselli, et al., GCN 14448
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 )
using  80cm TNT telescope located at Xinglong observatory, China
at 12:52:38 UT on 2013-04-28.  We obtained several B, V, R-band images.  

The optical counterpart is detected in all images. The prelinary analysis shows that
its brightness is about R=18.3 mag, calibrated with USNO B2 stars, 
at the mean time of  29.08 h after the trigger time.

This message may be cited.

D. A. Perley (Caltech) reports on behalf of a larger collaboration:

We observed the position of GRB 130427A (Maselli et al.; GCN 14448
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 ) with 
the Combined Array for Research in Millimeter Astronomy on two epochs on 
2013-04-28 (UT), between 01:45 and 02:46 and again between 07:22 and 
08:14, at a frequency of 93 GHz (3 mm).  The afterglow is well-detected 
in both epochs.  Preliminary reduction gives fluxes of:

3.7 +/- 0.4 mJy  (t_mid = 0.769 day)
2.6 +/- 0.4 mJy  (t_mid = 1.000 day)

These values are consistent with the flux reported by Zauderer et al. 
(GCN 14482
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 ) as also measured by CARMA at a time intermediate between 
these epochs.

The source location is:

RA = 11:32:32.82
Dec = +27:41:56.06
(J2000, uncertainty 0.4 arcsec)

Further observations are planned.

We thank the CARMA staff for their support in executing these observations.

J. Takahashi, K. Morihana, S. Honda and Y. Takagi (Univ. of Hyogo)
report on behalf of Nayuta team:

We observed the field of GRB 130427A (Masell et al., GCNC 14448)
in near-infared three bands (J, H and Ks) with NIC attached to the
Nayuta 2-m telescope at the Nishi-Harima Astronomical Observatory.

The observations were conducted from 2013-04-27 12:26 UT to 2013-04-27
17:25 UT (from ~4.7 h to ~9.6 h after the burst).
We detected the previously reported afterglow (Elenin et al., GCNC 14450)
in all the three bands.

Photometric results of our observations are listed below.
We used 2MASS 11323755+2743196 as the reference star for flux calibration.

# MID-UT   T-EXP[sec]  J    J_err H    H_err  Ks    Ks_err
-----------------------------------------------------------------------
12:26:56     150       14.7  0.2  13.5  0.2   13.5   0.2
13:27:59     150       14.7  0.2  14.2  0.2   13.2   0.3
14:39:42     150       14.7  0.3  14.3  0.2   13.4   0.3
17:17:31     150       --*   --*  14.8  0.2   14.6   0.2
17:24:52     300       15.2  0.2  14.8  0.2   14.0   0.2
------------------------------------------------------------------------
T-EXP: Total Exposure time [sec]
* Photometry was not possible because the signal from the reference star
was too low.

A. Trotter, D. Reichart, J. Haislip, A. LaCluyze, T. Berger, H. T. 
Cromartie, R. Egger, A. Foster, N. Frank, K. Ivarsen, M. Maples, J. 
Moore, M. Nysewander, E. Speckhard, and J. A. Crain report:

Skynet observed the Swift/XRT localization of GRB 130427A (Maselli et 
al., GCN 14448
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 , Swift trigger #554620) with four 16" telescopes of the 
PROMPT array at CTIO, Chile, starting at 2013-04-27, 23:28 UT, and 
continuing until 04-28, 05:30 UT (t=15.69h-21.67h post-trigger). It took 
~124 160-s exposures, simultaneously in each of the BVRI bands. We 
performed photometry on each exposure, calibrated to two SDSS stars in 
the field. We detect a fading afterglow in BVRI at the position reported 
by Elenin et al. (GCN 14450
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 ), which is ~50" south of the initial XRT 
localization.

A preliminary light curve of the first night's data is at:
http://skynet.unc.edu/grb/grb130427a.png

Further Skynet observations are ongoing.

D. Kuroda, (OAO, NAOJ), H. Hanayama (IAO, NAOJ), K. Yanagisawa,
Y. Shimizu, H. Toda (OAO, NAOJ), T. Miyaji J. Watanabe, (IAO, NAOJ),
S.Nagayama (NAOJ), M. Yoshida (Hiroshima), K. Ohta (Kyoto)
and N. Kawai(Tokyo Tech) report on behalf of the MITSuME collaboration:

We observed the field of GRB 130427A (Masell et al., GCNC 14448)
with the optical three color (g', Rc and Ic) CCD camera attached
to the MITSuME 50cm telescope of Okayama Astrophysical Observatory
and the Murikabushi 1m telescope of Ishigakijima Astronomical
Observatory.

We detected the previously reported afterglow (Elenin et al., GCNC 14450)
in all the three bands. Photometric results of the OT are listed below.
We used SDSS catalog for flux calibration.

Okayama Astrophysical Observatory:
The observation started on 2013-04-28 11:29:25 UT (~1.15 days after the burst).

#T0+[day]  MID-UT    T-EXP[sec]   g'   g'_err Rc    Rc_err Ic    Ic_err
-----------------------------------------------------------------------
1.16123    11:40:06    1440.0    18.4   0.2   17.9   0.1   17.7   0.1
------------------------------------------------------------------------
T0+ : Elapsed time after the burst [day]
T-EXP: Total Exposure time [sec]

Ishigakijima Astronomical Observatory:
The observation started on 2013-04-28 14:02:08 UT (~1.26 days after the burst).
#T0+[day]  MID-UT    T-EXP[sec]   g'   g'_err  Rc   Rc_err  Ic   Ic_err
-----------------------------------------------------------------------
1.31290    15:18:30    1020.0    18.61 0.06   18.22 0.04   17.37 0.05
------------------------------------------------------------------------
T0+ : Elapsed time after the burst [day]
T-EXP: Total Exposure time [sec]

P.A. Evans, K.L. Page (U. Leicester), A. Maselli, V. Mangano, M. Capalbi 
(INAF-IASFPA), D.N. Burrows (PSU) report on behalf of the Swift-XRT team:

We have reanalysed the XRT data for GRB 130427A (Maselli  et al. GCN 
Circ. 14448
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 ) from 144 s to 141 ks  after the BAT trigger. The data 
comprise 1.9 ks of  Windowed Timing (WT) mode data, with the rest taken 
in Photon Counting (PC) mode.

The star trackers on Swift failed to find a correct aspect solution 
during the first 1.9 ks of exposure (when the WT mode data were 
collected; see GCN Circ. 14448
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 ), and the GRB is seen to be drifting on 
the XRT detector. We have fitted this drift as a linear function, which 
is a good representation to the data. We have then rebuilt the GRB light 
curve and spectrum using this information. However, the time-tag of 
individual photon events may be incorrect by up to 1 second, and the 
method that we have used to correct the attitude may introduce low-level 
fluctuations. It is therefore inappropriate to perform any detailed 
timing or periodicity analysis with the current XRT data. We are working 
to rectify this situation.

The light curve can be modelled as a broken power-law with an initial 
decay index of 2.81 (+/- 0.04) with a break at T+421 s to a slope of 
alpha=1.17 (+/-0.01). This then breaks again at T+53.4ks to a final 
slope of 1.79 (+0.5, -0.05).

A spectrum formed from the WT mode data can be fitted with an absorbed 
power-law with a photon spectral index of 1.91 (+/-0.11). The best 
fitting absorption column is 2.8 (+/-0.05) x 10^21 cm^-2 at a redshift 
of 0.34, in addition to the Galactic value of 1.8 x 10^20 cm^-2 
(Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV 
flux conversion factor deduced from this spectrum  is 3.8 x 10^-11 (5.2 
x 10^-11) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of 
1.79, the count rate at T+3 days will be 0.14 count/sec, corresponding 
to an observed (unabsorbed) 0.3-10 keV flux of 5.3 x 10^-12 (7.3 x 
10^-12) erg cm^-2 s^-1.

The automated light curve of this GRB is online at 
http://www.swift.ac.uk/xrt_curves/00554620/


This circular is an official product of the Swift-XRT team.

L. Amati (INAF - IASF Bologna, Italy), S. Dichiara, F. Frontera, C. 
Guidorzi (University of Ferrara, Italy), report:

Based on the preliminary values of fluence and spectral parameters 
reported by the Fermi/GBM (Kienlin et al., GCN 14473
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 ) and Konus-Wind 
(Golenetskii et al., GCN 14487
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 ) teams, and by assuming a redshift of 0.34 
(Levan et al., CGN 14455; Flores et al., GCN 14491
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 ) and a standard 
Lambda_CDM cosmology with H_0 = 70 km/s/Mpc, Omega_M = 0.27, Omega_Lambda 
= 0.73, we estimate for GRB 130427A an intrinsic spectral peak energy Ep,i 
of 1250+/-150 keV and an isotropic-equivalent radiated energy of 
(1.05+/-0.15)x10^54 erg (1-10000 keV cosmological rest-frame).

These values are fully consistent with the best-fit power-law of the 
Ep,i-Eiso correlation holding for all long-bright cosmological GRBs (as 
determined, e.g., by Amati et al. 2009, A&A 508, 173). This is further 
evidence that the prompt emission properties of GRB 130427A, the most 
energetic GRB detected at z < 1, are the same as those of very bright, 
high-redshift events. Hence, its relatively low redshift makes it a unique 
case for investigating wether this class of events is associated to SNe 
with properties similar to those associated to "local" sub-luminous GRBs.

A. Volvach (CrAO), L. Volvach (CrAO), A. Pozanenko (IKI) on behalf of 
larger  collaboration report:

We observed the field of GRB 130427A (Maselli et al., GCN 14448
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 ) on Apr. 
28 between (UT) 16:22-18:41 with 22-m radio telescope RT-22 of
Crimean Astrophysical Observatory at mean frequency of 36 GHz. At the 
position of  the optical (Elenin et al., GCN 14450
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 ; Perley et al., GCN 
14451
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 ) and radio afterglow (Zauderer et al., GCN 14480
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 ; Zauderer et al., 
GCN 14482
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 ; Perley, GCN 14494
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 ) we detected a source with a flux 1.9 �� 0.4 
mJy  at T_mid = 1.405 days after burst trigger.

Nat Butler (ASU), Alan M. Watson (UNAM), Alexander Kutyrev (GSFC), William
H. Lee (UNAM), Michael G. Richer (UNAM), Chris Klein (UCB), Ori Fox (UCB)
J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (UCSC),
Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz (UCSC),
José A. de Diego (UNAM), Leonid Georgiev (UNAM), Jesús González (UNAM),
Carlos Román-Zúñiga (UNAM), Neil Gehrels (GSFC), and Harvey Moseley (GSFC)
report:

We again observed the field of GRB 130427A (Maselli, et al., GCN 14448
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 )
with the Reionization and Transients Infrared Camera (RATIR; www.ratir.org)
on the 1.5m Harold Johnson Telescope at the Observatorio Astronómico
Nacional on Sierra San Pedro Mártir from 2013/04 29.14 to 2013/04 29.27 UTC
(43.64 to 46.60 hours after the BAT trigger), obtaining a total of 3.02
hours exposure in the r' and i' bands and 1.57 hours exposure in the Z, Y,
J, and H bands.

We continue to detect the optical/NIR afterglow (Elenin et al; GCN 14450
Loading...
 
 )
in all bands.  In comparison with SDSS DR8 and 2MASS, we find:

  r'    18.90 +/- 0.01
  i'    18.67 +/- 0.01
  Z     18.38 +/- 0.03
  Y     18.19 +/- 0.02
  J     18.29 +/- 0.02
  H     18.01 +/- 0.02

These magnitudes are in the AB system and not corrected for Galactic
extinction in the direction of the GRB.  Uncertaints are 1-sigma.  The
source has faded by about 1 magnitude in all bands as compared to our
measurements on the previous night (Butler et al. 2013; GCN 14483
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 ).

We thank the staff of the Observatorio Astronómico Nacional in San Pedro
Mártir.

William C. Keel (U. of Alabama, Tuscaloosa), Dieter Hartmann and Aman Kaur
(Clemson University) report:

We observed the field of GRB 130427A (Maselli, et al., GCN 14448
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 ) with the
SARA 0.9m telescope at KPNO  (http://saraobservatory.org) from 0350-0420 UT
on April 29, obtaining a total of 30 min exposure in the B,V, and R band.

We detect the optical afterglow (Elenin et al; GCN14450
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 ) in all bands.
In comparison with Landolt (PG 0942) standard stars we find

B = 19.32
V = 19.00
R = 18.53

These magnitudes are not corrected for Galactic extinction, but include
KPNO airmass corrections at the time of observation.

This message may be cited

S. Zhu, J. Racusin (NASA/GSFC), J. Chiang (SLAC), G. Vianello (Stanford) report on behalf of the Fermi LAT team:

Using LAT source class events >100 MeV between T0+0 and 700 seconds after the GBM trigger, we find a LAT localization of RA = 173.148, Dec = +27.709, with a 68% containment radius of 0.068 degrees (statistical only). This localization is consistent with other reported positions.

The >100 MeV emission spectrum during the GBM T90 (T0+0 to 138 seconds) is fit by a power law with an index of -1.96 +/- 0.07. The fluence during this time is (1.1 +/- 0.1)E-4 erg/cm^2, making this the highest fluence LAT-detected burst in the LAT energy range (Fermi LAT Collaboration, arXiv:1303.2908). The >100 MeV peak flux, measured from 11.52 to 37.33 seconds, is (1.4 +/- 0.2)E-3 ph/cm^2/s.

The LAT Low Energy (LLE) emission during the bright structured peak (0 to 20 seconds) is roughly correlated with the GBM emission. A spike at T0+0 seconds is coincident in both LAT and LLE, but precedes the onset of the GBM emission. There are peaks in the LAT light curve at approximately 13 and 22 seconds; neither peak is coincident with the LLE or GBM.

Significant emission >100 MeV was detected throughout the first orbit until ~735 seconds, at which point the burst became occulted by the Earth. The LAT emission was still significantly detected when the burst emerged from occultation at ~3000 seconds, and remained detectable for about a day. The extended emission light curve can be fit by a broken power-law that has a power-law index = -0.89 +/- 0.04 at early times, and at late times, it has an index in the range -1.3 to -1.5; the temporal break occurs around 550-800 seconds after the GBM trigger.

We clarify that the Swift-BAT trigger time (Barthelmy et al., GCN 14470
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 ) is ~51 seconds after the GBM trigger, so the emission detected by RAPTOR beginning ~50 seconds before the Swift trigger with a peak magnitude of R~7.4 (Wren et al., GCN 14476
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 ) is therefore coincident with the GBM and LAT emission onset.

The Fermi LAT point of contact for this burst is Sylvia Zhu (s.jc.zhu@gmail.com).

The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.

X.-H. Zhao (YNAO and PSU), J. Mao (RIKEN and YNAO), J. G. Wang(YNAO), J.-M. Bai 
(YNAO) report on behalf of a larger collaboration:

We further observed the afterglow of GRB 130427A (Maselli et al., GCN 14448
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 ) with 2.4m Gao-Mei-Gu (GMG) telescope on Apr 28 and Apr 29. 
The sloan filter of r' was used. The magnitudes of the afterglow are 
r'~18.2  31.16 hrs after trigger
r'~18.9  53.37 hrs after trigger 

We thank the GMG staff, especially Hong-Yan Gao, Jian-Duo He and Gui-Hua He for performing these observations.

A. Trotter, D. Reichart, J. Haislip, A. LaCluyze, T. Berger, H. T. 
Cromartie, R. Egger, A. Foster, N. Frank, K. Ivarsen, M. Maples, J. 
Moore, M. Nysewander, E. Speckhard, and J. A. Crain report:

Skynet continued observing the Swift/XRT localization of GRB 130427A 
(Maselli et al., GCN 14448
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 , Swift trigger #554620) with four 16" 
telescopes of the PROMPT array at CTIO, Chile, starting at 2013-04-28, 
23:06 UT, and continuing until 04-29, 3:51 UT (t=39.28h-44.03h 
post-trigger). It took ~110 160-s exposures, simultaneously in each of 
the BVRI bands. We performed photometry on each exposure, calibrated to 
two SDSS stars in the field. B-band photometry was performed on stacks 
of 3-4 images in order to provide at least 3sigma detections.

A preliminary light curve of the second's data is at:
http://skynet.unc.edu/grb/grb130427a_2.png

A light curve of both the first and second nights' data is at:
http://skynet.unc.edu/grb/grb130427a_1_2.png

See Trotter et al. (GCN 14497
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 ) for a description of the first night's 
observations.  Further Skynet observations are ongoing.

Jay Norris, Daryl Macomb (Boise State U.) report:

We observed GRB 130427A with the Challis Observatory's 0.4-m telescope
(114.33 deg W, 44.5 deg N, 2165 m elevation) on April 29, 05:58 to 06:22 UT.
Using seventeen 10-second frames in the R filter acquired in a clear sky,
centered at ~ 06:06 UT, 22.3 hours after the Swift/BAT trigger (GCN 14448
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 ),
we clearly detected the optical afterglow.

Comparison with six stars in the field of view (R = 15.5 to 16.8 mag)
yielded an estimate of R = 17.6 mag for the afterglow with uncertainty
of ~ 0.15 mag.  Further observations were interrupted by clouds.

[GCN OPS NOTE(30apr13): Per author's request, the affiliation in the FROM-line
was changed to Boise State U.]

D. Kuroda (OAO, NAOJ),  H. Hanayama, T. Miyaji, J. Watanabe (IAO, NAOJ),
K. Yanagisawa (OAO, NAOJ), S.Nagayama (NAOJ), M. Yoshida (Hiroshima),
K. Ohta (Kyoto) and N. Kawai(Tokyo Tech)
report on behalf of the MITSuME collaboration:

We observed the field of GRB 130427A (Masell et al., GCNC 14448)
with the optical three color (g', Rc and Ic) CCD camera attached
to the Murikabushi 1m telescope of Ishigakijima Astronomical
Observatory.

We detected the previously reported afterglow (Elenin et al., GCNC 14450)
in all the three bands. The observation started on 2013-04-29 15:02:55 UT (~2.30
days after the burst).

Photometric results of the OT are listed below.
We used SDSS catalog for flux calibration.

#T0+[day]  MID-UT    T-EXP[sec]   g'   g'_err  Rc   Rc_err  Ic   Ic_err
-----------------------------------------------------------------------
2.31262    15:18:06   1500.0     19.19 0.05   18.87 0.04   18.65 0.09
2.33383    15:48:38   1440.0     19.29 0.06   18.90 0.05   18.76 0.11
------------------------------------------------------------------------
T0+ : Elapsed time after the burst [day]
T-EXP: Total Exposure time [sec]

Nat Butler (ASU), Alan M. Watson (UNAM), Alexander Kutyrev (GSFC), William
H. Lee (UNAM), Michael G. Richer (UNAM), Chris Klein (UCB), Ori Fox (UCB)
J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (UCSC),
Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz (UCSC),
José A. de Diego (UNAM), Leonid Georgiev (UNAM), Jesús González (UNAM),
Carlos Román-Zúñiga (UNAM), Neil Gehrels (GSFC), and Harvey Moseley (GSFC)
report:

We again observed the field of GRB 130427A (Maselli, et al., GCN 14448
Loading...
 
 )
with the Reionization and Transients Infrared Camera (RATIR; www.ratir.org)
on the 1.5m Harold Johnson Telescope at the Observatorio Astronómico
Nacional on Sierra San Pedro Mártir from 2013/04 30.14 to 2013/04 30.37 UTC
(67.58 to 73.15 hours after the BAT trigger), obtaining a total of 3.91
hours exposure in the r' and i' bands and 1.64 hours exposure in the Z, Y,
J, and H bands.

We continue to detect the optical/NIR afterglow (Elenin et al; GCN 14450
Loading...
 
 )
in all bands.  In comparison with SDSS DR8 and 2MASS, we find:

  r'    19.39 +/- 0.01
  i'    19.31 +/- 0.01
  Z     18.89 +/- 0.03
  Y     18.75 +/- 0.03
  J     18.87 +/- 0.02
  H     18.56 +/- 0.03

These magnitudes are in the AB system and not corrected for Galactic
extinction in the direction of the GRB.  Uncertaints are 1-sigma.  The
source has faded by about 0.5 magnitude in all bands as compared to our
measurements on the previous night (Butler et al. 2013; GCN 14506
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 ).

We thank the staff of the Observatorio Astronómico Nacional in San Pedro
Mártir.

=========================================================================
F. Verrecchia, C. Pittori (ASDC and INAF/OAR), A. Giuliani (INAF/IASF-Mi),
M. Marisaldi (INAF/IASF-Bo), F. Longo (University of Trieste and INFN
Trieste), F. Lucarelli (ASDC and INAF/OAR), E. Del Monte, F. Lazzarotto, I.
Donnarumma, Y. Evangelista, M. Feroci, L. Pacciani, P. Soffitta, E. Costa,
I. Lapshov, M. Rapisarda (INAF/IAPS Rome), G. Barbiellini, (INFN Trieste),
A. Bulgarelli, F. Gianotti, M. Trifoglio, G. Di Cocco, C. Labanti, V.
Fioretti, F. Fuschino, M. Galli (INAF/IASF-Bo), A. Chen, S. Mereghetti,
F. Perotti, P. Caraveo (INAF/IASF-Mi), M. Cardillo, E. Striani, M. Tavani
(INAF/IAPS Rome, and Univ. Roma Tor Vergata), A. Argan, G. Piano, S.
Sabatini, V. Vittorini (INAF/IAPS Rome), G. Pucella (ENEA Frascati), A.
Pellizzoni, A. Trois (INAF/OA Cagliari), M. Pilia (ASTRON), S. Vercellone
(INAF/IASF-Pa), P. W. Cattaneo, A. Rappoldi (INFN Pavia), A. Morselli, P.
Picozza (INFN Roma-2), M. Prest, E. Vallazza (Universita` 
dell'Insubria), P.
Lipari, D. Zanello (INFN Roma-1), P. Giommi (ASI), and G. Valentini (ASI),
on behalf of the AGILE Team, report:

The AGILE Gamma Ray Imaging Detector (GRID) detected high energy emission
from GRB 130427A (A. Maselli et al., GCN 14448
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 ), also reported
by Fermi-LAT (S. Zhu et al., GCN 14471
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 ).
A preliminary analysis of the AGILE-GRID data in temporal coincidence with
the GRB shows a significant excess of gamma-ray photons above 50 MeV at the
location of the event. The emission detected by the AGILE-GRID mostly
occurred between ~ t0 + 180 sec and t0 + 700 sec where t0 is 27 April 2013
at 07:47:15 UT. During this interval, the burst position was inside the
instrument FOV.

A maximum likelihood analysis of the AGILE-GRID data integrating over 12
hours, from 2013-04-27 05:00 UT to 2013-04-27 17:00 UT, using the standard
parameters used by AGILE quick look to detect persistent gamma-ray sources,
yields a detection at a significance level larger than 6 sigma, and a mean
flux F = (8.0+/-2.7) 10-6 ph/cm2/s (E > 100 MeV).
The preliminary photon spectral index obtained with this integration is
1.55 +/- 0.30.
Due to the exceptionally high fluence above 100 MeV of this burst, it is
possible for the first time to derive its properties using the maximum
likelihood techniques routinely used in the standard data analysis of
AGILE-GRID point sources.

The GRB also triggered the AGILE Minicalorimeter (MCAL), sensitive to
gamma-rays above 350 keV, at the time 07:47:06 UT.
According to the MCAL light curve, the emission lasts for about 20 s 
divided
into three main episodes. Although the large initial off-axis angle (more
than 120 degrees) prevents an accurate spectral analysis of MCAL data,
significant emission above 15 MeV is detected.

This measurement was obtained with AGILE observing a large portion of the
sky in spinning mode.

This message may be cited.
=========================================================================

A. Vonova (IKI), L. Elenin (KIAM), A. Pozanenko (IKI)

We have investigated photo-z of possible host galaxy (Melandri et al., GCN
14452
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 , Levan et al., 14455). Using LePhare sw (Arnouts et al. 1999, MNRAS,
310, 540; Ilbert et al. 2006, A&A, 457, 841) and SDSS  magnitudes  we
estimated a redshift of the galaxy z = 0.49 (+0.12,-0.24). This photo-z
value is compatible with a spectroscopy redshift (Levan et al., GCN 14455
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 ;
Xu et al., GCN 14478
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 ; Flores et al., 14491) and  confirms the SDSS galaxy
as the host.

A. Arai, J. Takahashi, K. Morihana, S. Honda and Y. Takagi (Univ. of Hyogo)
report on behalf of Nayuta team:

We performed low resolution ( R ~ 500) optical spectroscopic observations
of a source on the position of GRB 130427A (Masell et al., GCNC 14448)
with MALLS attached to the Nayuta 2-m telescope at the Nishi-Harima
Astronomical Observatory.

The observations were conducted from 2013-04-27 12:48 UT to 2013-04-27
22 : 09 UT (from ~5.1 h to ~ 5.4 h after the burst).

The resulting spectrum ( S/N ~ 5) shows a feature less continuum light
without any emission nor absorption line features in the wavelength
range between 450 nm and 680 nm.

[GCN OPS NOTE(30apr13): Per operator, I have corrected a mistake I made 
in assigning the affiliation for this submitor.]

X.-H. Zhao (YNAO and PSU), J. Mao (RIKEN and YNAO), L. Chang (YNAO), J.-M. Bai 
(YNAO) report on behalf of a larger collaboration:

We again observed the afterglow of GRB 130427A (Maselli et al., GCN 14448
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 ) with 2.4m Gao-Mei-Gu (GMG) telescope from 15:46:37 UT on 2013-04-30. 
We used the sloan filter of r' and obtained two images with the exposure time of 900s each image. The GRB afterglow decayed to r'~19.4 with a middle time of
80.2 hrs post-burst. 

We thank the GMG staff, especially Jian-Duo He and Gui-Hua He for performing these observations.

Poonam Chandra (NCRA-TIFR) reports on behalf of a larger team:

We carried out Giant Metrewave Radio Telescope (GMRT) observations of
GRB 130427A at 1390 GHz band on 2013 Apr 30.57 UT. In our preliminary
analysis we detect the GRB with a flux density of 500+/-94 uJy at
RA, Decl (J200) of 11 32 32.81, +27 41 56.00 which is consistent with
optical position reported by Elenin et al. (GCN 14450
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 ) within error bars.

We thank GMRT staff for making these observations possible. More
observations are planned.

Search for high-energy neutrinos in coincidence with GRB 130427A

The IceCube collaboration (icecube.wisc.edu) reports:

We used the data from IceCube to perform several searches for high-energy neutrinos in spatial and temporal 
coincidence with GRB 130427A (A. Maselli et al., GCN 14485
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 ). 

The first analysis is an automated online search for muon neutrino multiplets of two or more neutrinos in coincidence
within 100 seconds and 3.5 degrees. The search has a threshold of ~1 TeV and does not depend on an external GRB
trigger. No such multiplet was found. 

A second offline analysis is a likelihood based search for muon neutrinos with energies ~1 TeV and higher, with a primary 
background of atmospheric neutrinos and atmospheric muons. Data was scanned in the T90=162.83 s window reported by Swift BAT (S. D. 
Barthelmy et al., GCN 14470
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 ), and found no neutrinos.  We also performed a series of rolling time-window searches 
covering a window of interest of +/- 1 hour relative to the burst time and again found no neutrinos.

A final search focused on  neutrinos of all flavors with energies ~100 TeV and higher that have interaction vertices that fall within the detector in a window 
of interest of +-1 day. Because the intrinsic background for this search is very low only temporal information was used 
in this search. No neutrinos were found in this search.

Implications with respect to neutrino fluence for this GRB will be reported elsewhere. 

IceCube is a kilometer cubed neutrino telescope located at the geographic South Pole sensitive to neutrinos above 
~100 GeV. Previous limits on the emission of neutrinos by GRBs have been presented in R. Abbasi et al., Nature 484, 
351�354 (2012). A description of the multiplet search is found in R. Abbasi et al., A&A 539, A60 (2012). Funding 
acknowledgement and author list for IceCube are listed [http://icecube.wisc.edu/collaboration/authors/current here]

I. van de Stadt (AWSV Metius, the Netherlands), K. Wiersema (U. of Leicester),
T. Bekkers (AWSV Metius), M. Seynen (AWSV Metius) and
F. Nieuwenhout (AWSV Metius) report:

We observed the afterglow of GRB 130427A with the ABT, a 10-inch remote controlled observatory in Alkmaar,
The Netherlands. In a series of V band exposures the afterglow is detected with
V=18.70 +/- 0.13 mag at midtime 1.633 days after burst.

A. Corsi (GWU) reports on behalf of a larger collaboration:
 
We imaged the position of GRB 130427A (Maselli et al., GCN 14448
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 ) with
the Very Large Array in the 20-22 GHz frequency band, starting at about
1.9 days after the burst. A provisional reduction shows a source consistent
with the location of the GRB optical (e.g., Elenin et al., GCN 14450
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 ; Perley et al.,
GCN 14451
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 ) and radio (e.g., Zauderer et al., GCN 14480
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 ) afterglow. At this time
and frequency (~21 GHz), we estimate a preliminary flux of about 1.3 mJy.
 
Further observations are planned.
We thank the VLA staff for their support.

Aman Kaur, Dieter Hartmann (Clemson University), and William C. Keel (U.
of Alabama, Tuscaloosa),  report:

We continued our observations of GRB 130427A (Maselli, et al., GCN 14448
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 ) 
with the SARA 0.9m telescope at KPNO  (http://saraobservatory.org)
starting on April 29. We obtained a total of 190 min exposure in the R
band, with a sequence of 36 five min exposures,  and one ten  min
exposure.

We detect the optical afterglow (Elenin et al; GCN14450
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 ) decay between  R
= 19.2 and R = 19.4
during that sequence, which implies a temporal power law decay of the flux
density with a slope of
alpha ~ 0.8

Magnitudes are not corrected for Galactic extinction.

This message may be cited

WeiKang Zheng, S. Bradley Cenko, Alexei V. Filippenko and Adam Morgan
(UC Berkeley) report on behalf of the KAIT GRB team:

We observed the afterglow of GRB 130427A (e.g., Maselli et al., GCN
14448
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 ; Elenin et al., GCN 14450
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 ; Perley, GCN 14451
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 ) with the 0.76-m
Katzman Automatic Imaging Telescope (KAIT) located at Lick Observatory
on Apr. 28, 29, 30, and May 1 UT. Observations were carried out in the
B, V, R, and I filters. A series of images, lasting for about an hour,
was taken on each night. The afterglow is well detected in all filters,
and the estimated R-band magnitudes (calibrated to SDSS, transformed to
R) are as follows, with times relative to the BAT trigger:

t_mid(hr) exp(s) R(mag) err

20.2 60.0 17.4 +/- 0.1
45.1 60.0 18.5 +/- 0.1
69.1 360.0 19.1 +/- 0.1
93.1 600.0 19.6 +/- 0.2

A power-law fit to the data shows the afterglow continues to decay with
an index of roughly -1.0 (Itoh et al., GCN 14486
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 ).

Further observations are planned.

R. Ruffini, C.L. Bianco, M. Enderli, M. Muccino, A.V. Penacchioni, G.B. Pisani, J.A. Rueda, N. Sahakyan, Y. Wang, L. Izzo report:

The late x ray observations of GRB 130427A by Swift-XRT clearly evidence a pattern typical of a family of GRBs associated to supernova (SN) following the Induce Gravitational Collapse (IGC) paradigm (Rueda & Ruffini 2012; Pisani et al. 2013). We assume that the luminosity of the possible SN associated to GRB 130427A would be the one of 1998bw, as found in the IGC sample described in Pisani et al. 2013. Assuming the intergalactic absorption in the I-band (which corresponds to the R-band rest-frame) and the intrinsic one, assuming a Milky Way type for the host galaxy, we obtain a magnitude expected for the peak of the SN of I = 22 - 23 occurring 13-15 days after the GRB trigger, namely between the 10th and the 12th of May 2013.

Further optical and radio observations are encouraged.

D. Kuroda (OAO, NAOJ),  H. Hanayama, T. Miyaji, J. Watanabe (IAO, NAOJ),
K. Yanagisawa (OAO, NAOJ), S.Nagayama (NAOJ), M. Yoshida (Hiroshima),
K. Ohta (Kyoto) and N. Kawai(Tokyo Tech)
report on behalf of the MITSuME collaboration:

We observed the field of GRB 130427A (Masell et al., GCNC 14448)
with the optical three color (g', Rc and Ic) CCD camera attached
to the Murikabushi 1m telescope of Ishigakijima Astronomical
Observatory.

We detected the previously reported afterglow (Elenin et al., GCNC 14450)
in only Rc-band. The observation started on 2013-05-02 13:08:42 UT
(~5.2 days after the burst).

Photometric result and three sigma upper limits of the OT are listed below.
We used SDSS catalog for flux calibration.

#T0+[day]  MID-UT    T-EXP[sec]   g'   Rc   Rc_err  Ic
---------------------------------------------------------
5.25248    13:51:30    540.0    >20.7  20.3 0.3   >19.1
---------------------------------------------------------
T0+ : Elapsed time after the burst [day]
T-EXP: Total Exposure time [sec]

H. Flewelling, A. Schultz, N. Primak, K. C. Chambers, E. A. Magnier, W.
Sweeney, C. Z. Waters, S. Chastel, M. E. Huber, I. Smith, report on behalf
of Pan-STARRS 1:

Pan-STARRS 1, a 1.8 m survey telescope located at Haleakala, Hawaii,
observed GRB 130427A (Masell et al., GCNC 14448) a total of 4 times in the
z and y filters using the 1.4 Gigapixel camera, in the course of normal
survey operations. PS1 detected the afterglow (Elenin et al., GCNC 14450),
and the observations were calibrated with the "ubercal" technique (Schafly
et al 2011) and the PS1 reference catalog (Magnier et al 2012). The results
are:

t_mid(hr) exp(s) filter      mag
108.18    80.0    y      19.52 +/- 0.10
108.21    80.0    y      19.66 +/- 0.09
111.35    60.0    z      19.84 +/- 0.07
111.37    60.0    z      19.81 +/- 0.06

Note the host galaxy pre-outburst has been observed in the PS1 3pi survey
and thus photometry of the host galaxy is available.

This discovery was made possible by the PS1 system operated by the
 PS1 Science Consortium and its member institutions (
http://www.ps1sc.org/PS1_System_GCN.shtml ). We thank the telescope
 operators of the PS1 telescope for their support.

D. Lennarz, I. Taboada (Georgia Tech) report on behalf of the HAWC
collaboration (http://www.hawc-observatory.org/collaboration/):

We used data from the partially built HAWC detector to perform a search
for VHE emission in temporal coincidence with GRB 130427A (A. Maselli et
al., GCN 14448
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 ). This search was conducted using the scaler data
acquisition only, as the main data acquisition was not operational at
the time. At the time of the GBM trigger, the elevation of the burst in
HAWC's field of view, was only 33.13 degrees and setting. The
sensitivity of HAWC at this elevation is more than 2 orders of magnitude
poorer than near the zenith. Furthermore, while near zenith the nominal
threshold of the scaler system is a few GeV, towards the horizon the
nominal threshold is much higher.

We used six search windows with respect to the GBM trigger time: one in
the range 0 s to 20 s, which covers the bright structured peak seen by
Fermi-GBM that seems to be correlated with the Fermi-LAT emission (S.
Zhu et al., GCN 14508
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 ), an extended window from -5 s to 55 s and a
window from -5 s to 145 s, which is slightly larger than the T90
reported by GBM. We also searched around the second peak in the GBM
light curve (120 s to 300 s), -10 s to 10 s around the time of the
highest energy LAT photon and in an extended window from -10 s to 290 s.
We find a deviation of +38960 / -77884 / -337877 / -165991 / -519485 /
-1036 of the global PMT count with respect to a moving average in the
six time windows mentioned above. The p-value for these deviations
assuming background hypothesis are 17 % / 78 % / 95 % / 71 % / 90 % and
50 % respectively. Our observations are consistent with background only.

The implications of this non-detection with respect to the VHE fluence
of this GRB will be reported elsewhere.

HAWC is a gamma-ray detector under construction in Central Mexico. It
currently consists of 29 operational Water Cherenkov Detectors out of
300 planned. A detailed description of the sensitivity of HAWC to GRBs
can be found in A.U. Abeysekara et al., Astroparticle Physics 35,
641-��650 (2012).

D. Frederiks, on behalf of the Konus-Wind team, reports:

We have also found a typo in our GCN Circ. 14487
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  "Konus-Wind observation of GRB 130427A":
the correct energy range for the total K-W energy fluence and the peak flux estimations
for GRB 130427A is 20-10000 keV, not "20 - 1200 keV", as mentioned.

The 20-10000 keV range is a standard for K-W estimations of GRB energetics
in the observer frame.

We thank D.A.Kann for his help in clearing this issue and sorry again for the inconvenience.

V. Rumyantsev (CrAO), K.Antoniuk (CrAO),  D. Shakhovskoy (CrAO), A. 
Pozanenko (IKI) report on  behalf of larger GRB follow-up collaboration:

We continue observation of the Swift GRB 130427A (Maselli et  al., GCN 
14448
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 )  with AZT-11 telescope of CrAO observatory. Preliminary 
photometry the optical afterglow of GRB 130427A (Maselli et al., GCN 
14448
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 , Elenin et al. GCN 14450
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 , Perley GCN 14451
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 ) is following

T_start (UT)             T0+     Filter Exp.  OT
                          mid,days       s     mag
2013-05-02T19:34:45.35   5.5618  R      6120  19.75 +/- 0.15
2013-05-03T20:41:27.17   6.5581  R      3600  20.30 +/- 0.25
2013-05-04T18:26:22.25   7.4747  R      5400  20.20 +/- 0.18

Photometry is based on SDSS star
SDSS id             B      eB    V      eV    R      eR    I      eI
J113220,11+274133,5 17,421 0,020 16,911 0,020 16,561 0,015 16,124 0,016

David M. Smith (UC Santa Cruz), Andre Csillaghy (Fachhochschule Nordwestschweiz),
Kevin Hurley (UC Berkeley), Hugh Hudson (UC Berkeley, U. Glasgow), Steven Boggs
(UC Berkeley), and Andrew Inglis (NASA Goddard/CUA) report:

The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI)
satellite observed the prompt emission of GRB 130427A (Maselli et
al. , GCN 14448
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 ), with usable data over the approximate range 50 keV to
15 MeV, and a time resolution of 1us on each photon recorded.
Absolute timing accuracy is ~1ms.  Since the GRB was 122 degrees from
the Sun, the photons entered through the rear of the spectrometer.

The lightcurve of the main outburst in three energy bands is shown binned to 25ms 
resolution at:
http://scipp.ucsc.edu/~dsmith/rhessi/grb/130427a_longplot.gif
The black, red, and orange lightcurves represent the energy ranges 50 keV to 1 MeV,
1-5 MeV, and 5-15 MeV, respectively.  

The lightcurve of the slower, fainter peak of the GRB, about two minutes after
the primary outburst, is shown here:
http://scipp.ucsc.edu/~dsmith/rhessi/grb/130427a_lateplot.gif
The black, red, and orange lightcurves in this case represent the energy ranges 
50-200 keV, 200-500 keV, and 500 keV to 1 MeV, respectively.

RHESSI data are publicly available.  We invite anyone intending to use the RHESSI
data to consult with us about instrumental issues associated with the observation.

D. A. Kann, B. Stecklum, and F. Ludwig (TLS Tautenburg) report:

We observed the optical afterglow (Elenin et al., GCN 14450
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 ) of the nearby
(Levan et al., GCN 14455
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 ), extremely bright GRB 130427A (Maselli et al.,
GCN 14448
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 ; Zhu et al., GCN 14471
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 ; von Kienlin, GCN 14473
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 ; Golenetskii et
al., GCN 14487
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 ) with the 1.34m Schmidt telescope of the Thueringer
Landessternwarte Tautenburg equipped with a 4k CCD camera under good
weather conditions. We obtained 3 x 600 sec frames in the Rc band. The
afterglow is detected in each frame.

Using the nearby star given in Rumyantsev et al. (GCN 14582
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 ), we derive a
preliminary magnitude of Rc = 20.27 +/- 0.07 at 8.55173 days after the
GRB.

This value is in good agreement with detections from the last few days
(Kuroda et al., GCN 14534
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 ; Rumyantsev et al., GCN 14582
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 ). The afterglow
does not yet show clear signs of flattening associated with either a
rising supernova component or a significant contribution from the
underlying host galaxy.

This message may be cited.

L. Hermansson, P. Holmstr�m, M. Johansson (Sandvreten Observatory, Sweden)

We observed the field of GRB 130427A (Maselli et al., GCN 14448
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 ; Elenin et
al., GCN 14450
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 ) with the 0.45 m f/4.5 Newton located at Sandvreten
Observatory, Sweden.

Observations were obtained between 2013-Apr-27 22:29:03 and 23:48:43 UT. Two
images each were obtained in each of B, V, Rc and Ic bands with Sch�ler
Johnson-Cousins photometric filters and an SBIG ST-7E CCD.

The following magnitudes were obtained from the observations with Maxim DL
software using four SDSS comparison stars transformed to the BVRI system
using Lupton (2005). Magnitudes are not corrected for extinction.

Filter    Tmid (T0+day)   Exp (s)    Mag         Err
B         0.65231         2x600      17.74       0.06
V         0.63317         2x300      17.38       0.03
Rc        0.61726         2x300      16.99       0.03
Ic        0.66821         2x300      16.78       0.05

We wish to thank D. A. Kann and the Cosmoquest forum for alerting us to GRB
130427A. We are also grateful for the guidance D.A. Kann provided during the
preparation of this report.

D. Xu (DARK/NBI), A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), T.
Kruehler, D. Malesani (DARK/NBI), G. Leloudas (OKC, Stockholm and
DARK/NBI), J.P.U. Fynbo, J. Hjorth, (DARK/NBI), S. Schulze (PUC and
MCSS), P. Jakobsson, Z. Cano (U. Iceland), J. Gorosabel
(IAA-CSIC/UPV-EHU), report:

We have been monitoring the optical counterpart of GRB 130427A
(Maselli et al., GCN 14448
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 ; Elenin et al., 14450) starting 12.85 hr
after the GRB trigger (Xu et al., GCN 14478
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 ), mainly using the 2.5
Nordic  Optical Telescope (NOT) equipped with the ALFOSC camera.
Observations were carried out using the SDSS filters.

The light curve between ~1.0 and 5.0 days after the trigger (observer
frame) is well fit by a power law with decay index 1.3. Starting from
day 5.0, however, the light curve gradually flattens. The flattening,
albeit reduced, is still evident after subtracting the (known) flux
contribution of the host galaxy. In particular, clear flux in excess
of the afterglow and host contribution is apparent on May 5 and 6,
that is 8.6 and 9.6 days after the GRB.

Photometry in the Sloan griz filters was secured during the night of
May 6. After subtracting from the observed flux the host contribution,
and correcting for the (small) Galactic extinction, the SED clearly
deviates from a power-law, in sharp contrast with our earlier
measurements and the typical spectrum of GRB afterglows. Instead, the
griz SED shows a broad hump peaking in the i and r bands, which is
roughly consistent with the spectrum of other broad-lined SNe
associated with GRBs at comparable epochs (e.g., SN 1998bw: Patat et
al. ApJ, 555 900; SN 2006aj: Pian et al., Nat. 442,1011).

The flattening in the decay, the change of the spectral shape, and the
overall flux level are all consistent with the emergence of a SN,
though detailed spectroscopy and long-term monitoring will be required
to fully assess the nature of the flux excess.

Martin Topinka, Lorraine Hanlon, Pete Tisdall, Seamus Meehan, Antonio Martin-Carrillo (UCD, Dublin) and Petr Kub��nek (FZ�� AV��R, Praha) on behalf of the Watcher telescope team, report:

We followed the Swift detection of GRB 130427A (Maselli et al., GCN 14448
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 , Swift trigger 554620) with the Watcher robotic telescope (D=40cm) located at Boyden Observatory, near Bloemfontein in South Africa.

We started imaging the field at 16:43:55 UT on 2013-04-27 (i.e. 8.9 hr after the BAT trigger) taking 2 minute exposures in the clear filter using an Andor CCD camera.

The optical afterglow (Elenin et al., GCN 14450
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 ) is clearly detected. Preliminary analysis of some of the initial images gives a magnitude for the optical afterglow of 16.59+/-0.04 @ 17:00:35 UT and 16.79+/-0.07 @ 18:59:13 UT.

Magnitudes were estimated using several nearby USNO-B1 stars as references and are not corrected for galactic dust extinction. Further analysis of these images is on-going. The field continues to be regularly observed with Watcher.

This message is quotable in publications.

P. Garnavich (Notre Dame) reports:

A spectrum of the GRB 130427A afterglow was obtained with the
Large Binocular Telescope (LBT+MODS1 instrument) on 2013 May 7.15 (UT),
9.8 days after the burst. The spectrum covers 340 nm to 950 nm and
is dominated by a power-law continuum. Narrow Balmer, [OII], [OIII]
emission lines, and MgII and MgI absorption lines from the host
galaxy are present at a redshift of 0.340.

The LBT spectrum shows no obvious undulations characteristic of a
broad-lined type Ic supernova such as SN 1998bw. In contrast,
a 98bw-like supernova was detectable from GRB 030329 around
seven days after its burst (Stanek et al. 2003, ApJ, 591, L17).
This early detection was primarily due to the prominent peak
seen around 500 nm (rest frame) in broad-lined type Ic events.

Adding a pre-maximum spectrum of SN 1998bw
(Patat et al. 2001, ApJ, 555, 900) to a power-law continuum
suggests that any 98bw-like supernova is at least an R-band
magnitude fainter than the afterglow 10 days after the
GRB 130427A burst.

I thank Rick Pogge, Paul Martini and Scott Adams for help in
obtaining the spectra.
The LBT is an international collaboration among institutions in the
United States, Italy and Germany. LBT Corporation partners are:
The University of Arizona on behalf of the Arizona university system;
Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft,
Germany, representing the Max-Planck Society, the Astrophysical
Institute Potsdam, and Heidelberg University; The Ohio State University,
and The Research Corporation, on behalf of The University of Notre Dame,
University of Minnesota and University of Virginia.

Alan M. Watson (UNAM), Nat Butler (ASU), Alexander Kutyrev (GSFC),
William H. Lee (UNAM), Michael G. Richer (UNAM), Chris Klein (UCB), Ori
Fox (UCB) J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino
Cucchiara (UCSC), Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico
Ramirez-Ruiz (UCSC), José A. de Diego (UNAM), Leonid Georgiev (UNAM),
Jesús González (UNAM), Carlos Román-Zúñiga (UNAM), Neil Gehrels (GSFC),
and Harvey Moseley (GSFC) report:

We have been monitoring GRB 130427A with the Reionization and
Transients Infrared Camera (RATIR; www.ratir.org) on the 1.5m Harold
Johnson Telescope at the Observatorio Astronómico Nacional on Sierra San
Pedro Mártir. We have precise and homogeneous photometry for all nights
except 2013 May 6.

During the first night the optical afterglow is well-fitted by a power
law with an index of -1. However, around 1 day after the burst there is
a break, and the power law steepens.

Our photometry in gri from 2 to 11 days is well-fitted by a power law
with an index very close to -1.5 plus a constant contribution with i =
21.23 ± 0.05, g-i =  0.74 ± 0.12, and r-i =  0.05 ± 0.05, consistent
with the SDSS DR9 photometry of the presumed host galaxy.

We see no evidence for an additional component such as the one mentioned
by Xu et al. (GCN Circular 14597
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 ).

Further observations are planned.

We thank the staff of the Observatorio Astronómico Nacional in San Pedro
Mártir.

A. Trotter, D. Reichart, J. Haislip, A. LaCluyze, K. McLin, L. Cominsky, 
T. Berger, H. T. Cromartie, R. Egger, A. Foster, N. Frank, K. Ivarsen, 
M. Maples, J. Moore, M. Nysewander, E. Speckhard, and J. A. Crain report:

Skynet continued observing the Swift/XRT localization of GRB 130427A 
(Maselli et al., GCN 14448
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 , Swift trigger #554620) with four 16" 
telescopes of the PROMPT array at CTIO, Chile (BVRI bands), and with the 
14" GLAST Optical Robotic Telescope (GORT) at the Hume Observatory in 
California (RcIc bands). Our observations span 10 nights, from t=0.65 to 
10.8 days post-trigger. Skynet has taken 2684 160-second exposures on 
the 4 PROMPT telescopes, and 360 160-second exposures on GORT, or a 
total of over 135 hours on source. We performed photometry on each 
exposure, calibrated to two SDSS stars in the field. We stacked 
exposures to improve sensitivity, in groups ranging from 3 exposures on 
night 1, to 60 exposures on night 10.

We detect a fading afterglow in BVRI at the position reported by Elenin 
et al. (GCN 14450
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 ), which is ~50" south of the initial XRT localization. 
From night 2 onwards, the light curves fade with an approximate power 
law index alpha=-1 (with no corrections for the known host galaxy flux). 
We see some evidence for flattening of the I-band light curve beginning 
at t~8 days, and of the R-band curve at t~10 days, though it is not 
clear whether this is due to host galaxy contamination or to an 
intrinsic re-brightening.

A preliminary light curve including all Skynet observations through 
t=10.8 days is at:
http://skynet.unc.edu/grb/grb130427a_10.png

See Trotter et al. (GCN 14497
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 , GCN 14510
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 ) for descriptions and light 
curves of the first and second nights' observations. Further Skynet 
observations are ongoing.

D. A. Perley and S. Tang (Caltech) report:

On the night of 2013-05-09 UT we observed the location of GRB 130427A 
with the Low Resolution Imaging Spectrometer (LRIS) on the Keck I 10m 
telescope, during excellent weather conditions (clear skies and 0.7 
arcsecond seeing).

In a pair of 90 second g-band images we clearly detect the transient 
superimposed on a faint, extended source that we identify as the host 
galaxy.  While blended with the light of the transient, the diameter of 
this extended emission is approximately 3 arcseconds, corresponding to a 
physical size of ~14 kpc at a redshift of z=0.34.  The magnitude of the 
transient at this time (within a 1" aperture centered on the optical 
position) is:

  g = 21.23 +/- 0.04 mag  (t = 12.00 days)

This is consistent (within uncertainties) with the rate of decay seen in 
recent P60 observations between 1-8 days post-GRB after subtraction of 
the host galaxy.

We also acquired a deep sequence of spectroscopic observations (2000 sec 
total integration) with LRIS, covering a wavelength range from 
approximately 3250 to 10300 Angstroms.   We observe no broad features or 
other evidence of contribution of a supernova to the spectrum at this 
time, similar to as reported from LBT observations two nights previously 
(Garnavich et al., GCN 14605
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 .)

We thank and S. R. Kulkarni and the PTF collaboration for these 
observations.

K. Wiersema (U. of Leicester), O. Vaduvescu (ING), N. Tanvir (U. of Leicester), 
A. Levan (Warwick) and O. Hartoog (U. of Amsterdam) report:

We observed the position of GRB 130427A with the 4.2m William Herschel telescope,
using the PFIP camera, on May 8th, under good seeing conditions (0.7 arcseconds). 
Exposures of 4x600 seconds were obtained using a narrowband filter covering 
the [O II] emission line doublet (3728 A) at the redshift of the 
GRB (z=0.3399; Levan et al., Xu et al. and Flores et al.; GCN 14455
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 , 14478
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 , 14491
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 ).
We used this filter to obtain the best visibility of the host galaxy against the bright afterglow
and possible supernova contribution.
The resulting data show a clear detection of the host galaxy. The GRB is located near, but
somewhat offset from, a brighter patch in the host. The host is an irregular galaxy, with
a broadly elliptical shape. The GRB is located North-West of the
majority of extended, smooth, host emission - a convenient choice in spectrograph slit
position angle may minimize host contamination and aid in identification of SN signatures.
The long axis of the host is approximately oriented along 70 degrees position angle (where
North=0, East = 90 degrees), and is approximately 3.4 arcseconds in length.

A jpg finder chart of the [OII] imaging can be found here:
http://www.star.le.ac.uk/~kw113/grb130427A/hostgalaxy_130427a.jpg

[GCN OPS NOTE(09may13): Per author's request, the "Apr 8" was changed to "May 8"
in the first sentence.]

D. A. Kann, B. Stecklum, and C. Hoegner (TLS Tautenburg) report:

We observed the optical afterglow position (Elenin et al., GCN 14450
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 ) of
GRB 130427A (Maselli et al., GCN 14448
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 ) with the 1.34m Schmidt telescope
of the Thueringer Landessternwarte Tautenburg equipped with the 2k CCD
camera under good weather conditions. We obtained 3 x 600 sec frames in
the Rc band. The afterglow is detected in each frame.

Using the nearby star given in Rumyantsev et al. (GCN 14582
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 ) and also used
in our first epoch observations (Kann et al., GCN 14592
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 ), we derive a
preliminary magnitude of Rc = 20.37 +/- 0.07 at 15.54806 days after the
GRB.

This magnitude is only insignificantly fainter than the one we derived in
the first epoch, evidencing a clear flattening (see also Xu et al., GCN
14597
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 ). The host galaxy is expected to have about 21st magnitude in Rc
(Vega) following r' = 21.26 from SDSS (see, e.g., Watson et al., GCN
14606
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 ). Subtracting this magnitude from our detection yields a magnitude
for the optical transient of ~ 21.2 +/- 0.2.  This value agrees well with
an extrapolation of the earlier slope, implying that no further break has
occurred in the optical light curve (in agreement with the X-ray decay,
which shows a very similar slope). This implies either that the post-jet
break decay is among the most shallow known, or that a jet break has still
not occurred, pushing GRB 130427A further into the territory of hyper-
luminous events (Fan et al., arXiv:1305.1261, though see Laskar et al.,
arXiv:1305.2453).

The situation concerning a rising supernova is still unclear. Xu et al.
(GCN 14597
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 ) claimed a host-independent flattening and spectral change,
which was afterwards disputed on photometric (Watson et al., GCN 14606
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 ;
Perley & Tang, GCN 14615
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 ) and spectroscopic (Garnavich, GCN 14605
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 ; Perley
& Tang, GCN 14615
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 ) grounds. Our measurement offers no solution to this
conundrum, but it is possible that the SN, even if as luminous as SN
1998bw, will peak at a magnitude significantly fainter than the host
galaxy and afterglow (Ruffini et al., GCN 14526
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 ), making detection more
difficult than even in the case of GRB 030329/SN 2003dh.

We wish to thank T. Kruehler for discussions relating to the host galaxy.

This message may be cited.

A. Volnova (IKI), I. Korobtsev (ISTP), E. Klunko (ISTP), A. Pozanenko 
(IKI)  on behalf of  larger GRB  follow-up collaboration:

We continue observation of the Swift GRB 130427A (Maselli et  al., GCN 
14448
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 )  with AZT-33IK telescope of Sayan observatory (Mondy). The 
afterglow (Elenin et al. GCN 14450
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 , Perley GCN 14451
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 ) is clearly 
detected. The brightness of the afterglow+host  is following:

UT start,            t-t0       Filter Exp.   OT
                     (mid, days)        (s)    (mag.)

2013-05-13T15:20:00  16.3346   R      3600    20.66 +/- 0.06
2013-05-14T14:24:35  17.2968   R      3600    20.63 +/- 0.07

The photometry is based on the same star  reported by Rumyantsev et al. 
(GCN 14582
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 ). After subtraction  of a suggested brightness (R, Vega)of 
the host galaxy (e.g. Watson et al., GCN 14606
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 ; Kann et al., GCN 14631
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 ) 
from our photometric values, our light curve can be approximated (in 
general)  by a single power law   starting at ~ 0.6 days after burst 
trigger.  Indeed our early observations suggest some flattening between 
6.5 - 13 days (Xu et al., GCN 14597
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 ; Kann et al., GCN 14631
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 ). It could 
be due to a SN or due to a wide bump analogous to bumps observed early 
in the light curve.  However photometry is still preliminary and more 
detailed calibration/intercalibration is necessary.

A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), D. Xu (DARK/NBI), 
G. Leloudas (OKC, Stockholm, DARK/NBI),  T. Kruehler, 
D. Malesani (DARK/NBI), J. Gorosabel (IAA-CSIC, UPV/EHU), Z. Cano (U. Iceland),
C.C. Thoene, R. Sanchez-Ramirez (IAA-CSIC), S. Schulze (PUC and MCSS), 
J.P.U. Fynbo, J. Hjorth (DARK/NBI), P. Jakobsson (U. Iceland) and 
A. Cabrera-Lavers (IAC-ULL) report on behalf of a larger collaboration:

We obtained spectroscopy of the optical counterpart and host galaxy of 
GRB 130427A (Maselli et al., GCN 14448
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 ; Elenin et al., GCN 14450
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 ) with the 
10.4m GTC telescope, 16.7 days after the GRB onset. This is 12.5 days in the 
host galaxy rest-frame (z = 0.34; Levan et al. GCN 14455
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 , Xu et al. GCN 14478
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and Flores et al. GCN 1449
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 ). Observations consisted of 4x1200s with the 
R500R grism, covering the range between 4800 and 10000 AA with a 
resolution of ~600. The slit was oriented to cover both the afterglow and the 
host galaxy centre.

The spectrum has a strong contribution from the host galaxy. To overcome this, 
we built a synthetic host galaxy spectrum based on the SDSS (DR9) photometry 
using LePhare (version 2.2, Arnouts et al. 1999, MNRAS, 310, 540; Ilbert et al. 
2006, A&A, 457, 841). We then subtracted this host galaxy template from the 
GTC spectrum to obtain a "clean" spectrum of the counterpart associated to GRB 
130427A.

The resulting spectrum is that of a broad-lined Ic SN, with a prominent bump at 
~6800 A observer frame. In particular, we obtain an excellent match with the 
spectrum of  SN 2010bh at 12.7 (rest-frame) days after GRB 100316D 
(Bufano et al. 2012, ApJ 753, 67).

We stress that this conclusion is independent of the host galaxy model 
adopted. By running SNID (Blondin & Tonry 2007, ApJ, 666, 1024) on the 
original spectrum (i.e. including host contamination), we still obtain good 
matches with a series of broad-lined Type Ic SNe, including SNe 1998bw, 
1997ef, 2002ap and 2006aj, albeit at a lower redshift. The fact that SNID 
suggests a lower redshift  is explained by the fact that SN 2010bh had high 
expansion velocities, reaching ~34000 km/s at similar phases (Bufano et al. 
2012, ApJ 753, 67), which we suggest is also the case for the SN associated 
with GRB 130427A.

A figure of our preliminary analysis can be seen at:

http://www.iaa.es/~deugarte/GRBs/130427A/130427A_GTC.jpg

We acknowledge excellent support from the GTC staff.

[GCN OPS NOTE(14may13): Per author's request, ZCwas added to the author list.]

A. Trotter, D. Reichart, J. Haislip, A. LaCluyze, K. McLin, L. Cominsky, 
A. Smith, D. Caton, L. Hawkins, B. Holmes, T. Linder, T. Berger, H. T. 
Cromartie, R. Egger, A. Foster, N. Frank, K. Ivarsen, M. Maples, J. 
Moore, M. Nysewander, E. Speckhard, and J. A. Crain report:

Skynet has continued observing the Swift/XRT localization of GRB 130427A 
(Maselli et al., GCN 14448
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 , Swift trigger #554620) with: four 16" 
telescopes of the PROMPT array at CTIO, Chile (BVRI bands); the 14" 
GLAST Optical Robotic Telescope (GORT) at the Hume Observatory in 
California (RcIc bands); the 14" Deep Sky Observatory (DSO-14) telescope 
at Pisgah National Forest, NC; and the 30" telescope at the Astronomical 
Research Observatory (ARO-30) in Westfield, IL. Our observations now 
span 18 nights, from t=0.65 to 17.6 days post-trigger. Skynet has taken 
3420 160-second exposures on the 4 PROMPT telescopes, 420 160-second 
exposures on GORT, 91 160s exposures on DSO-14 and 133 60-160s exposures 
on ARO-30, or a total of over 178 hours on source. We performed 
photometry on each exposure, calibrated to two SDSS stars in the field. 
We stacked exposures to improve sensitivity, in groups ranging from 3 
exposures on night 1, to 60 exposures on night 18.

In Trotter et al. (GCN 14608
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 ) we reported a flattening of the light 
curve at t~10 days.  That flattening has continued, with possible 
chromatic bumps in V, R and I bands at ~14d, 11d and 10d, respectively.  
Our most recent observations, at t=17.8d, show a rebrightening in V, R 
and I bands; we speculate that this may be the onset of the classical 
supernova, which was detected spectroscopically by de Ugarte Postigo et 
al. (GCN 14646
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 ) at t=16.7 days.

A preliminary light curve including all Skynet observations through 
t=17.8 days is at:
http://skynet.unc.edu/grb/grb130427a_17.png

Further observations are scheduled.

Alan M. Watson (UNAM), Nat Butler (ASU), Alexander Kutyrev (GSFC),
William H. Lee (UNAM), Michael G. Richer (UNAM), Chris Klein (UCB), Ori
Fox (UCB), J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino
Cucchiara (UCSC), Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico
Ramirez-Ruiz (UCSC), José A. de Diego (UNAM), Leonid Georgiev (UNAM),
Jesús González (UNAM), Carlos Román-Zúñiga (UNAM), Neil Gehrels (GSFC),
and Harvey Moseley (GSFC) report:

We have continued to monitor GRB 130427A with the Reionization and
Transients Infrared Camera (RATIR; www.ratir.org) on the 1.5m Harold
Johnson Telescope at the Observatorio Astronómico Nacional on Sierra San
Pedro Mártir, obtaining homogenous photometry in griZYJH. We have
photometry for every night except 2013 May 6. On most nights our
photometric uncertainties in gri are about 2%.

As we reported earlier in Watson et al. (GCN Circular 14606
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 ), the
optical afterglow during the first day is well-fitted by a power law
with a temporal index of -1. However, around T+1d there is a break, and
the power law steepens. From T+2.5d to T+14.9d our gri photometry is
well-fitted by a power law with a temporal index close to -1.5 plus a
constant component consistent with the presumed SDSS host galaxy.

However, our observations at T+15.9d, T+16.9d, and T+17.9d are
systematically brighter than this fit. Adding a new component starting
at T+15.5d with zero colors and constant magnitude significantly
improves the fit (with a confidence level of better than 99.5%). The
constant component has

  g = r = i = 24.53 ± 0.25.

We do not mean to suggest that the new component actually has zero color
or constant magnitude. However, at this moment our data cannot usefully
constrain anything other than a characteristic brightness.

Our data, model, and residuals are shown at

  https://dl.dropboxusercontent.com/u/528672/GCN/2013-05-16-GRB-130427A.pdf

Assuming a distance modulus of 41.26, the new component corresponds to
an absolute magnitude of -16.7 ± 0.25. If the new component is a Type 1c
supernova, as suggested by de Ugarte Postigo et al. (GCN Circular
14646
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 ), we might expect the peak extinction-corrected absolute magnitude to be
around -18 (Drout et al. 2011, ApJ, 741, 97). Thus, depending on the
host galaxy extinction, we might be seeing this possible supernova at or just
before its peak.

We caution that the new component is currently about 2 magnitudes
fainter than the afterglow component, which at 18.0 days is predicted to
have

  i = 22.21 ± 0.04

and even fainter then the galaxy, which is predicted to have

  i = 21.23 ± 0.03.

The relative brightnesses of the new component, the fading afterglow,
and the host galaxy also have significant implications for unveiling the
spectrum of the possible supernova.

We further caution that from our data alone we cannot exclude the
possibility that the new component might simply be a significant
flattening of the late afterglow component.

The largest residuals of our observations from the model (with or
without the new component) are at the level of 0.05 magnitudes. We do
not see the large variations reported by Trotter et al. (GCN Circular
14662
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 ).

Further observations are planned.

We thank the staff of the Observatorio Astronómico Nacional in San Pedro
Mártir.

Vladimir V. Sokolov (SAO-RAS), Alberto J. Castro-Tirado (IAA-CSIC),
Alexander S. Moskvitin, Elena A. Barsukova, Viktoria N. Komarova, Nikolaj
V. Borisov, Azamat F. Valeev, Tatyana N. Sokolova (SAO-RAS) and Vitaly P.
Goranskij (SAI-MSU, SAO-RAS), report:

"We have obtained 3 epochs of spectroscopy for the GRB 130427A optical
afterglow (Maselli et al. GCNC 14448, Elenin et al. GCNC 14450) with the
6-meter BTA equipped with Scorpio. The spectra (with exposure times of
4 x 900 s, 5 x 900 s and 4 x 1200 s) were taken on May 2/3, 5/6 and
10/11 respectively. We used the VPHG 550G grating which covers the
3700-7900 A spectral range and provides a 13 A spectral resolution.

Narrow host galaxy lines such as 3727 A [OII], [OIII] 4959 A, 5007 A and
Balmer lines are noticeable in all spectra. The measured redshift is 0.3393,
in good agreement with the previously reported values (Levan et al. GCNC
14455; Xu et al. GCNC 14478; Flores et al. GCNC 14491; Garnavich GCNC 14605
and Perley & Tang, GCNC 14615). Particularly, we detect marginal excess
emission in the range 6000-7000 A on the later spectrum obtained on May
10/11, which can be interpreted as evidence of the underlying SN (de
Ugarte Postigo et al., GCNC 14646), what is also supported by the
long-term photometric observations (Trotter et al., GCNC 14662; Watson et
al., GCNC 14666).

Our preliminary flux-calibrated spectra can be seen at:

http://www.sao.ru/hq/grb/GRB130427A/GRB130427A_BTA_May2-10.jpg

We thanks S.N. Fabrika, O.P. Zhelenkova, Yu.Yu. Balega, V.V. Vlasyuk and
A.S. Moiseev for their help in obtaining the observations."

J. Kelemen (kelemen at konkoly.hu) on behalf of the GRB OT observing program
at the Konkoly Observatory.

Starting on the evening of 15/05/2013 we observed the field of GRB 130427A
(Maselli et al., GCN 14448
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 ) 18.5435 days after the burst, using a 60/90/180 cm
Schmidt telescope located at the Mountain Station of the Konkoly
Observatory equipped with an Apogee CCD camera through R filter. On the coadded 
R images (total exp.time 1120 sec) we detected the OT and the host galaxy as
well. 
Based on the nearby UCAC-4 stars we provide 20.90 +/- 0.05 magnitude in the R
band for the OT. The brightness of the host galaxy was not subtracted. 

time from GRB    exp    filter     Mag.       

18.5435        1120 s     R       20.9 +/-0.05

A. Melandri (INAF-OAB), V. D'Elia (ASDC), P. D'Avanzo (INAF-OAB), M.  
Della Valle (INAF-OAC), E. Pian (INAF-OAT/SNS), G. Tagliaferri (INAF- 
OAB) on behalf of a larger collaboration report:

We have observed the field of GRB 130427A (Maselli et al., GCN 14448
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 )  
for 9 epochs from t-t0=6.7 to t-t0=18.8 days after the burst event.   
The preliminary light curve in BVR and I bands does not show any  
evidence of a bump related to a SN, and it is marginally consistent  
with a SN component, which should be at least 2 mag fainter than 98bw  
at maximum.

The spectrum (2x1800s) of 13 May, after comparison with SN2010bh  
template, is suggestive, in the range 5000A-7500A of broad GRB-SN  
features, but altogether, we don't find the good similarity that the  
GTC finds (de Ugarte-Postigo et al., CBET 3529). The spectrum of the  
transient (after subtraction of a galaxy template) is available at:

http://www.brera.inaf.it/utenti/davanzo/grb/GRB130427A/GRB130427A.png

Observations have been taken in the framework of the ESO-Program  
091.D-0291 (PI E. Pian).

A.J. Levan (U. Warwick), A.S. Fruchter, J. Graham (STScI), N.R. Tanvir (U. Leicester), Jens Hjorth, Johan Fynbo (Dark Cosmology Centre, Copenhagen), D. Perley (Caltech), S.B. Cenko (U.C. Berkeley), E. Pian (Trieste), Z. Cano (U. Iceland) A. Pe'er (Cork), R. Hounsell (STScI), K. Mishra (ARIES, India), C. Kouveliotou (MSFC) report:

We observed the optical/NIR counterpart of GRB 130427A (Maselli et al. GCN 14448
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 ) with the Hubble Space Telescope beginning at 02:23 UT on 20 May 2013. The afterglow is well detected in our multi-band observations in the UV (F336W), optical (F606W) and NIR (F160W) and is offset approximately 0.8" from the optical centroid of its host. The host itself also contains additional star forming complexes including a bright UV source approximately 0.25" from the GRB position.

In the three bands we measure preliminary magnitudes of the afterglow + supernova of

F336W=23.10 +/- 0.02
F606W=21.85 +/- 0.02
F160W=21.34 +/- 0.03

These magnitudes show significant curvature in the optical likely due to the underlying supernova SN 2013cq (de Ugarte Postigo CBET 3529; Xu et al. GCN 14597
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 ).    If the optical light were entirely dominated by supernova emission the absolute magnitude at z=0.34 would be M_B~ -19.1 at 17 rest-frame days post burst. However, SNe are weaker UV and IR emitters and so under the naive assumption that the UV and IR bands are dominated by power-law afterglow emission with minimal supernova contribution the inferred magnitude of the supernova in the V-band (rest frame B-band) is V~23. This corresponds to an absolute magnitude of M_B ~ -17.9, approximately a magnitude fainter than the B-band peak of SN 1998bw (which occurred at a comparable epoch of 15 days post burst).     However, the SN could contribute as much as one half of the flux we are seeing in the NIR and UV and there may be substantial host emission underneath the object in the optical and UV.   Thus the SN magnitude should be considered very approximate.  
Images of the field are posted at

http://www.star.le.ac.uk/~anl/GRB130427A

We thank the staff of STScI for their work in rapidly scheduling these observations.