GRB 130427B

A. Maselli (INAF-IASFPA), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
N. Gehrels (NASA/GSFC), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL),
A. Y. Lien (NASA/GSFC/ORAU), V. Mangano (INAF-IASFPA),
C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC),
C. J. Mountford (U Leicester), D. M. Palmer (LANL) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 13:20:41 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 130427B (trigger=554635).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 314.891, -22.530 which is 
   RA(J2000)  =  20h 59m 34s
   Dec(J2000) = -22d 31' 49"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows 3 peaks
with a total duration of about 40 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 13:21:59.1 UT, 77.4 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 314.89766, -22.54548 which
is equivalent to:
   RA(J2000)  = 20h 59m 35.44s
   Dec(J2000) = -22d 32' 43.7"
with an uncertainty of 4.4 arcseconds (radius, 90% containment). This
location is 59 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 4.41
x 10^20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 1.15e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 87 seconds after the BAT trigger. There is a candidate afterglow in
the list of sources generated on-board at
  RA(J2000)  =	20:59:35.64 = 314.89848
  DEC(J2000) = -22:32:46.5  = -22.54626
with a 90%-confidence error radius of about 1.10 arc sec. This position is 7.6
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.19. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.07. 

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.)

C. Guidorzi (U. Ferrara), J. Japeli, A. Gomboc
(U. Ljubljana), A. Melandri (INAF-OAB) on behalf of a
large collaboration report:

The 2-m Faulkes Telescope North automatically began observing
GRB 130427B (Maselli et al. GCN Circ. 14460
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 ) at 4.6 minutes from
the Swift/BAT trigger time.
The automatic LT-TRAP procedure identified a bright uncatalogued
object at the following position consistent with that reported
by Maselli et al.:

RA=   20:59:35.62
Dec= -22:32:46.9

(J2000.0).
Its magnitude is estimated to be 15.5 in the R band.

J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 1221 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 130427B, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 314.89849, -22.54670 which is equivalent
to:

RA (J2000): 20h 59m 35.64s
Dec (J2000): -22d 32' 48.1"

with an uncertainty of 1.8 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

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 130427B (trigger #554635)
(Maelli, et al., GCN Circ. 14460
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 ).  The BAT ground-calculated position is
RA, Dec = 314.900, -22.548 deg, which is 
   RA(J2000)  =  20h 59m 36.0s 
   Dec(J2000) = -22d 32' 51.6" 
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 22%.
 
The mask-weighted light curve shows shows a very small peak at ~T-3 sec,
the main peak starting at ~T-1 sec, peaking at ~T+2 sec, and ending at
~T+40 sec.  T90 (15-350 keV) is 27.0 +- 7.3 sec (estimated error
including systematics).
 
The time-averaged spectrum from T-1.29 to T+32.71 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.64 +- 0.15.  The fluence in the 15-150 keV band is 1.5 +- 0.1 x 10^-06 erg/cm2.
The 1-sec peak photon flux measured from T+0.71 sec in the 15-150 keV band
is 3.0 +- 0.4 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/554635/BA/

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 130427B (Maselli et al., GCNC 14460)
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 2013-04-27 18:47:30 UT (~5.4 h after the burst).
We did not find any new point source within the enhanced XRT circle
(Osborne et al., GCNC 14467) in all the three bands.

Photometric results of the OT are listed below.
We used GSC2.3 catalog for flux calibration.

#T0+[day]  MID-UT    T-EXP[sec]   g'     Rc     Ic
-----------------------------------------------------
0.22892    18:50:19    2160.0   >18.0  >18.3  >17.9
-----------------------------------------------------
T0+ : Elapsed time after the burst [day]
T-EXP: Total Exposure time [sec]

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

We have analysed 7.3 ks of XRT data for GRB 130427B (Maselli  et al.
GCN Circ. 14460
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 ),  from 67 s to 24.2 ks after the  BAT trigger. The
data comprise 120 s in Windowed Timing (WT) mode (the first 8 s were
taken while Swift was slewing) with the remainder in Photon Counting
(PC) mode. The enhanced XRT position for this burst was given by
Osborne et al. (GCN. Circ 14467).

The late-time light curve (from T0+11.6 ks) can be modelled with  a
power-law decay with a decay index of alpha=1.9 (+0.6, -0.5).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.93 (+0.14, -0.13). The
best-fitting absorption column is  8.3 (+2.8, -2.6) x 10^20 cm^-2, in
excess of the Galactic value of 4.4 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 1.79 (+0.23, -0.11)
and a best-fitting absorption column consistent with the Galactic
value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.9 x 10^-11 (4.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     0 (+4.7, -0) x 10^20 cm^-2
Galactic foreground: 4.4 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.79 (+0.23, -0.11)

If the light curve continues to decay with a power-law decay index of
1.9, the count rate at T+24 hours will be 9.7 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.8 x
10^-14 (4.2 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00554635.

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

H. Flores (GEPI/Obs. de Paris),  S. Covino (INAF),  A. de Ugarte
Postigo (IAA-CSIC, DARK/NBI), J. Fynbo, T. Kruehler, D. Xu (DARK/NBI)
and N. Tanvir (U. Leicester)  report on behalf of the X-Shooter GRB
collaboration:

We have observed the afterglow of the GRB 130427B (Maselli et al., GCN
14460
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 ) with the ESO VLT equipped with the X-shooter spectrograph. The
observation started on  2013-04-28 at 9:45 UT. A total exposure of
2x600 s was obtained just before morning twilight, covering the
spectral range from 300 to 2500 nm.

In the spectrum we detect a  feature consistent with a damped
Lyman-alpha line in the UVB arm at redshift of z=2.78 +/-  0.02. Given
the low S/N of the spectrum, this redshift measurement should be
considered tentative.

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

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 130427B (Maselli, et al., GCN 14460
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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 28.44 to 2013/04 28.48 UTC (21.26 to
22.19 hours after the BAT trigger), obtaining a total of 0.71 hours
exposure in the r' and i' bands and 0.29 hours exposure in the Z, Y, J, and
H bands.

For a source within the Swift-XRT error circle (Osborne, et al., GCN
14467
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 ), in comparison with USNO-B1 and 2MASS, we obtain the following upper
limits (3-sigma):

  r' > 22.08
  i' > 22.06
  Z  > 21.28
  Y  > 20.77
  J  > 20.57
  H  > 19.84

These limits,  in the AB system and not corrected for Galactic extinction
in the direction of the GRB, were derived from PSF-fitting photometry,
because of the presence of a 16.5 magnitude star only 5 arcseconds to the
south of the SWIFT-XRT source position.

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

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 130427B (Maselli et al., GCNC 14460)
with the optical three color (g', Rc and Ic) CCD camera attached
to the Murikabushi 1m telescope of Ishigakijima Astronomical
Observatory.

The observation started on 2013-04-28 18:47:24 UT (~1.23 days after the burst).
We did not find any new point source within the enhanced XRT circle
(Osborne et al., GCNC 14467) in all the three bands.

Three sigma upper limits of the OT are listed below.
We used GSC2.3 catalog for flux calibration.

#T0+[day]  MID-UT    T-EXP[sec]   g'     Rc     Ic
-----------------------------------------------------
1.25418    19:26:42    3540.0   >20.6  >20.7  >20.1
-----------------------------------------------------
T0+ : Elapsed time after the burst [day]
T-EXP: Total Exposure time [sec]