GRB 120913B

E. A. Helder (PSU), S. D. Barthelmy (GSFC), N. Gehrels (NASA/GSFC),
D. Grupe (PSU), S. T. Holland (STScI), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf
of the Swift Team:

At 23:55:58 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 120913B (trigger=533613).  Swift did not slew because of the
Sun observing constraint. 
The BAT on-board calculated location is 
RA, Dec 213.644, -14.535 which is 
   RA(J2000)  =  14h 14m 35s
   Dec(J2000) = -14d 32' 04"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows several peaks
with a total duration of about 70 sec.  The peak count rate
was ~3000 counts/sec (15-350 keV), at ~5 sec after the trigger. 

Due to a Sun observing constraint, Swift cannot slew to the BAT
position until 20:57 UT on 2012 December 14. There will thus be no XRT
or UVOT data for this trigger before this time. 

Burst Advocate for this burst is E. A. Helder (helder AT psu.edu). 
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.)

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

Skynet observed the field of GRB120913B (GCN 13763
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 , Swift trigger #533613) 
with four of the PROMPT telescopes located at CTIO in Chile in BVRI 
beginning ~58 seconds after the burst. We detect a fading source at RA 
14:14:38.46 DEC -14:30:57.2, within the BAT error circle.

The object peaked at a magnitude of ~15.9 in R (calibrated to nearby SDSS 
catalog stars) at ~4.2 minutes after the burst before beginning to fade.

Further observations are ongoing.

Klotz A. (IRAP-CNRS-OMP), Gendre B. (ASDC/INAF-OAR),
Boer M. (UNS-CNRS-OCA), Atteia J.L. (IRAP-CNRS-OMP) report:

We imaged the field of GRB 120913B detected by SWIFT
(trigger 533613) with the TAROT robotic telescope (D=25cm)
located at the European Southern Observatory,
La Silla observatory, Chile.

The observations started 30.6s after the GRB trigger
(17.7s after the notice). The elevation of the field decreased
from 26 degrees above horizon and weather conditions
were good.

We detect a new fading source in the error box given
by SWIFT. We detected the candidate couterpart
mentioned by LaCluyze et al. (GCNC 13764)
at the following position (+/- 2 arcsec):

RA(J2000.0) = 14h 14m 38.57s
DEC(J2000.0) -14d 30' 55.8"

OT raised until R=15.8 5 min after the GRB.
Now the afterglow is fading.

Magnitudes were estimated with the nearby NOMAD1 star
0754-0289300 (R.A.=213.6332611 Decl=-14.5238889 J2000)
R=15.99. MAgnitutes are not corrected for galactic dust
extinction.

N.B. Galactic coordinates are lon=331.3041 lat=+43.7095
and the galactic extinction in R band is about 0.2 magnitude
estimated from D. Schlegel et al. 1998ApJ...500..525S.

This message may be cited.

Klotz A. (IRAP-CNRS-OMP), Gendre B. (ASDC/INAF-OAR),
Boer M. (UNS-CNRS-OCA), Atteia J.L. (IRAP-CNRS-OMP) report:

We continue to observe the afterglow of GRB 120913B
(LaCluyze et al. GCNC 13764). A rapid photometric
analysis gives the following magnitudes:

t0+Mid(s)  Rmag
    159     16.5
    200     16.3
    281     15.8
    351     15.9
    451     16.2
    652     16.6
   1054     17.0

We measure an optical decay alpha=-1 from
observations after 300s, i.e. after the
optical peak.

Considering the optical decay, optical
predicted magnitudes are:

2012-09-14T00:56 UT R=18.4 (t0+1h)
2012-09-14T01:56 UT R=19.1 (t0+2h)
2012-09-14T02:56 UT R=19.6 (t0+3h)
2012-09-14T03:56 UT R=20.0 (t0+4h)
2012-09-15T00:56 UT R=21.0 (t0+1day)

These predictions can help spectrometry
observations.

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

Skynet continued observing the field of GRB120913B (GCN 13763
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 , Swift 
trigger #533613) in BVRI using four of the PROMPT telescopes located at 
CTIO in Chile until the object set.

A preliminary light curve of the first night's data from PROMPT can be 
found at: http://skynet.unc.edu/grb/grb120913b.png

S. Covino, D. Fugazza, A. Melandri,  on behalf of the REM team report:

We imaged the field of GRB120913B (Helder et al., GCN 13763
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 ) with the NIR camera only with the H filter. The REM telescope is at present under substantial maintenance and the optical camera and some of the subsystems are still off-line.

Observations started at UT 23:56:47, about 49 sec after the GRB, and frames were unfortunately not properly focussed. Yet the afterglow (LaClyyze et al., GCN 13764
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 ; Klotz et al., GCN 13765
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 ) is detectable and approximately 2 min after the GRB was at H = 13.72 +- 0.15, calibrated by means of isolated unsaturated 2MAS stars in the field.

Martin Jelinek (IAA Granada, Spain),
Petr Kubanek, Martin Masek, Jakub Cerny, Jan Ebr and
Michael Prouza (FZU Praha, Czech Rep.)
on behalf of the FRAM team, coordinated by FZU Praha

report:

The 30cm FRAM telescope, located at Pierre Auger
observatory in Malargue, Argentina, followed the GRB
120913B (Helder et al. GCNC13763, Swift trigger
#533613). Observations started 24.4s after the GRB
trigger.

10s R-band (lambda_eff = 640nm) images with a limit of
~16.0 were obtained. We identify the optical transient
reported by LaCluyze et al. (GCNC13764) and Klotz et
al. (GCNC13765) with a magnitude R = 16.3 +/- 0.2 on a
sum of 8x10s images obtained between 271 and 364s after
the GRB.

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), E. A. Helder (PSU),
H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC), D. M. Palmer (LANL),
T. Sakamoto (GSFC/UMBC), 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-240 to T+882 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 120913B (trigger #533613)
(Helder, et al., GCN Circ. 13763
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 ).  The BAT ground-calculated position is
RA, Dec = 213.660, -14.508 deg, which is 
   RA(J2000)  =  14h 14m 38.3s 
   Dec(J2000) = -14d 30' 27.9" 
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 59%.
 
The mask-weighted light curve shows two main pulses riding on top of a broad peak.
The burst starts at ~T-50 sec, with peaks at ~T+5 and ~T+39 sec, and ending at ~T+140 sec.
T90 (15-350 keV) is 126 +- 4 sec (estimated error including systematics).
 
The time-averaged spectrum from T-51.70 to T+111.79 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.63 +- 0.04.  The fluence in the 15-150 keV band is 1.11 +- 0.02 x 10^-5 erg/cm2.
The 1-sec peak photon flux measured from T+38.46 sec in the 15-150 keV band
is 3.2 +- 0.2 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/533613/BA/

V. Pelassa (UAH) reports on behalf of the Fermi GBM Team:

"At 23:55:58.77 UT on 13 september 2012, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 120913B (trigger 369273361 / 120913997), which
was also detected by the Swift/BAT (E.A. Helder et al. 2008, GCN 13763
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 )
The GBM on-ground location is consistent with the Swift position.
 The angle from the Fermi LAT boresight is 66 degrees.

The GBM light curve consists of 2 peaks at T0-10 and T0+30s on top of a
longer plateau, with a duration (T90) of about 110 s (50-300 keV).
The time-averaged spectrum from T0-40.961 s to T0+98.306 s is
best fit by a Band function with Epeak = 163 +/- 23 keV,
alpha = -1.19 +/- 0.06, and beta = -1.97 +/- 0.07
(Cstat 604 for 483 d.o.f.).

The event fluence (10-1000 keV) in this time interval is
(2.9 +/- 0.7)E-05 erg/cm^2. The 1.024-sec peak photon flux measured
starting from T0+37.632 s in the 10-1000 keV band
is 5.0 +/- 0.3 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."