[vsnet-grb-info 13825] GRB 130831A: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  15139
SUBJECT: GRB 130831A: Swift detection of a burst with an optical counterpart
DATE:    13/08/31 13:21:11 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

L. M. Z. Hagen (PSU), M. M. Chester (PSU), J. R. Cummings (NASA/UMBC),
N. Gehrels (NASA/GSFC), D. Grupe (PSU), J. A. Kennea (PSU),
V. Mangano (PSU), F. E. Marshall (NASA/GSFC),
C. J. Mountford (U Leicester), P. T. O'Brien (U Leicester),
D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/PSU), M. H. Siegel (PSU),
E. Troja (NASA/GSFC/UMCP) and B.-B. Zhang (UAH) report on behalf of
the Swift Team:

At 13:04:16 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 130831A (trigger=568849).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 358.649, +29.448 which is 
   RA(J2000) = 23h 54m 36s
   Dec(J2000) = +29d 26' 53"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 50 sec.  The peak count rate
was ~10000 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 13:06:22.3 UT, 125.8 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 358.62398,
29.42849 which is equivalent to:
   RA(J2000)  = 23h 54m 29.76s
   Dec(J2000) = +29d 25' 42.6"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 105 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 4.80
x 10^20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 8.87e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
190 seconds after the BAT trigger. There is a candidate afterglow in the
rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	23:54:29.89 = 358.62453
  DEC(J2000) = +29:25:46.2  =  29.42950
with a 90%-confidence error radius of about 0.74 arc sec. This position is 5.8
arc sec. from the center of the XRT error circle. The estimated magnitude is
14.56 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.04. 

Burst Advocate for this burst is L. M. Z. Hagen (lea.zernow.hagen AT gmail.com). 
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.)