[vsnet-grb-info 17187] GRB 151031A: Swift detection of a burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  18538
SUBJECT: GRB 151031A: Swift detection of a burst
DATE:    15/10/31 06:06:57 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

E. Sonbas (Adiyaman Univ.), V. D'Elia (ASDC), L. Izzo (URoma/ICRA),
J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA) and
D. M. Palmer (LANL) report on behalf of the Swift Team:

At 05:50:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 151031A (trigger=662330).  Swift slewed to the burst after
a 6 minute delay due to viewing constraints. 
The BAT on-board calculated location is 
RA, Dec 83.199, -39.092 which is 
   RA(J2000) = 05h 32m 48s
   Dec(J2000) = -39d 05' 28"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 10 sec.  The peak count rate
was ~2500 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 05:57:43.9 UT, 433.6 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 83.19532, -39.12049 which is equivalent
to:
   RA(J2000)  = 05h 32m 46.88s
   Dec(J2000) = -39d 07' 13.8"
with an uncertainty of 4.1 arcseconds (radius, 90% containment). This
location is 103 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 2.62
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 439 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers none of
the XRT error circle. The 8'x8' region for the list of sources generated
on-board covers 100% of the XRT error circle. The list of sources is typically
complete to about 18 mag. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.03. 

Burst Advocate for this burst is E. Sonbas (edasonbas AT yahoo.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.)