[vsnet-grb-info 19920] GRB 171013A: Swift detection of a possible burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22007
SUBJECT: GRB 171013A: Swift detection of a possible burst
DATE:    17/10/13 00:51:52 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

B. Sbarufatti (PSU), J. R. Cummings (NASA/UMBC), J.D. Gropp (PSU),
J. A. Kennea (PSU), H. A. Krimm (NSF/USRA), S. J. LaPorte (PSU),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL) and A. Tohuvavohu (PSU) report on behalf of the
Swift Team:

At 00:22:20 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a possible GRB 171013A (trigger=778435).  Swift slewed immediately 
to the location. 
The BAT on-board calculated location is 
RA, Dec 84.085, +34.449 which is 
   RA(J2000) = 05h 36m 20s
   Dec(J2000) = +34d 26' 57"
with an uncertainty of 3 arcmin (radius, 90% containment, including systematic
uncertainty).  The BAT light curve showed a weak, short (<1 s) peak. 
The peak count rate was ~1500 counts/sec (15-350 keV), at ~0 sec 
after the trigger. 

The XRT began observing the field at 00:24:07.6 UT, 107.2 seconds after
the BAT trigger. No source was detected in 521 s of promptly downlinked
data, which covered 72% of the BAT error circle. We are waiting for the
full dataset to detect and localise the XRT counterpart. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 110 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 25% of
the BAT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
BAT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the large, but uncertain, extinction expected. 

Based on the marginal significance of this detection (5.74 sigma in rates,
6.9 sigma in the on-board image, 6.5 in the ground-processed image),
the weak structure in the light curve, and the lack of a detection by XRT, 
we cannot verify that this is a true GRB.  In addition, there is considerable
background from the source Swift J0243.6+6124 in the field of view, with 
strong variation at its ~9.86 s period, which makes analysis of the light curve 
difficult.  The determination of the reality of this source will require 
the full downlinked data. 

Burst Advocate for this burst is B. Sbarufatti (bxs60 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.)