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GCN Circular 18611

Subject
GRB 151118A: Swift detection of a burst
Date
2015-11-18T03:24:39Z (8 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@milkyway.gsfc.nasa.gov>
A. Amaral-Rogers (U Leicester), S. D. Barthelmy (GSFC),
J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of
the Swift Team:

At 03:06:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 151118A (trigger=664078).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 57.158, +65.869, which is 
   RA(J2000)  =  03h 48m 38s
   Dec(J2000) = +65d 52' 07"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a few peaks
with a duration of about 25 sec.  The peak count rate
was ~2600 counts/sec (15-350 keV), at ~6 sec after the trigger. 

The XRT began observing the field at 03:07:36.2 UT, 66.2 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 57.1719,
65.9012 which is equivalent to:
   RA(J2000)  = 03h 48m 41.25s
   Dec(J2000) = +65d 54' 04.4"
with an uncertainty of 2.4 arcseconds (radius, 90% containment). This
location is 117 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 in excess of the Galactic value (4.54 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 5.9
(+4.29/-3.55) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
281 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 18.0 mag. No correction has been made for the expected extinction
corresponding to E(B-V) of 1.10. 

Burst Advocate for this burst is A. Amaral-Rogers (aar14 AT le.ac.uk). 
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.)
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