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GCN Circular 12253

Subject
GRB 110807A: Swift detection of a burst or a posible new SGR
Date
2011-08-07T20:22:17Z (13 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@milkyway.gsfc.nasa.gov>
V. D'Elia (ASDC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
M. M. Chester (PSU), C. Guidorzi (U Ferrara),
S. T. Holland (CRESST/USRA/GSFC), N. P. M. Kuin (UCL-MSSL),
C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC),
A. Melandri (INAF-OAB), D. M. Palmer (LANL), C. J. Saxton (UCL-MSSL),
G. Stratta (ASDC), C. A. Swenson (PSU) and T. N. Ukwatta (MSU) report
on behalf of the Swift Team:

At 19:57:46 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 110807A (trigger=458907).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 278.696, -8.760 which is 
   RA(J2000)  =  18h 34m 47s
   Dec(J2000) = -08d 45' 35"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single timebin spike
structure with a duration of less than 0.064 sec.  The peak count rate
was ~3500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 20:03:13.2 UT, 326.8 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source
located at RA, Dec 278.71751, -8.76597 which is equivalent to:
   RA(J2000)  = 18h 34m 52.20s
   Dec(J2000) = -08d 45' 57.5"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 79 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. This
position is 6.6 arcseconds from that of a known X-ray source: 2XMM
J183452.4-084603 in the XMM-NEWTON XMMSSC catalogue. 

A power-law fit to a spectrum formed from promptly downlinked event
data does not constrain the column density. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 332 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 100% of
the XRT 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
XRT 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. 

Due to the short, soft nature of the burst, and the proximity
of the source to the Galactic plane (0.3 degrees) we suggest
that this source may be a previously unknown Soft Gamma Repeater. 
Verification of this could come if a second burst is seen or
the XRT counterpart shows periodicity. 

Burst Advocate for this burst is V. D'Elia (delia AT asdc.asi.it). 
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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