[vsnet-grb-info 10954] GRB 110921A: Swift detection of a burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  12373
SUBJECT: GRB 110921A: Swift detection of a burst
DATE:    11/09/21 14:01:54 GMT
FROM:    David Burrows at PSU/Swift  <burrows at astro.psu.edu>

W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), S. Campana (INAF-OAB),
J. R. Cummings (NASA/UMBC), V. D'Elia (ASDC),
P. A. Evans (U Leicester), C. Guidorzi (U Ferrara),
S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU),
J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA),
O. M. Littlejohns (U Leicester), V. Mangano (INAF-IASFPA),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
P. T. O'Brien (U Leicester), C. Pagani (U Leicester),
K. L. Page (U Leicester), J. L. Racusin (NASA/GSFC),
T. Sakamoto (NASA/UMBC), B. Sbarufatti (INAF-OAB/IASFPA),
M. H. Siegel (PSU), E. Sonbas (GSFC/USRA/Adiyaman Univ.),
R. L. C. Starling (U Leicester) and B.-B. Zhang (PSU) report on behalf
of the Swift Team:

At 13:51:20 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 110921A (trigger=503652).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 294.078, +36.377 which is 
   RA(J2000) = 19h 36m 19s
   Dec(J2000) = +36d 22' 36"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a ragged, single-peaked
structure with a duration of about 10 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~4 sec after the trigger. 

The XRT began observing the field at 13:53:22.5 UT, 122.4 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 294.09838, 36.32882 which is equivalent
to:
   RA(J2000)  = 19h 36m 23.61s
   Dec(J2000) = +36d 19' 43.8"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 182 arcseconds from the BAT onboard position, outside 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 (1.43 x
10^21 cm^-2, Kalberla et al. 2005), with an excess column of 3.1
(+2.28/-1.97) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 125 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 0.14. 

Burst Advocate for this burst is W. H. Baumgartner (wayne AT milkyway.gsfc.nasa.gov). 
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.)