TITLE:   GCN CIRCULAR
NUMBER:  11135
SUBJECT: GRB 100823A: Swift detection of a burst
DATE:    10/08/23 17:38:11 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

V. Mangano (INAF-IASFPA), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
M. M. Chester (PSU), N. Gehrels (NASA/GSFC), J. M. Gelbord (PSU),
E. A. Hoversten (PSU), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), F. E. Marshall (NASA/GSFC),
P. T. O'Brien (U Leicester), T. Sakamoto (NASA/UMBC),
M. C. Stroh (PSU) and E. Troja (NASA/GSFC/ORAU) report on behalf of
the Swift Team:

At 17:25:35 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100823A (trigger=432420).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 20.703, +5.842, which is 
   RA(J2000)  =  01h 22m 49s
   Dec(J2000) = +05d 50' 31"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single FRED-like peak
with a duration of about 25 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 17:26:44.6 UT, 68.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 20.70383, 5.83361
which is equivalent to:
   RA(J2000)  = 01h 22m 48.92s
   Dec(J2000) = +05d 50' 01.0"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 30 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 (3.73e+20
cm^-2, Kalberla et al. 2005), with an excess column of 2.1
(+1.87/-1.66) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 5.93e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 77 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 3-sigma
upper limit is 19.3 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 expected extinction corresponding to E(B-V) of  0.04. 

Burst Advocate for this burst is V. Mangano (vanessa AT ifc.inaf.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.)