TITLE:   GCN CIRCULAR
NUMBER:  13302
SUBJECT: GRB 120521A: Swift detection of a short burst
DATE:    12/05/21 06:12:55 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

M. M. Chester (PSU), J. R. Cummings (NASA/UMBC), V. D'Elia (ASDC),
B. Gendre (ASDC), D. M. Palmer (LANL) and G. Stratta (ASDC) report on
behalf of the Swift Team:

At 05:59:42 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 120521A (trigger=522578).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 148.693, -49.419 which is 
   RA(J2000) = 09h 54m 46s
   Dec(J2000) = -49d 25' 08"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single short FRED
structure with a duration of about 0.7 sec.  The peak count rate
was ~5000 counts/sec (15-350 keV), at ~0.25 sec after the trigger. 

The XRT began observing the field at 06:00:56.6 UT, 73.8 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 148.72544, -49.41760 which
is equivalent to:
   RA(J2000)  = 09h 54m 54.11s
   Dec(J2000) = -49d 25' 03.4"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 76 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 consistent with the Galactic value of 2.08
x 10^21 cm^-2 (Kalberla et al. 2005). 

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 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.