[vsnet-grb-info 11609] GRB 120305A: Swift detection of a short burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  13004
SUBJECT: GRB 120305A: Swift detection of a short burst
DATE:    12/03/05 20:03:38 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

G. Stratta (ASDC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC), B. Gendre (ASDC),
C. Guidorzi (U Ferrara), S. T. Holland (STScI), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), A. Y. Lien (NASA/GSFC/ORAU),
V. Mangano (INAF-IASFPA), C. B. Markwardt (NASA/GSFC),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
T. Sakamoto (NASA/UMBC), C. J. Saxton (UCL-MSSL),
B. Sbarufatti (INAF-OAB/PSU), C. A. Swenson (PSU),
G. Tagliaferri (INAF-OAB), E. Troja (NASA/GSFC/ORAU) and
B.-B. Zhang (PSU) report on behalf of the Swift Team:

At 19:37:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 120305A (trigger=516997).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 47.546, +28.493 which is 
   RA(J2000)  =  03h 10m 11s
   Dec(J2000) = +28d 29' 36"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single spike
with a duration of about 200 msec.  The peak count rate
was ~32000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

Using promptly downlinked data we find a bright, fading, uncatalogued
X-ray source with an enhanced position: RA, Dec 47.5368, 28.4915 which
is equivalent to:
   RA(J2000)  = 03h 10m 08.84s
   Dec(J2000) = +28d 29' 29.3"
with an uncertainty of 2.6 arcseconds (radius, 90% containment). This
location is 29 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 (1.13 x
10^21 cm^-2, Kalberla et al. 2005), with an excess column of 4
(+3.38/-2.63) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 1.03e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 71 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 expected extinction corresponding to E(B-V) of
0.43. 

We note quasi-periodic variability in the BAT lightcurve before and after the
burst.  We believe it is unrelated to the GRB and could be coming from a
different source, but will further examine the data to determine the origin. 

Burst Advocate for this burst is G. Stratta (giulia.stratta 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.)