GCN Circular 13017

W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
M. M. Chester (PSU), V. D'Elia (ASDC), A. Y. Lien (NASA/GSFC/ORAU),
C. B. Markwardt (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the
Swift Team:

At 06:13:38 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 120308A (trigger=517234).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is RA, Dec 219.078, +79.703 which is 
   RA(J2000) = 14h 36m 19s
   Dec(J2000) = +79d 42' 12"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a broad peak
structure with a duration of about ~30 sec.  The peak count rate
was ~500 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 06:15:11.3 UT, 92.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 219.08492, 79.68629 which
is equivalent to:
   RA(J2000)  = 14h 36m 20.38s
   Dec(J2000) = +79d 41' 10.6"
with an uncertainty of 4.1 arcseconds (radius, 90% containment). This
location is 60 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 2.89
x 10^20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 1.17e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
157 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 19.2 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 18.0 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.03. 

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.)