GCN Circular 8948

P. A. Evans (U Leicester), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), D. N. Burrows (PSU),
N. Gehrels (NASA/GSFC), S. T. Holland (CRESST/USRA/GSFC),
R. Margutti (Univ Bicocca&OAB), C. B. Markwardt (CRESST/GSFC/UMD),
F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester),
C. Pagani (PSU), K. L. Page (U Leicester), A. M. Parsons (GSFC),
P. Romano (INAF-IASFPA), B. Sbarufatti (INAF-IASFPA),
R. L. C. Starling (U Leicester), M. C. Stroh (PSU),
T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the
Swift Team:

At 18:01:23 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090308 (trigger=345777).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 183.526, -48.833 which is 
   RA(J2000) = 12h 14m 06s
   Dec(J2000) = -48d 49' 57"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  This is an image trigger, but even so
the BAT light curve shows a weak peak at T+20 to T+35 sec. 
The peak count rate was ~350 counts/sec (15-350 keV), at ~25 sec
after the trigger. 

The XRT began observing the field at 18:03:30.4 UT, 126.9 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 183.5017,
-48.8169 which is equivalent to:
   RA(J2000)  = 12h 14m 0.41s
   Dec(J2000) = -48d 49' 00.9"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 81 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 (8.54e+20
cm^-2, Kalberla et al. 2005), with an excess column of 1.8
(+1.51/-1.36) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 131 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. Because part of the 2.7'x2.7'
sub-image was not received, there is no overlap with the XRT error circle. 
The 8'x8' region for the list of sources generated on-board covers the entire
XRT error circle. No correction has been
made for the expected extinction corresponding to E(B-V) of 0.11. 

Burst Advocate for this burst is P. A. Evans (pae9 AT star.le.ac.uk). 
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.)