GCN Circular 8939

O. Godet (U Leicester), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
S. Campana (INAF-OAB), P. A. Evans (U Leicester), J. A. Kennea (PSU),
R. Margutti (Univ Bicocca&OAB), J. P. Osborne (U Leicester),
C. Pagani (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL),
M. Perri (ASDC), P. Romano (INAF-IASFPA),
B. A. Rowlinson (U Leicester), G. Stratta (ASDC), M. C. Stroh (PSU),
G. Tagliaferri (INAF-OAB) and L. Vetere (PSU) report on behalf of the
Swift Team:

At 13:54:24 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090306 (trigger=345396).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 16.740, -50.065 which is 
   RA(J2000) = 01h 06m 58s
   Dec(J2000) = -50d 03' 54"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a short, 
hard structure with a duration less than a single 64 msec timebin. 
The peak count rate was ~2000 counts/sec (15-350 keV), at ~0 sec
after the trigger. 

The XRT began observing the field at 13:55:57.4 UT, 95.5 seconds
after the BAT trigger. Using promptly downlinked data we find an
uncatalogued X-ray source located at RA, Dec 16.74294, -50.00206 which
is equivalent to:
   RA(J2000)  = 01h 06m 58.31s
   Dec(J2000) = -50d 00' 07.4"
with an uncertainty of 4.9 arcseconds (radius, 90% containment). 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
1.37e+20 cm^-2 (Kalberla et al. 2005). 

At this time we do not have UVOT data. 

This trigger was the first of a new triggering function
within BAT (the so called Subthreshold trigger).  While there
is a real source in the XRT initial image, this would say
the trigger is a real GRB.  However because we did not receive
the usual BAT Position message (for reasons yet unknown),
we caution that we can not say with confidence that this is a
real burst.  We will not have the full data downlinked
until ~17:00 UT. 

Burst Advocate for this burst is O. Godet (og19 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.)