GCN Circular 7351

T. Sakamoto (NASA/UMBC), W. H. Baumgartner (GSFC/UMBC),
A. P. Beardmore (U Leicester), G. Cusumano (INAF-IASFPA),
P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), C. Gronwall (PSU),
E. A. Hoversten (PSU), N. P. M. Kuin (MSSL), J. Mao (INAF-OAB),
A. Moretti (INAF-OAB), P. T. O'Brien (U Leicester),
S. R. Oates (UCL-MSSL), J. P. Osborne (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL),
D. Perez (U Leicester), P. Romano (INAF-IASFPA),
M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester),
G. Stratta (ASDC), G. Tagliaferri (INAF-OAB),
E. Troja (U Leicester/INAF-IASFPa) and H. Ziaeepour (UCL-MSSL) report
on behalf of the Swift Team:

At 09:10:35 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 080303 (trigger=304549).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 112.073, -70.240 which is 
   RA(J2000) = 07h 28m 18s
   Dec(J2000) = -70d 14' 23"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a FRED-like 
structure with a duration of about 30 sec.  The peak count rate
was ~2600 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 09:11:47.7 UT, 72.4 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
112.05892, -70.23448 which is equivalent to:
   RA(J2000)  = 07h 28m 14.14s
   Dec(J2000) = -70d 14' 04.1"
with an uncertainty of 2.6 arcseconds (radius, 90% containment). This
location is 26 arcseconds from the BAT onboard position, within the BAT
error circle. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
1.26e+21 cm^-2 (Kalberla et al. 2005), so we cannot constrain the
redshift at this time using the relation from Grupe et al. (2007). A
summary of the promptly downlinked data is given at
http://www.swift.ac.uk/sper/304549/. 

The initial flux in the 2.5 s image was 4.53e-10 erg cm^-2 s^-2 (0.2-10
keV). 

UVOT took a finding chart exposure of 100 seconds with the White
(160-650 nm) filter starting 80 seconds after the BAT trigger. There
is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image
at
    RA(J2000) =	07:28:14.14 = 112.0589  
    DEC(J2000) = -70:14:01.7 = -70.2338 
with a 1-sigma error radius of about 0.6 arc sec. This
position is 2.4 arc sec. from the center of the XRT error circle. The
estimated magnitude is 18.4 with a 1-sigma error of about 0.5 mag. A
400s V band image was taken beginning 186 s after the burst. The
estimated magnitude is 18.3 with a 1 sigma error of 0.5 magnitudes. 
At 986 s after the burst the source is undetected in a 108 s V band
exposure. The typical limiting magnitude is 18.0 magnitudes in an
exposure of this length. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.19. 

Burst Advocate for this burst is T. Sakamoto (Taka.Sakamoto AT 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.)