GCN Circular 7647

C. Guidorzi (INAF-OAB), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), D. N. Burrows (PSU),
J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC),
O. Godet (U Leicester), S. T. Holland (CRESST/USRA/GSFC),
J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA),
W.L Landsman (GSFC), C. B. Markwardt (CRESST/GSFC/UMD),
K. M. McLean (GSFC/UMD), P. T. O'Brien (U Leicester),
D. M. Palmer (LANL), D. Perez (U Leicester), P. Romano (INAF-IASFPA),
T. Sakamoto (NASA/UMBC), M. Stamatikos (NASA/ORAU),
R. L. C. Starling (U Leicester), G. Tagliaferri (INAF-OAB),
T. N. Ukwatta (GSFC/GWU), D. E. Vanden Berk (PSU) and
H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team:

At 19:53:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 080430 (trigger=310613).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 165.322, +51.689 which is 
   RA(J2000) = 11h 01m 17s
   Dec(J2000) = +51d 41' 22"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single FRED peak
structure with a duration of about 20 sec.  The peak count rate
was ~6000 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 19:53:50.9 UT, 48.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
165.31041, 51.68546 which is equivalent to:
   RA(J2000)  = 11h 01m 14.50s
   Dec(J2000) = +51d 41' 07.7"
with an uncertainty of 2.7 arcseconds (radius, 90% containment). This
location is 28 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 in excess of the Galactic value (9.59e+19
cm^-2, Kalberla et al. 2005), with an excess column of 3 (+1.33/-1.17)
x 10^21 cm^-2 (90% confidence). The relation of Grupe et al. (2007)
implies that this burst has a redshift z<3.5, although high redshift
fits to the absorbed XRT spectrum are possible if paired with an
anomalously large column. A summary of the promptly downlinked data is
given at http://www.swift.ac.uk/sper/310613/. 

The initial flux in the 2.5 s image was 1.26e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 400 seconds with the v filter starting
164 seconds after the BAT trigger. There is a candidate afterglow in the
rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	11:01:14.71 = 165.3113
  DEC(J2000) = +51:41:08.5  =  51.6857
with a 1-sigma error radius of about 0.5 arc sec. This position is 2.2 arc sec. 
from the center of the XRT error circle. The estimated magnitude is 17.6 with a
1-sigma error of about 0.5 mag. A white filter finding chart exposure was also
taken before the v filter, showing that the afterglow is fading.  No correction
has been made for the expected extinction of about 0.04 magnitudes. 

Burst Advocate for this burst is C. Guidorzi (cristiano.guidorzi AT brera.inaf.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.)