[vsnet-grb-info 5875] GRB 080328: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  7525
SUBJECT: GRB 080328: Swift detection of a burst with an optical counterpart
DATE:    08/03/28 08:28:44 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

M. Perri (ASDC), D. N. Burrows (PSU), S. Campana (INAF-OAB),
P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC),
C. Guidorzi (INAF-OAB), C. B. Markwardt (CRESST/GSFC/UMD),
K. L. Page (U Leicester), D. M. Palmer (LANL),
D. Perez (U Leicester), P. Romano (INAF-IASFPA),
R. L. C. Starling (U Leicester), G. Stratta (ASDC),
D. E. Vanden Berk (PSU), P. A. Ward (MSSL-UCL) and
H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team:

At 08:03:04 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 080328 (trigger=307931).  Swift slewed immediately to the 
burst.  The BAT on-board calculated location is 
RA, Dec 80.479, +47.524 which is 
   RA(J2000) = 05h 21m 55s
   Dec(J2000) = +47d 31' 26"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 110 sec.  The peak count rate
was ~9500 counts/sec (15-350 keV), at ~10 sec after the trigger. 

The XRT began observing the field at 08:04:44.0 UT, 99.4 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
80.48189, 47.51035 which is equivalent to:
   RA(J2000)  = 05h 21m 55.65s
   Dec(J2000) = +47d 30' 37.3"
with an uncertainty of 2.6 arcseconds (radius, 90% containment). This
location is 49 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
3.38e+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/307931/. 

The initial flux in the 2.5 s image was 1.08e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of nominal 400 seconds with the V filter
starting 216 seconds after the BAT trigger. There is a candidate afterglow in
the list of sources generated on-board at
  RA(J2000)  =	05:21:55.85 =  80.4827
  DEC(J2000) = +47:30:39.2  =  47.5109
with a 1-sigma error radius of about 0.6 arc sec. This position is 2.8 arc sec. 
from the center of the XRT error circle. The estimated magnitude is 17.7 with a
1-sigma error of about 0.5 mag. No correction has been made for the expected
extinction of about 1.9 magnitudes. 

Burst Advocate for this burst is M. Perri (perri AT asdc.asi.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.)