TITLE:   GCN CIRCULAR
NUMBER:  10712
SUBJECT: GRB 100504A: Swift detection of a burst
DATE:    10/05/04 20:29:03 GMT
FROM:    Jamie A. Kennea at PSU/Swift-XRT  <kennea@astro.psu.edu>

J. Mao (INAF-OAB), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
A. de Ugarte Postigo (INAF-OAB), J. M. Gelbord (PSU),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), F. E. Marshall (NASA/GSFC),
P. T. O'Brien (U Leicester), D. M. Palmer (LANL),
P. Romano (INAF-IASFPA), B. Sbarufatti (INAF-OAB/IASFPA),
M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC),
G. Tagliaferri (INAF-OAB) and E. Troja (NASA/GSFC/ORAU) report on
behalf of the Swift Team:

At 19:20:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100504A (trigger=421124).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 255.572, -35.599 which is 
   RA(J2000) = 17h 02m 17s
   Dec(J2000) = -35d 35' 55"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a double-peaked
structure with a duration of about 30 sec.  The peak count rate
was ~2500 counts/sec (15-350 keV), at ~0 and ~10 sec after the trigger. 

The XRT began observing the field at 19:22:02.1 UT, 62.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright, fading,
uncatalogued X-ray source located at RA, Dec 255.57053, -35.58712 which
is equivalent to:
   RA(J2000)  = 17h 02m 16.93s
   Dec(J2000) = -35d 35' 13.6"
with an uncertainty of 4.1 arcseconds (radius, 90% containment). This
location is 42 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.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
3.61e+21 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 5.58e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 71 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any afterglow in the
sub-image. The coverage of the XRT error circle by the 8'x8' region for the
list of sources generated on-board is uncertain because the large number of
sources filled the available telemetry. Because of the density of catalogued
stars, further analysis is required to report an upper limit for any afterglow
in the region. No correction has been made for the large, but uncertain
extinction expected. 

Burst Advocate for this burst is J. Mao (jirong.mao 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.)