GCN Circular 18000

M. G. Bernardini (INAF-OAB), S. D. Barthelmy (GSFC),
P. D'Avanzo (INAF-OAB), A. Melandri (INAF-OAB), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf
of the Swift Team:

At 00:28:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a short burst consistent with the location of the
ROSAT faint source 1RXS J125754.7+14185 (trigger=648437).  Swift 
slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 194.497, +14.305 which is 
   RA(J2000) = 12h 57m 59s
   Dec(J2000) = +14d 18' 18"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single spike
structure with a duration of about 0.25 sec.  The peak count rate
was ~8000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 00:28:51.6 UT, 49.2 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source
with an enhanced position: RA, Dec 194.4710, 14.3183 which is
equivalent to:
   RA(J2000)  = 12h 57m 53.03s
   Dec(J2000) = +14d 19' 05.8"
with an uncertainty of 3.5 arcseconds (radius, 90% containment). This
location is 102 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. This position is 28.6 arcseconds from that
of a known X-ray source: 1RXS J125754.7+141850 in the ROSAT RASSFSC
catalogue. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 53 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 none of
the XRT error circle. The 8'x8' region for the list of sources generated
on-board covers 100% of the XRT error circle. The list of sources is typically
complete to about 18 mag. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.03. 

This source location is consistent with the source 1RXS J125754.7+141850,
an object in the ROSAT RASSFSC.  This catalog has 106k sources,
and so the possibility of a chance coincidence cannot be ruled out. 
This burst is comparable in hardness and other aspects to a typical
short GRB.  For this reason, we cannot confirm whether this burst
comes from the previously-known source or a new sGRB with the 
immediately-available data. 

The complete downlinked dataset and further observations will be required
to determine the nature of this source. 

Burst Advocate for this burst is M. G. Bernardini (grazia.bernardini 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.)