GCN Circular 8311

H. A. Krimm (CRESST/GSFC/USRA), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), M. Capalbi (ASDC), P. A. Evans (U Leicester),
N. Gehrels (NASA/GSFC), O. Godet (U Leicester),
C. Guidorzi (INAF-OAB), S. T. Holland (CRESST/USRA/GSFC),
S. D. Hunsberger (PSU), J. A. Kennea (PSU), J. Mao (INAF-OAB),
C. B. Markwardt (CRESST/GSFC/UMD), P. T. O'Brien (U Leicester),
S. R. Oates (UCL-MSSL), K. L. Page (U Leicester),
D. M. Palmer (LANL), P. Romano (INAF-IASFPA),
B. Sbarufatti (INAF-IASFPA), R. L. C. Starling (U Leicester),
G. Tagliaferri (INAF-OAB) and E. Troja (INAF-IASFPA) report on behalf
of the Swift Team:

At 09:28:08 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a short, soft burst (trigger=330353).  Swift slewed 
immediately to the location.  The BAT on-board calculated location is 
RA, Dec 237.711, -54.319 which is 
   RA(J2000) = 15h 50m 51s
   Dec(J2000) = -54d 19' 08"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peaked
structure with a duration of less than 0.1 sec, with most of the flux
below 100 keV.  The peak count rate was ~4000 counts/sec (15-350 keV), 
at ~0 sec after the trigger. 

The XRT began observing the field at 09:29:26.6 UT, 77.9 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source
with an enhanced position: RA, Dec 237.72546, -54.30659 which is
equivalent to:
   RA(J2000)  = 15h 50m 54.11s
   Dec(J2000) = -54d 18' 23.7"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 54 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. 

This position is 1.5 arcsec from (and hence consistent with) a source in the
2XMM catalogue: 2XMM J155054.2-541824. The XRT count rate is significantly 
higher than that catalogued by XMM, however. 

A power-law fit to a spectrum formed from promptly downlinked event
data does not constrain the column density. 

UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm)
filter starting 82 seconds after the BAT trigger. No afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 18.5 mag. 
No correction has been made for the large, but uncertain extinction expected. 

The enhanced brightness of the 2XMM source immediately following
a BAT detection argues that this is the correct source identification. 
However, we note that there are additional catalogued X-ray sources in 
the BAT error circle which are also plausible candidates for the
burst source.  Given the location of the burst, within 0.14 degrees
of the Galactic plane, these are all expected to be Galactic sources. 

Because this is a short soft burst from a Galactic source,
this may be a magnetar, such as an Soft Gamma Repeater (SGR) or
an Anomalous X-ray Pulsar (AXP).