[vsnet-grb-info 3602] Swift detection of an outburst from the 4U 1246-58 LMXB burster

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5436
SUBJECT: Swift detection of an outburst from the 4U 1246-58 LMXB burster
DATE:    06/08/11 03:44:16 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at lheamail.gsfc.nasa.gov>

P. Romano (INAF-OAB), S. D. Barthelmy (GSFC), N. Gehrels (NASA/GSFC),
C. B. Markwardt (GSFC/UMD), F. E. Marshall (NASA/GSFC),
K. M. McLean (LANL/UTD), A. Moretti (INAF-OAB), D. M. Palmer (LANL),
G. Sato (GSFC/ISAS), M. Stamatikos (NASA/ORAU) and
D. E. Vanden Berk (PSU) report on behalf of the Swift Team:

At 02:59:55 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a soft burst from near the Galactic plane (trigger=223918). 
Swift slewed immediately to the burst. 
The BAT on-board calculated location is RA,Dec 192.420, -59.093
{12h 49m 41s, -59d 05' 32"} (J2000) with an uncertainty of 3 arcmin
(radius, 90% containment, including systematic uncertainty). 
The BAT light curve shows a weak soft peak with a duration of about 20 sec. 
The peak count rate was ~500 counts/sec (15-350 keV), at ~5 sec after
the trigger. 

The XRT began observing the field at 03:03:08 UT, 193 seconds after
the BAT trigger. XRT found a bright, possibly fading, X-ray source
located at RA(J2000) = 12h 49m 39.8s, Dec(J2000) = -59d 05' 16.0",
with an estimated uncertainty of 4.7 arcseconds (90% confidence
radius). This location is 20 arcseconds from the BAT on-board
position, within the BAT error circle.  This position is consistent
(3.1 arcsec) with the best known X-ray position  of the 4U 1246-58 LMXB burster 
(Bassa et al. 2006, A&A, 446, L17). The initial flux in the 2.5s image was 4.5e-09
erg/cm2/s (0.2-10 keV). 

UVOT took a finding chart exposure of 100 seconds with the White
(160-650 nm) filter starting 202 seconds after the BAT trigger. No
UV/optical counterpart 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, no counterpart can be confirmed
at this time. There is large, but uncertain extinction in the direction
of the source.