A. P. Beardmore (U Leicester), H. A. Krimm (CRESST/GSFC/USRA),
C. J. Mountford (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the
At 04:03:55 UT, the Swift Burst Alert Telescope (BAT) triggered and
located trigger 556533. Swift slewed immediately to the location.
The BAT on-board calculated location is
RA, Dec 118.580, -28.134 which is
RA(J2000) = 07h 54m 19s
Dec(J2000) = -28d 08' 03"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed no significant
structure. The peak count rate was ~1400 counts/sec (15-350 keV),
at ~4 sec after the trigger.
The XRT began observing the field at 04:05:29.5 UT, 94.7 seconds after
the BAT trigger. No source was detected in 574 s of promptly downlinked
data, which covered 92% of the BAT error circle. We are waiting for the
full dataset to detect and localise the XRT counterpart.
UVOT took a finding chart exposure of nominal 250.000 seconds with the
U filter starting 154 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 25% of the BAT error circle. The typical
3-sigma upper limit has been about 19.6 mag. The 8'x8' region for
the list of sources generated on-board covers 100% of the BAT error
circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the large, but uncertain extinction
This was a marginal detection (5.85 sigma) in an image
at a location which was almost 12 arcminutes from
the known source A0753-2802. Under these circumstances,
Swift attempts to verify or refute the reality of the
possible event using the narrow field instruments. In
this case, we believe that this was due to a statistical
fluctuation and not an astrophysical event.