J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC), C. Gronwall (PSU),
J. A. Kennea (PSU), D. M. Palmer (LANL) and
B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team:
At 18:34:11 UT, the Swift Burst Alert Telescope (BAT) located a
marginal significance (5.84 sigma) image peak in the vicinity of the
known source IGR J16185-5928 (trigger=609694) in an untriggered image.
Swift slewed immediately to the location in order to confirm or
refute the event. The BAT on-board calculated location is
RA, Dec 244.998, -59.389 which is
RA(J2000) = 16h 19m 59s
Dec(J2000) = -59d 23' 21"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). As is usual for images without a corresponding
rate trigger, no obvious variation is visible in the BAT rates lightcurve.
The XRT began observing the field at 18:36:33.1 UT, 141.5 seconds after
the BAT trigger. No source was detected in 443 s of promptly downlinked
data, which covered 93% 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 150 seconds with the White filter
starting 144 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. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any afterglow in the
sub-image. The 8'x8' region for the list of sources generated on-board covers
53% of the BAT error circle. 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 expected extinction corresponding
to E(B-V) of 0.26.
Due to the marginal significance of the detection (5.84 sigma), the
lack of a corresponding rate trigger, the large distance of the image
peak from the known source (12 arcminutes), and the lack of a
corresponding source in the XRT and UVOT instruments, we believe that
this is a statistical fluctuation in image space rather than an