S. D. Barthelmy (GSFC), D. N. Burrows (PSU), V. D'Elia (ASDC),
S. T. Holland (STScI), J. A. Kennea (PSU),
C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC) and
D. M. Palmer (LANL) report on behalf of the Swift Team:
At 02:22:21 UT, the Swift Burst Alert Telescope (BAT) triggered and
located the recently-discovered source IGR J18245-2452 (ATel #4925,
Eckert et al) (trigger=552336). Swift slewed immediately to the source.
The BAT on-board calculated location is
RA, Dec 276.104, -24.875 which is
RA(J2000) = 18h 24m 25s
Dec(J2000) = -24d 52' 27"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). As is typical for an image trigger, the
immediately available lightcurve data shows no obvious variation.
The XRT began observing the field at 02:25:32.34 UT, 190.7 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source
located at RA, Dec 276.13519, -24.86810 which is equivalent to:
RA(J2000) = 18h 24m 32.45s
Dec(J2000) = -24d 52' 05.2"
with an uncertainty of 4.0 arcseconds (radius, 90% containment). This
location is 104 arcseconds from the BAT onboard position, within the
BAT error circle; and 8.7 arcseconds from the location of the center of
globular cluster M28. The XRT position may be improved as more data are
received; the latest position is available at
http://www.swift.ac.uk/sper. This position is 41.6 arcseconds from the
center of the 1.4 arcminute INTEGRAL error circle, and therefore consistent
with the source being IGR J18245-2452. This position is also 2.4 arcseconds from
that of a known X-ray source: 1RXH J182432.6-245205 in the ROSAT ROSHRI
catalogue. The catalogued count-rate of this source is equivalent to
approximately 0.013 XRT count/sec; the mean count-rate in the
promptly-available XRT data is 0.80 count/sec.
A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.73
x 10^21 cm^-2 (Kalberla et al. 2005).
UVOT took a finding chart exposure of 150 seconds with the U filter starting
868 seconds after the BAT trigger. No credible 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, further analysis
is required to report an upper limit for any counterpart in the sub-image. The
8'x8' region for the list of sources generated on-board covers 0.00% of the XRT