S. D. Barthelmy (GSFC), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), V. Mangano (INAF-IASFPA),
C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC) and
A. Maselli (INAF-IASFPA) report on behalf of the Swift Team:
At 19:15:25 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a transient position consistent with Sgr A* (trigger=554491).
Swift did not slew immediately because the current target had a higher merit
value. The slew was delayed by 15 min. The BAT on-board calculated location
is RA, Dec 266.412, -28.990 which is
RA(J2000) = 17h 45m 39s
Dec(J2000) = -28d 59' 22"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve shows shows a single 64 msec bin.
The peak count rate was ~2500 counts/sec (15-350 keV), at ~0 sec
after the trigger.
The BAT position is within 65 arcsec of the galactic center and Sgr A*. Given
the recent activity reported from Sgr A* (ATEL #5006), it's quite possible that
this is the source of X-ray flaring. Also, this is a confused region, so
we cannot rule out other candidates.
The XRT began observing the field at 19:31:27.6 UT, 962.8 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source
located at RA, Dec 266.41715, -29.00866 which is equivalent to:
RA(J2000) = 17h 45m 40.12s
Dec(J2000) = -29d 00' 31.2"
with an uncertainty of 4.1 arcseconds (radius, 90% containment). This
location is 69 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 3.2 arcseconds from that of Sgr A*, and is consistent with
being that source. The X-ray source shows an elevated emission level,
consistent with that reported from observations taken yesterday in
UVOT took a finding chart exposure of 150 seconds with the White filter
starting 967 seconds after the BAT trigger. No credible optical counterpart 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 19.6 mag.
No correction has been made for the large,
but uncertain extinction expected.