S. D. Barthelmy (GSFC), A. Deich (PSU), J.D. Gropp (PSU),
J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
B. Sbarufatti (PSU) and A. Tohuvavohu (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:
At 19:27:11 UT, the Swift Burst Alert Telescope (BAT) triggered and
located on CXOU J164710.2-455216/PSR J1647-4552 (trigger=808755).
Swift slewed immediately to the source.
The BAT on-board calculated location is
RA, Dec 251.749, -45.850 which is
RA(J2000) = 16h 47m 00s
Dec(J2000) = -45d 51' 00"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed a single-peaked
structure with a duration of about 0.4 sec. The peak count rate
was ~4000 counts/sec (15-350 keV), at ~0 sec after the trigger.
The XRT began observing the field at 19:27:50.6 UT, 38.9 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source
with an enhanced position: RA, Dec 251.7918, -45.8713 which is
RA(J2000) = 16h 47m 10.17s
Dec(J2000) = -45d 52' 16.2"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 132 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 1.6 arcseconds from a
known X-ray source: CXOU J164710.2-455216/PSR J1647-4552, a known
magnetar (e.g. Woods et al., 2011, ApJ, 726, 37), which has triggered
BAT previously (GCNs 5581, 12359, 21095, 22024 and 22025). This source
is in the Swift XRT 1SXPS catalogue with a mean 0.3-10 keV count-rate
of 0.2290 +/-0.0028 ct/sec;
see http://www.swift.ac.uk/1SXPS/1SXPSJ164710.3-455217 for details
of these previous observations.
A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (2.03 x
10^22 cm^-2, Willingale et al. 2013), with an excess column of 2.2
(+2.52/-1.78) x 10^22 cm^-2 (90% confidence).
UVOT took a finding chart exposure of 250 seconds with the U filter
starting 99 seconds after the BAT trigger. No credible counterpart
has been found in the initial data products. The 2.7'x2.7' sub-image
covers none of the XRT error circle. The 8'x8' region for the list
of sources generated on-board covers 100% of the XRT error circle.
The typical upper limit has been about 18th mag. No correction has
been made for the large, but uncertain, extinction expected.