L. M. McCauley (PSU), P. A. Evans (U Leicester),
H. A. Krimm (CRESST/GSFC/USRA), K. L. Page (U Leicester) and
B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team:
At 08:33:23 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 151111A (trigger=663074). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec 56.828, -44.164 which is
RA(J2000) = 03h 47m 19s
Dec(J2000) = -44d 09' 49"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed a single FRED-like
structure with a duration of about 40 sec. The peak count rate
was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger.
The XRT began observing the field at 08:34:35.8 UT, 72.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
56.8452, -44.1615 which is equivalent to:
RA(J2000) = 03h 47m 22.86s
Dec(J2000) = -44d 09' 41.3"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 45 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.
A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.07 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 2.6
(+1.82/-1.61) x 10^21 cm^-2 (90% confidence).
The initial flux in the 2.5 s image was 1.85e-09 erg cm^-2 s^-1 (0.2-10
UVOT took a finding chart exposure of 150 seconds with the White filter
starting 81 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 100% of
the XRT 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
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
Burst Advocate for this burst is L. M. McCauley (marissamc AT swift.psu.edu).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)