TITLE:   GCN CIRCULAR
NUMBER:  18436
SUBJECT: GRB 151022A: Swift detection of a burst
DATE:    15/10/22 14:22:47 GMT
FROM:    David Burrows at PSU/Swift  <burrows@astro.psu.edu>

P. D'Avanzo (INAF-OAB), D. N. Burrows (PSU),
H. A. Krimm (CRESST/GSFC/USRA), A. Melandri (INAF-OAB),
P. T. O'Brien (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB/PSU) report on behalf
of the Swift Team:

At 14:06:32 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 151022A (trigger=661036).  Swift slewed to the burst at 
T0+300 seconds.  The BAT on-board calculated location is 
RA, Dec 349.213, +55.813 which is 
   RA(J2000) = 23h 16m 51s
   Dec(J2000) = +55d 48' 48"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 20 sec.  The peak count rate
was ~1400 counts/sec (15-350 keV), at ~6 sec after the trigger. 

The XRT began observing the field at 14:11:32.6 UT, 300.5 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 349.19815, 55.81207 which
is equivalent to:
   RA(J2000)  = 23h 16m 47.56s
   Dec(J2000) = +55d 48' 43.5"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 30 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 consistent with the Galactic value of 4.74
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 304 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 large, but uncertain extinction expected. 

Burst Advocate for this burst is P. D'Avanzo (paolo.davanzo AT brera.inaf.it). 
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.)