TITLE:   GCN CIRCULAR
NUMBER:  18425
SUBJECT: GRB 151021A: Swift detection of a burst
DATE:    15/10/21 02:05:25 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

A. Melandri (INAF-OAB), S. D. Barthelmy (GSFC),
P. D'Avanzo (INAF-OAB), H. A. Krimm (CRESST/GSFC/USRA),
N. P. M. Kuin (UCL-MSSL), D. M. Palmer (LANL) and
B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team:

At 01:29:12 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 151021A (trigger=660671).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 337.611, -33.199 which is 
   RA(J2000) = 22h 30m 27s
   Dec(J2000) = -33d 11' 55"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex multi-peaked
structure with a duration of at least 100 sec.  The peak count rate
was 2500 counts/sec (15-350 keV), at 10 sec after the trigger,
although the lightcurve around the time of trigger is not
immediately available. 

The XRT began observing the field at 01:30:43.2 UT, 90.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 337.64619, -33.19907 which
is equivalent to:
   RA(J2000)  = 22h 30m 35.09s
   Dec(J2000) = -33d 11' 56.7"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 106 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 destiny in excess of the Galactic value (1.13 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.9
(+2.55/-2.23) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 0.1 s image was 2.76e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 99 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. 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 0.01. 

Burst Advocate for this burst is A. Melandri (andrea.melandri 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.)