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THE ASTRONOMER Electronic Circular No 3117      2015 Oct 21 10.03UT
Ed:Guy M Hurst, 16,Westminster Close, Kempshott Rise,  Basingstoke,
Hants, RG22 4PP,England.Telephone/FAX(01256)471074Int:+441256471074
INTERNET: GUY@TAHQ.DEMON.CO.UK      Backup: gmh@wdcc1.bnsc.rl.ac.uk
WORLD WIDE WEB                         http://www.theastronomer.org
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GRB 151021A: OPTICAL COUNTERPART
A Melandri (INAF-OAB and Burst Advocate) advises on GRB 18425 that at
01:29:12 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB
151021A.  Swift slewed immediately to the burst. 
The BAT on-board calculated location is:
RA 22h 30m 27s Dec -33d 11' 55" (2000).
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 finding an uncatalogued and bright, uncatalogued X-ray source
located at:
RA 22h 30m 35.09s Dec -33d 11' 56.7" (2000)

On GCN 18427 V. Lipunoc et. al. Moscow State University, Sternberg
Astronomical Institute reports that MASTER II  robotic telescope located in
SAAO was pointed to the  GRB151021A,  25 sec after notice time and 50 sec
after trigger time at 2015-10-21 01:30:06 UT in two polarizations. On their
first (10s exposure)  set they found 1 optical transient within SWIFT
error-box (Melandri et. al GCN 18425) brighter than 16:

date       time      Tmid-T0      Coord2000                  Mag
2015-10-21 01:30:06   55.0  22h 30m 34.42s , -33d 11m 50.3s 14.86

TA observers are encouraged, as discussed at the recent TA meeting, to
investigate if any robotic telescopes are available to obtain further urgent
photometry. Please relay any results to the editor.

Guy M Hurst