TITLE:   GCN CIRCULAR
NUMBER:  18186
SUBJECT: GRB 150821A: Swift detection of a burst with an optical afterglow
DATE:    15/08/21 09:59:12 GMT
FROM:    Kim Page at U.of Leicester  <klp5@leicester.ac.uk>

E. Troja (NASA/GSFC/UMCP), M. G. Bernardini (INAF-OAB),
S. B. Cenko (GSFC), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC),
P. A. Evans (U Leicester), A. Melandri (INAF-OAB) and
K. L. Page (U Leicester) report on behalf of the Swift Team:

At 09:44:00 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150821A (trigger=652847).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 341.906, -57.875 which is 
   RA(J2000) = 22h 47m 37s
   Dec(J2000) = -57d 52' 30"
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 at least 100 sec.  The peak count rate
was ~2840 counts/sec (15-350 keV), at ~35 sec after the trigger. 

The XRT began observing the field at 09:48:04.2 UT, 243.3 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 341.9129, -57.8941 which is equivalent to:
   RA(J2000)  = +22h 47m 39.10s
   Dec(J2000) = -57d 53' 38.8"
with an uncertainty of 4.9 arcseconds (radius, 90% containment). This
location is 70 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 251 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	22:47:39.18 = 341.91326
  DEC(J2000) = -57:53:38.1  = -57.89393
with a 90%-confidence error radius of about 0.61 arc sec. This position is 0.9
arc sec. from the center of the XRT error circle. The estimated magnitude is
16.48 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.02. 

Burst Advocate for this burst is E. Troja (eleonora.troja AT nasa.gov). 
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.)