TITLE:   GCN CIRCULAR
NUMBER:  8237
SUBJECT: GRB 080916: Swift detection of a burst with optical afterglow
DATE:    08/09/16 10:03:53 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

H. Ziaeepour (UCL-MSSL), L. Vetere (PSU),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
S. Campana (INAF-OAB), P. A. Evans (U Leicester), C. Gronwall (PSU),
H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (MSSL),
V. Mangano (INAF-IASFPA), J. Mao (INAF-OAB), S. R. Oates (UCL-MSSL),
K. L. Page (U Leicester), D. M. Palmer (LANL), B. Preger (ASDC),
P. Romano (INAF-IASFPA), T. Sakamoto (NASA/UMBC),
M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester) and
T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team:

At 09:45:20 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 080916 (trigger=324895).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 336.302, -57.022 which is 
   RA(J2000) = 22h 25m 12s
   Dec(J2000) = -57d 01' 17"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a double-peaked
structure with a duration of about 60 sec.  The peak count rate
was ~5000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 09:46:30.8 UT, 70.2 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 336.27802, -57.02190 which is
equivalent to:
   RA(J2000)  = 22h 25m 6.72s
   Dec(J2000) = -57d 01' 18.8"
with an uncertainty of 3.3 arcseconds (radius, 90% containment). This
location is 46 arcseconds from the BAT onboard position, within the BAT
error circle. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
1.83e+20 cm^-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm)
filter starting 76 seconds after the BAT trigger. There is a candidate
afterglow in the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	22:25:06.19 = 336.2758
  DEC(J2000) = -57:01:22.8  = -57.0230
with a 1-sigma error radius of about 0.5 arc sec. This position is 5.9 arc
sec. from the center of the XRT error circle. The estimated magnitude is 18.3
with a 1-sigma error of about 0.5 mag. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.02. 

Burst Advocate for this burst is H. Ziaeepour (hz AT mssl.ucl.ac.uk). 
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.)