[vsnet-grb-info 8243] GRB 090812: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9768
SUBJECT: GRB 090812: Swift detection of a burst with an optical counterpart
DATE:    09/08/12 06:20:46 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

M. Stamatikos (NASA/ORAU), J. R. Cummings (NASA/UMBC),
P. A. Evans (U Leicester), C. Gronwall (PSU),
C. Guidorzi (U Ferrara), C. B. Markwardt (CRESST/GSFC/UMD),
P. T. O'Brien (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL) and R. L. C. Starling (U Leicester) report on
behalf of the Swift Team:

At 06:02:08 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090812 (trigger=359711).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 353.194, -10.595 which is 
   RA(J2000) = 23h 32m 47s
   Dec(J2000) = -10d 35' 39"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a triple-peaked
structure with a duration of about 70 sec.  The peak count rate
was ~6500 counts/sec (15-350 keV), at ~25 sec after the trigger. 

The XRT began observing the field at 06:03:25.7 UT, 76.8 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 353.20133, -10.60457 which
is equivalent to:
   RA(J2000)  = 23h 32m 48.32s
   Dec(J2000) = -10d 36' 16.5"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 43 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
2.26e+20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 4.95e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
298 seconds after the BAT trigger. There is a candidate afterglow in the
rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	23:32:48.53 = 353.20222
  DEC(J2000) = -10:36:17.3  = -10.60481
with a 90%-confidence error radius of about 0.78 arc sec. This position is 5.1
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.62 with a 1-sigma error of about 0.16. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.02. 

Burst Advocate for this burst is M. Stamatikos (michael AT milkyway.gsfc.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.)