[vsnet-grb-info 8589] GRB 091029: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  10097
SUBJECT: GRB 091029: Swift detection of a burst with an optical counterpart
DATE:    09/10/29 04:05:12 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

D. Grupe (PSU), J. R. Cummings (NASA/UMBC),
F. E. Marshall (NASA/GSFC), C. Pagani (PSU), D. M. Palmer (LANL),
M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC),
T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the
Swift Team:

At 03:53:22 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 091029 (trigger=374210).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 60.183, -55.994 which is 
   RA(J2000) = 04h 00m 44s
   Dec(J2000) = -55d 59' 37"
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 50 sec.  The peak count rate
was ~3000 counts/sec (15-350 keV), at ~18 sec after the trigger. 

The XRT began observing the field at 03:54:41.9 UT, 79.3 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 60.17774, -55.95574 which
is equivalent to:
   RA(J2000)  = 04h 00m 42.66s
   Dec(J2000) = -55d 57' 20.7"
with an uncertainty of 4.4 arcseconds (radius, 90% containment). This
location is 138 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
1.14e+20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 1.80e-09 erg cm^-2 s^-1 (0.2-10
keV). 


UVOT took a finding chart exposure of 150 seconds with the White filter
starting 90 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	04:00:42.58 = 60.17742
  DEC(J2000) = -55:57:20.0  = -55.95555
with a 90%-confidence error radius of about 0.70 arc sec. This position is 2.3
arc sec. from the center of the XRT error circle. The estimated magnitude is
20.31 with a 1-sigma error of about 0.18. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.02. 




Burst Advocate for this burst is D. Grupe (grupe AT astro.psu.edu). 
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.)