[vsnet-grb-info 12341] GRB 120907A: Swift detection of a burst with optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  13716
SUBJECT: GRB 120907A: Swift detection of a burst with optical counterpart
DATE:    12/09/07 00:38:43 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

J. L. Racusin (NASA/GSFC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
N. Gehrels (NASA/GSFC), S. T. Holland (STScI), J. A. Kennea (PSU),
N. P. M. Kuin (UCL-MSSL), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL),
T. Sakamoto (NASA/UMBC), B. Sbarufatti (INAF-OAB/PSU) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 00:24:23 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 120907A (trigger=532871).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 74.741, -9.314 which is 
   RA(J2000)  =  04h 58m 58s
   Dec(J2000) = -09d 18' 48"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single pulse
with a duration of about 5 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 00:25:45.1 UT, 82.0 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 74.7494, -9.3143 which
is equivalent to:
   RA(J2000)  = 04h 58m 59.86s
   Dec(J2000) = -09d 18' 51.3"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 29 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 5.35
x 10^20 cm^-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
142 seconds after the BAT trigger. There is a candidate afterglow in the
rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	04:59:00.01 =  74.75005
  DEC(J2000) = -09:18:54.2  =  -9.31505
with a 90%-confidence error radius of about 0.63 arc sec. This position is 4.6
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.63 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.09. 

Burst Advocate for this burst is J. L. Racusin (judith.racusin 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.)