[vsnet-grb-info 12237] GRB 120811C: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  13622
SUBJECT: GRB 120811C: Swift detection of a burst with an optical counterpart
DATE:    12/08/11 15:52:41 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

B. N. Barlow (PSU), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), D. Grupe (PSU),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL) and B.-B. Zhang (PSU)
report on behalf of the Swift Team:

At 15:34:52 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 120811C (trigger=530689).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 199.709, +62.291 which is 
   RA(J2000)  =  13h 18m 50s
   Dec(J2000) = +62d 17' 29"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single bright peak
with a duration of about 35 sec.  The peak count rate
was ~7000 counts/sec (15-350 keV), at ~6 sec after the trigger. 

The XRT began observing the field at 15:36:00.8 UT, 68.7 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 199.6833, 62.3027 which is equivalent to:
   RA(J2000)  = +13h 18m 43.99s
   Dec(J2000) = +62d 18' 09.7"
with an uncertainty of 5.0 arcseconds (radius, 90% containment). This
location is 60 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. 

The initial flux in the 2.5 s image was 2.97e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 75 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	13:18:43.81 = 199.68253
  DEC(J2000) = +62:18:02.7  =  62.30076
with a 90%-confidence error radius of about 0.75 arc sec. This position is 7.1
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.40 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.03. 

Burst Advocate for this burst is B. N. Barlow (bnb2 AT 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.)