TITLE:   GCN CIRCULAR
NUMBER:  13845
SUBJECT: GRB 121011A: Swift detection of a burst with optical afterglow
DATE:    12/10/11 11:36:05 GMT
FROM:    Judith Racusin at GSFC  <judith.racusin@nasa.gov>

J. L. Racusin (NASA/GSFC), D. N. Burrows (PSU), V. D'Elia (ASDC),
P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC),
B.P. Gompertz (U Leicester), J. A. Kennea (PSU),
N. P. M. Kuin (UCL-MSSL), C. Pagani (U Leicester),
K. L. Page (U Leicester), B. Sbarufatti (INAF-OAB/PSU) and
R. L. C. Starling (U Leicester) report on behalf of the Swift Team:

At 11:15:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 121011A (trigger=535764).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 260.200, +41.137 which is 
   RA(J2000) = 17h 20m 48s
   Dec(J2000) = +41d 08' 12"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 10 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 11:17:07.9 UT, 97.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 260.21504,
41.11179 which is equivalent to:
   RA(J2000)  = 17h 20m 51.61s
   Dec(J2000) = +41d 06' 42.4"
with an uncertainty of 4.3 arcseconds (radius, 90% containment). This
location is 99 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 2.34
x 10^20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 6.36e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 108 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	17:20:51.22 = 260.21340
  DEC(J2000) = +41:06:36.9  =  41.11024
with a 90%-confidence error radius of about 0.63 arc sec. This position is 9.1
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.76 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 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.)