TITLE:   GCN CIRCULAR
NUMBER:  15402
SUBJECT: GRB 131030A: Swift detection of a burst with an optical counterpart
DATE:    13/10/30 21:08:51 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

E. Troja (NASA/GSFC/UMCP), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
J. R. Cummings (NASA/UMBC), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), V. Mangano (PSU),
C. B. Markwardt (NASA/GSFC), A. Maselli (INAF-IASFPA),
K. L. Page (U Leicester), D. M. Palmer (LANL), T. Sakamoto (AGU),
B. Sbarufatti (INAF-OAB/PSU), C. A. Swenson (PSU) and
T. N. Ukwatta (MSU) report on behalf of the Swift Team:

At 20:56:18 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 131030A (trigger=576238).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 345.080, -5.389 which is 
   RA(J2000) = 23h 00m 19s
   Dec(J2000) = -05d 23' 20"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed multiple peaks
structure with a duration of about 45 sec.  The peak count rate
was ~31500 counts/sec (15-350 keV), at ~6 sec after the trigger. 

The XRT began observing the field at 20:57:37.1 UT, 78.4 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 345.0657, -5.3659 which is equivalent to:
   RA(J2000)  = +23h 00m 15.77s
   Dec(J2000) = -05d 21' 57.2"
with an uncertainty of 5.1 arcseconds (radius, 90% containment). This
location is 97 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 0.1 s image was 5.43e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 89 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	23:00:16.13 = 345.06720
  DEC(J2000) = -05:22:05.1  =  -5.36808
with a 90%-confidence error radius of about 0.74 arc sec. This position is 9.5
arc sec. from the center of the XRT error circle. The estimated magnitude is
14.97 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.06. 

Burst Advocate for this burst is E. Troja (eleonora.troja 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.)