[vsnet-grb-info 17173] GRB 151029A: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  18522
SUBJECT: GRB 151029A: Swift detection of a burst with an optical counterpart
DATE:    15/10/29 08:49:18 GMT
FROM:    Jamie A. Kennea at PSU/Swift-XRT  <kennea at swift.psu.edu>

T. N. Ukwatta (LANL), A. D'Ai (INAF-IASFPA), J. A. Kennea (PSU),
A. Maselli (INAF-IASFPA), P. T. O'Brien (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Swift Team:

At 07:49:54 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 151029A (trigger=662086).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 38.529, -35.354, which is
   RA(J2000)  =  02h 34m 07s
   Dec(J2000) = -35d 21' 14.4"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Lightcurve data is not immediately available. 

Using promptly downlinked data we find an uncatalogued X-ray source
with an enhanced position: RA, Dec 38.5280, -35.3855 which is
equivalent to:
   RA(J2000)  = 02h 34m 06.72s
   Dec(J2000) = -35d 23' 07.7"
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
position is 113.4 arcsec from the BAT position. This position may be
improved as more data are received; the latest position is available at
http://www.swift.ac.uk/sper. 
We cannot determine whether the source is fading at the present time.  

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 3.41
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 209 seconds after the BAT trigger. The initial
2.7'x2.7' sub-image does not cover the XRT error circle. There is a candidate
afterglow in the UVOT srclist at:
RA(J2000) =   02:34:08.08 = 38.527964
DEC(J2000) = +35:23:08.8 = -35.385784
with a 90%-confidence error radius of about 0.61 arc sec. This position is 1.06
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.34 with a 1-sigma error of about 0.04. No correction has been made for the
expected extinction. 

Burst Advocate for this burst is T. N. Ukwatta (tilan.ukwatta AT gmail.com). 
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.)