[vsnet-grb-info 9538] GRB 100728B: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  11009
SUBJECT: GRB 100728B: Swift detection of a burst with an optical counterpart
DATE:    10/07/28 11:42:40 GMT
FROM:    Kim Page at U.of Leicester  <kpa at star.le.ac.uk>

D. C. Morris (GWU/GSFC), A. P. Beardmore (U Leicester),
V. D'Elia (ASDC), P. A. Evans (U Leicester), J. M. Gelbord (PSU),
C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC),
H. A. Krimm (CRESST/GSFC/USRA), O. M. Littlejohns (U Leicester),
C. B. Markwardt (CRESST/GSFC/UMD), J. P. Osborne (U Leicester),
C. Pagani (U Leicester), K. L. Page (U Leicester),
A. Rowlinson (U Leicester), B. Sbarufatti (INAF-OAB/IASFPA),
M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC) and
R. L. C. Starling (U Leicester) report on behalf of the Swift Team:

At 10:31:55 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100728B (trigger=430172).  Swift slewed immediately to the 
burst. The BAT on-board calculated location is RA, Dec 44.053, +0.280 
which is
    RA(J2000) = 02h 56m 13s
    Dec(J2000) = +00d 16' 46"
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 plus some possible precursor
activity.  The peak count rate was ~2000 counts/sec (15-350 keV),
at ~0 sec after the trigger.

The XRT began observing the field at 10:33:32.9 UT, 97.0 seconds after
the BAT trigger. No source was detected in the first 2.5 seconds image.
The XRT observed the field for approximately 80 seconds in WT mode.
The prompt spectrum from the WT data shows evidence of a source.
Due to an observing constraint no additional PC data were taken to allow
XRT to centroid on the afterglow and derive a position.
We are waiting for the full dataset to detect and localise the
XRT counterpart.

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 100 seconds after the BAT trigger. There is a candidate afterglow 
in the rapidly available 2.7'x2.7' sub-image at
   RA(J2000)  =	02:56:13.46 = 44.05607
   DEC(J2000) = +00:16:51.9  =  0.28109
with a 90%-confidence error radius of about 0.75 arc sec. This position is 
11.7 arc sec. from the center of the BAT error circle. The estimated 
magnitude is 17.30 with a 1-sigma error of about 0.14. No correction has 
been made for the expected extinction corresponding to E(B-V) of 0.07.

Burst Advocate for this burst is D. C. Morris (david.c.morris 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.)