[vsnet-grb-info 23848] GRB 191004B: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25947
SUBJECT: GRB 191004B: Swift detection of a burst with an optical counterpart
DATE:    19/10/04 21:43:36 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

S. B. Cenko (GSFC), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), A. D'Ai (INAF-IASFPA),
V. D'Elia (SSDC), J.D. Gropp (PSU), J. A. Kennea (PSU),
N. J. Klingler (PSU), N. P. M. Kuin (UCL-MSSL),
S Laha (CRESST/GSFC/UMD), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), M. J. Moss (GWU), D. M. Palmer (LANL) and
A. Tohuvavohu (Toronto) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 21:33:41 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 191004B (trigger=927839).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 49.177, -39.613, which is 
   RA(J2000)  =  03h 16m 42s
   Dec(J2000) = -39d 36' 46"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows multiple peaks
with a total duration of about 40 sec.  The peak count rate
was ~9500 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 21:34:39.3 UT, 57.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 49.20403, -39.63494 which
is equivalent to:
   RA(J2000)  = 03h 16m 48.97s
   Dec(J2000) = -39d 38' 05.8"
with an uncertainty of 3.9 arcseconds (radius, 90% containment). This
location is 108 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. No spectrum from the promptly downlinked
event data is yet available to determine the column density. 

The initial flux in the 2.5 s image was 1.81e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 68 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	03:16:49.10 =  49.20458
  DEC(J2000) = -39:38:03.3  = -39.63425
with a 90%-confidence error radius of about 0.62 arc sec. This position is 2.9
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.00 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.02. 

Burst Advocate for this burst is S. B. Cenko (brad.cenko 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/)