TITLE:   GCN CIRCULAR
NUMBER:  42322
SUBJECT: GRB 251017A: Swift detection of a burst with an optical counterpart
DATE:    25/10/17 08:54:42 GMT
FROM:    K.L. Page at U Leicester <klp5@leicester.ac.uk>

R. Gupta (NASA GSFC), R. Brivio (INAF-OAB), S. Dichiara (PSU),
R. A. J. Eyles-Ferris (U Leicester), A. Mei (INAF-OAB),
K. L. Page (U Leicester), C. Salvaggio (INAF-OAB) and
T. Sbarrato (INAF-OAB) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 08:40:44 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 251017A (trigger=1404904).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 335.386, +7.051 which is 
   RA(J2000) = 22h 21m 33s
   Dec(J2000) = +07d 03' 02"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 25 sec.  The peak count rate
was ~2136 counts/sec (15-350 keV), at ~6 sec after the trigger.

The XRT began observing the field at 08:42:34.1 UT, 109.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 335.40109, 7.04043 which
is equivalent to:
   RA(J2000)  = 22h 21m 36.26s
   Dec(J2000) = +07d 02' 25.5"
with an uncertainty of 4.0 arcseconds (radius, 90% containment). This
location is 65 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 https://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 in excess of the Galactic value (8.90 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 6.7
(+3.92/-3.27) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 2.03e-09 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 120 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	22:21:36.24 = 335.40098
  DEC(J2000) = +07:02:23.6  =	7.03990
with a 90%-confidence error radius of about 0.77 arc sec. This position is 4.6
arc sec. from the center of the XRT error circle. The estimated magnitude is
20.42 with a 1-sigma error of about  0.21. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.108.

Burst Advocate for this burst is R. Gupta (rahulbhu.c157 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/)