GCN Circular 42343

R. Gupta (NASA GSFC), R. Brivio (INAF-OAB), S. Dichiara (PSU),
M. Ferro (INAF-OAB), D. M. Palmer (LANL), T. Sbarrato (INAF-OAB) and
M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 03:12:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 251018A (trigger=1405083).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 282.071, -37.028 which is 
   RA(J2000) = 18h 48m 17s
   Dec(J2000) = -37d 01' 41"
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 60 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~1 sec after the trigger.

The XRT began observing the field at 03:13:16.7 UT, 74.4 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 282.0433, -37.0265 which is equivalent to:
   RA(J2000)  = 18h 48m 10.39s
   Dec(J2000) = -37d 01' 35.4"
with an uncertainty of 6.3 arcseconds (radius, 90% containment). This
location is 79 arcseconds from the BAT onboard position, within the BAT
error circle. 
The initial flux in the 2.5 s image was 5.19e-10 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 83 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	18:48:10.04 = 282.04183
  DEC(J2000) = -37:01:42.2  = -37.02840
with a 90%-confidence error radius of about 0.62 arc sec. This position is 8.0
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.96 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.100.

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/)