TITLE:   GCN CIRCULAR
NUMBER:  38752
SUBJECT: GRB 250101A: Swift detection of a burst with an optical counterpart
DATE:    25/01/01 13:39:49 GMT
FROM:    K.L. Page at U Leicester <klp5@leicester.ac.uk>

K. L. Page (U Leicester), R. Gupta (NASA GSFC), H. A. Krimm (NSF),
A. Y. Lien (U Tampa) and M. H. Siegel (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 13:22:50 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250101A (trigger=1278305).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 37.061, +19.154 which is 
   RA(J2000) = 02h 28m 15s
   Dec(J2000) = +19d 09' 14"
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 ~400 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 13:24:57.4 UT, 127.4 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 37.06963, 19.19521 which is equivalent
to:
   RA(J2000)  = 02h 28m 16.71s
   Dec(J2000) = +19d 11' 42.8"
with an uncertainty of 3.9 arcseconds (radius, 90% containment). This
location is 151 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 consistent with the Galactic value of 1.44
x 10^21 cm^-2 (Willingale et al. 2013).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 132 seconds after the BAT trigger. There is a candidate afterglow in
the list of sources generated on-board at
  RA(J2000)  =	02:28:16.50 =  37.06873
  DEC(J2000) = +19:11:39.9  =  19.19442
with a 90%-confidence error radius of about 1.10 arc sec. This position is 3.8
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.30. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.164.

Burst Advocate for this burst is K. L. Page (klp5 AT leicester.ac.uk).
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/)