GCN Circular 39396

R. Caputo (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL), T. M. Parsotan (GSFC)
and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 03:34:37 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250221A (trigger=1290305).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 59.477, -15.139 which is 
   RA(J2000) = 03h 57m 54s
   Dec(J2000) = -15d 08' 18"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peak
structure with a duration of about 5 sec.  The peak count rate
was ~2200 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 03:36:39.9 UT, 122.9 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 59.46251, -15.13379
which is equivalent to:
   RA(J2000)  = 03h 57m 51.00s
   Dec(J2000) = -15d 08' 01.6"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 53 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (3.73 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.1
(+2.63/-2.31) x 10^21 cm^-2 (90% confidence).

UVOT took a finding chart exposure of 250 seconds with the U filter starting
340 seconds after the BAT trigger. There is a candidate afterglow in the
rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	03:57:51.06 =  59.46274
  DEC(J2000) = -15:07:59.2  = -15.13311
with a 90%-confidence error radius of about 0.76 arc sec. This position is 2.4
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.82 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.046.

Burst Advocate for this burst is R. Caputo (regina.caputo 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/)