TITLE:   GCN CIRCULAR
NUMBER:  40292
SUBJECT: GRB 250430A: Swift detection of a burst with optical counterpart
DATE:    25/04/30 18:07:07 GMT
FROM:    Tyler Parsotan  at NASA GSFC <tyler.parsotan@nasa.gov>

T. M. Parsotan (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC),
N. J. Klingler (GSFC/UMBC/CRESSTII), M. J. Moss (GSFC),
K. L. Page (U Leicester), M. H. Siegel (PSU) and M. A. Williams (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 17:31:21 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250430A (trigger=1308754). Swift slewed immediately to the burst.
The BAT on-board calculated location is RA, Dec 233.428, -18.083 which is 
   RA(J2000) = 15h 33m 43s
   Dec(J2000) = -18d 04' 59"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi peak
structure with a duration of about 20 sec.  The peak count rate
was ~3000 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 17:33:37.5 UT, 157.3 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 233.38611, -18.11784 which is
equivalent to:
   RA(J2000)  = 15h 33m 32.67s
   Dec(J2000) = -18d 07' 04.2"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 190 arcseconds from the BAT onboard position, outside 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 (9.57 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.5
(+2.62/-2.27) x 10^21 cm^-2 (90% confidence).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 161 seconds after the BAT trigger. There is a candidate afterglow in
the list of sources generated on-board at
RA(J2000) = 15:33:32.85 = 233.38688
DEC(J2000) = -18:07:06.2 = -18.11840
with a 90%-confidence error radius of about 1.10 arc sec. This position is 3.65
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.44 with a 1-sigma error of about 0.35. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.092.

Burst Advocate for this burst is T. M. Parsotan (tyler.parsotan 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/)