TITLE:   GCN CIRCULAR
NUMBER:  43252
SUBJECT: GRB 251230A: Swift detection of a burst with a bright optical counterpart
DATE:    25/12/30 01:48:56 GMT
FROM:    Simone Dichiara at Pennsylvania State University <sbd5667@psu.edu>

S. Lanava (PSU), S. Dichiara (PSU) and R. Gupta (NASA GSFC) report on
behalf of the Neil Gehrels Swift Observatory Team:

At 01:21:41 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 251230A (trigger=1429020).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 179.894, +11.415 which is 
   RA(J2000) = 11h 59m 35s
   Dec(J2000) = +11d 24' 55"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). The BAT light curve showed a multiple-peaked
structure with a duration of about 80 sec. The peak count rate
was ~5000 counts/sec (15-350 keV), at ~30 sec after the trigger.

The XRT began observing the field at 01:22:49.3 UT, 68.1 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
179.89781, 11.43828 which is equivalent to:
   RA(J2000)  = 11h 59m 35.47s
   Dec(J2000) = +11d 26' 17.8"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 84 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 (1.88 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.4
(+1.47/-1.34) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 4.73e-09 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 76 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	11:59:35.42 = 179.89760
  DEC(J2000) = +11:26:19.3  =  11.43870
with a 90%-confidence error radius of about 0.74 arc sec. This position is 4.7
arc sec. from the center of the XRT error circle. The estimated magnitude is
13.74 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.024.

Burst Advocate for this burst is S. Lanava (sml7284 AT psu.edu).
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/)