GCN Circular 39686

A. Melandri (INAF-OAR), V. D'Elia (SSDC & INAF-OAR),
J. J. DeLaunay (PSU), S. Dichiara (PSU), R. Gupta (NASA GSFC),
J. A. Kennea (PSU), M. J. Moss (GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB) and M. A. Williams (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 13:24:17 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250312A (trigger=1294765).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 29.510, -42.404 which is 
   RA(J2000) = 01h 58m 02s
   Dec(J2000) = -42d 24' 15"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Only the first 8 seconds of the BAT light
curve are immediately available.  This limited data shows a complex peak
structure that extends beyond the T+8s cut-off. The peak count rate
was ~1800 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 13:26:33.0 UT, 135.4 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
29.51911, -42.39599 which is equivalent to:
   RA(J2000)  = 01h 58m 04.59s
   Dec(J2000) = -42d 23' 45.6"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 37 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 (1.79 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3
(+2.65/-2.31) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 7.29e-09 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 427 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag.
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.017.

Burst Advocate for this burst is A. Melandri (andrea.melandri AT inaf.it).
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/)