GCN Circular 40585

R. Caputo (GSFC), R. Caputo (NASA/GSFC), R. Gupta (NASA GSFC),
J. A. Kennea (PSU), M. J. Moss (GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL), T. M. Parsotan (GSFC), C. Salvaggio (INAF-OAB),
T. Sbarrato (INAF-OAB) and B. Sbarufatti (INAF-OAB) report on behalf
of the Neil Gehrels Swift Observatory Team:

At 18:58:42 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250530C (trigger=1319125). Swift slewed immediately to the burst.
The BAT on-board calculated location is RA, Dec 55.143, -22.459 which is 
   RA(J2000) = 3h 40m 34s
   Dec(J2000) = -22d 27' 32"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Due to a telemetry gap, the immediately-available
BAT light curve does not start until T+8s.  This shows possible activity out
to T+40 s.  The count rate was ~200 counts/sec (15-350 keV), at
the start of available light curve. A more complete description of
the emission, including the trigger time, will require the full
downlinked data from the next ground pass.

The XRT began observing the field at 19:00:29.8 UT, 106.9 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 55.17029,
-22.44825 which is equivalent to:
   RA(J2000)  = 03h 40m 40.87s
   Dec(J2000) = -22d 26' 53.7"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 98 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 (1.70 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.6
(+2.00/-1.76) x 10^21 cm^-2 (90% confidence).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 111 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
0.025.

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/)