TITLE:   GCN CIRCULAR
NUMBER:  40491
SUBJECT: GRB 250520A: Swift detection of a burst
DATE:    25/05/20 02:59:58 GMT
FROM:    K.L. Page at U Leicester <klp5@leicester.ac.uk>

R. A. J. Eyles-Ferris (U Leicester), N. P. M. Kuin (UCL-MSSL),
A. Y. Lien (U Tampa), M. J. Moss (GSFC) and K. L. Page (U Leicester)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 02:42:16.84 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250520A (trigger=1315630). Swift slewed immediately to the burst.
The BAT on-board calculated location is RA, Dec 282.268, -11.847 which is 
   RA(J2000) = 18h 49m 04s
   Dec(J2000) = -11d 50' 49"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a short burst
with a duration of about 1 sec.  The peak count rate
was ~10,000 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 02:44:40.8 UT, 144.1 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 282.28588, -11.86811 which
is equivalent to:
   RA(J2000)  = 18h 49m 08.61s
   Dec(J2000) = -11d 52' 05.2"
with an uncertainty of 3.6 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 (3.68 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 8
(+6.16/-4.83) x 10^21 cm^-2 (90% confidence).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 148 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. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any afterglow in the
sub-image. The 8'x8' region for the list of sources generated on-board covers
100% of the XRT error circle. Because of the density of catalogued stars,
further analysis is required to report an upper limit for any afterglow in the
region. No correction has been made for the large, but uncertain, extinction
expected.

Burst Advocate for this burst is R. A. J. Eyles-Ferris (raje1 AT leicester.ac.uk).
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/)