TITLE:   GCN CIRCULAR
NUMBER:  40407
SUBJECT: GRB 250509A: Swift detection of a burst
DATE:    25/05/09 22:49:14 GMT
FROM:    Mike Siegel at PSU/Swift MOC <siegel@swift.psu.edu>

M. H. Siegel (PSU), S. Dichiara (PSU), R. Gupta (NASA GSFC) and
B. Sbarufatti (INAF-OAB) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 22:33:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250509A (trigger=1311764). Swift slewed immediately to the burst.
The BAT on-board calculated location is RA, Dec 46.455, -38.853 which is 
   RA(J2000) = 03h 05m 49s
   Dec(J2000) = -38d 51' 11"
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 60 sec.  The peak count rate
was ~2800 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 22:34:41.7 UT, 71.1 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 46.45032, -38.84173 which
is equivalent to:
   RA(J2000)  = 03h 05m 48.08s
   Dec(J2000) = -38d 50' 30.2"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 42 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 (2.03 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 9.2
(+3.98/-3.41) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 7.21e-09 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of nominal 150.000 seconds with the White
filter starting 80 seconds after the BAT trigger. No credible afterglow
candidate has been found in the initial data products. Data from the 2.7'x2.7'
sub-image are not available at this time. 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 0.021.

Burst Advocate for this burst is M. H. Siegel (siegel AT swift.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/)