GCN Circular 37397

S. Dichiara (PSU), J. J. DeLaunay (PSU), R. Gupta (NASA GSFC),
J. A. Kennea (PSU), K. L. Page (U Leicester), T. M. Parsotan (GSFC)
and M. A. Williams (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 18:26:03 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 240905E (trigger=1252695).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 345.799, +35.495 which is 
   RA(J2000) = 23h 03m 12s
   Dec(J2000) = +35d 29' 42"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex double-peaked
structure with a duration of about 10 sec.  The peak count rate
was ~3000 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 18:27:50.0 UT, 106.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright X-ray
source located at RA, Dec 345.80347, 35.52378 which is equivalent to:
   RA(J2000)  = 23h 03m 12.83s
   Dec(J2000) = +35d 31' 25.6"
with an uncertainty of 4.3 arcseconds (radius, 90% containment). This
location is 104 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.  We cannot determine whether the source
is fading at the present time. This position is 22.7 arcseconds from
that of a known X-ray source: 1RXS J230313.6+353105 in the ROSAT
RASSFSC catalogue. Assuming a typical GRB power-law spectrum (NH=3e21
cm^-2, gamma=2), the catalogue entry for this object  is equivalent to
a 0.2-10 keV flux of 1.4e-12 erg cm^-2 s^-1; the flux in the 2.5 s XRT
image was 1.11e-09 erg cm^-2 s^-1. No spectrum from the promptly
downlinked event data is yet available to determine the column density.

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 115 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 none of
the XRT error circle. 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 0.076.

We note that the X-ray source is near to a catalogued X-ray source, 1RXS 
J230313.6+353105. However, this is not a known transient source, and is likely a 
chance coincidence. Therefore at this time, we believe that this is a GRB.

Burst Advocate for this burst is S. Dichiara (sbd5667 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/)