TITLE:   GCN CIRCULAR
NUMBER:  36976
SUBJECT: GRB 240730A: Swift detection of a burst
DATE:    24/07/30 08:11:21 GMT
FROM:    David Palmer at LANL <palmer@lanl.gov>


S. B. Cenko (GSFC), P. A. Evans (U Leicester),
R. A. J. Eyles-Ferris (U Leicester), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 07:52:06 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 240730A (trigger=1245636).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 37.829, +59.074 which is 
   RA(J2000) = 02h 31m 19s
   Dec(J2000) = +59d 04' 27"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 80 sec.  The peak count rate
was ~300 counts/sec (15-350 keV), at ~30 sec after the trigger. 

The XRT began observing the field at 07:54:14.8 UT, 128.7 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 37.83844,
59.02878 which is equivalent to:
   RA(J2000)  = 02h 31m 21.22s
   Dec(J2000) = +59d 01' 43.6"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 163 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 consistent with the Galactic value of 7.80
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 133 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 coverage of the XRT error circle by the 8'x8' region
for the list of sources generated on-board is uncertain because the large
number of sources filled the available telemetry. The list of sources is
typically complete to about 18 mag. No correction has been made for the large,
but uncertain, extinction expected. 

Although this appears to be a long GRB, its long duration and
its position near the Galactic plane (lat=-1.35) raise the possibility
that this is a Galactic transient instead.  If that is the case
then we give it the name Swift J0231.3+5902 . 

Burst Advocate for this burst is S. B. Cenko (brad.cenko 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/)