GCN Circular 35849

A. Y. Lien (U Tampa), V. D'Elia (SSDC & INAF-OAR),
A. Melandri (INAF-OAR), K. L. Page (U Leicester) and
M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 06:33:18 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 240303A (trigger=1218452).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 229.748, -57.375 which is 
   RA(J2000) = 15h 19m 00s
   Dec(J2000) = -57d 22' 30"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual with an image trigger, the
BAT light curve does not show any significant structure. 

The XRT began observing the field at 06:35:35.6 UT, 136.7 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 229.7375, -57.3633 which is equivalent to:
   RA(J2000)  = 15h 18m 57.00s
   Dec(J2000) = -57d 21' 47.9"
with an uncertainty of 4.8 arcseconds (radius, 90% containment). This
location is 46 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
143 seconds after the BAT trigger. No credible afterglow candidate has been
found in the initial data products. 

The source location is on the Galactic plane (Glon=321.816 deg, Glat=-0.003 deg). 
At this moment we are unable to tell if this is a GRB or a galactic transient,
which we would name Swift J151857.0-572147. We await further ground data to
determine the nature of the source. 

Burst Advocate for this burst is A. Y. Lien (yarleen AT gmail.com). 
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/)