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GCN Circular 35858

Subject
Swift detection of Swift J151857.0-572147
Date
2024-03-04T09:14:44Z (2 months ago)
From
P.A. Evans at U. Leicester <pae9@leicester.ac.uk>
Via

E. Ambrosi (INAF-IASFPA), V. D'Elia (SSDC & INAF-OAR),
P. A. Evans (U Leicester), J. A. Kennea (PSU), 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 08:53:10 UT, the Swift Burst Alert Telescope (BAT) triggered and
located Swift J151857.0-572147 (trigger=1218604).  Swift slewed
immediately to the object. The BAT on-board calculated location is 
RA, Dec 229.732, -57.354 which is 
   RA(J2000) = 15h 18m 56s
   Dec(J2000) = -57d 21' 15"
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 08:56:20.6 UT, 189.7 seconds after
the BAT trigger. XRT found a bright X-ray source located at RA, Dec
= 229.7358, -57.3617 which is equivalent to:
   RA(J2000)  = 15h 18m 56.59s
   Dec(J2000) = -57d 21' 42.1"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 28 arcseconds from the BAT onboard position, within the BAT
error circle, and consistent with the localisation of Swift J151857.0-572147
reported in GCN Circs. 35849Not found
 
 
 & 35853. 

UVOT took a finding chart exposure of 250 seconds with the U filter
starting 195 seconds after the BAT trigger. No credible afterglow
candidate has been found in the initial data products. 

This object is the same as BAT trigger 1218452, which was originally
identified as GRB 240303A (Lien et al., GCN Circ. 35849Not found
 
 
) but later confirmed
as a Galactic transient Swift J151857.0-572147 (Kennea et al., GCN Circ. 35853Not found
 
 
).