TITLE: GCN CIRCULAR NUMBER: 40079 SUBJECT: GRB 250331D: Swift-XRT afterglow detection DATE: 25/04/05 12:24:26 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has conducted further observations of the field of the Swift/BAT-detected burst GRB 250331D. The observations now extend from T0+45.3 ks to T0+406.9 ks and have a total exposure time of 8.6 ks.

Of the sources previously reported, "Source 2" is fading with >3-sigma significance, and is therefore likely the GRB afterglow. Using 3064 s of PC mode data and 2 UVOT images, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 48.65718, +62.28326 which is equivalent to:

RA (J2000): 03h 14m 37.72s Dec(J2000): +62d 16' 59.8"

with an uncertainty of 3.5 arcsec (radius, 90% confidence). This position is 3.9 arcmin from the Swift/BAT position. There is evidence that the source may be fading, however we cannot constrain the decay at the current time.

The results of the XRT-team automatic analysis of the likely afterglow are at https://www.swift.ac.uk/ToO_GRBs/00021825/Source2.php. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021825.

This circular is an official product of the Swift-XRT team.

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