TITLE: GCN CIRCULAR NUMBER: 41239 SUBJECT: GRB 250804B: Swift-XRT observations DATE: 25/08/05 14:17:29 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

S. Dichiara (PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), S. Campana (INAF-OAB), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), D.N. Burrows (PSU) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the IPN-detected burst GRB 250804B in a series of observations tiled on the sky. The total exposure time is 3.3 ks, distributed over 3 tiles; the maximum exposure at a single sky location in the tiling was 2.2 ks. The data were collected between T0+79.5 ks and T0+92.2 ks, and are entirely in Photon Counting (PC) mode.

Three uncatalogued X-ray sources have been detected, however none of them is above the RASS limit or shows definitive signs of fading. Therefore, at the present time we cannot identify which, if any, is the afterglow. Details of these sources are given below:

Source 5: RA (J2000.0): 40.9795 = 02:43:55.09 Dec (J2000.0): -46.5917 = -46:35:30.2 Error: 6.8 arcsec (radius, 90% conf.) Count-rate: (6.1 [+3.1, -2.3])e-3 ct s^-1 Distance: 1117 arcsec from IPN position. Flux: (1.60 [+0.81, -0.62])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 6: RA (J2000.0): 40.8694 = 02:43:28.64 Dec (J2000.0): -46.4818 = -46:28:54.4 Error: 7.1 arcsec (radius, 90% conf.) Count-rate: (9.6 [+4.0, -3.2])e-3 ct s^-1 Distance: 1217 arcsec from IPN position. Flux: (1.78 [+0.75, -0.59])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 7: RA (J2000.0): 40.5489 = 02:42:11.74 Dec (J2000.0): -46.5143 = -46:30:51.5 Error: 8.8 arcsec (radius, 90% conf.) Count-rate: (8.8 [+4.8, -3.6])e-3 ct s^-1 Distance: 924 arcsec from IPN position. Flux: (3.0 [+1.6, -1.2])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Four catalogued sources were also detected.

The results of the XRT-team automatic analysis of the tiled XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/xrt_products/TILED_GRB00138.

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

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