TITLE: GCN CIRCULAR NUMBER: 41300 SUBJECT: GRB 250808A: Swift-XRT candidate counterpart detection DATE: 25/08/09 09:08:58 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi (INAF-IASFPA) , A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N. Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), M. Ferro (INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the SVOM/ECLAIRs-detected source GRB 250808A, collecting 1.7 ks of Photon Counting (PC) mode data between T0+3.7 ks and T0+5.6 ks after the trigger. A candidate counterpart has been found. The details of this source are:

Source 1 (SWIFT J041412.2-224923): ================================== RA (J2000.0): 63.5510 = 04h 14m 12.24s Dec (J2000.0): -22.8231 = -22d 49' 23.2" Error: 3.9 (arcsec, radius, 90% confidence). Detect flag: GOOD Distance: 4.9 arcmin from the SVOM/ECLAIRs position. Mean rate: 0.140 +/- 0.013 ct s^-1 Mean flux: (4.58 +/- 0.43)e-12 erg cm^-2 s^-1 Peak rate: 0.204 +/- 0.046 ct s^-1 Peak flux: (6.7 +/- 1.5)e-12 erg cm^-2 s^-1 ECF: 3.27e-11 erg cm^-2 ct^-1 assuming NH=1.35e+21 cm^-2, gamma=2.07 determined from a spectral fit. XMM UL: 2.2e-12 erg cm^-2 s^-1, (0.3-10 keV) so the source is 3.0-sigma above this 3-sigma upper limit. The source may be fading, at the 2.1-sigma level.

All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with catalogues and upper limits from other missions, we assumed a power-law spectrum with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated.

The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/SVOM/SVOM_FIELD00027.

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

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