TITLE: GCN CIRCULAR NUMBER: 40951 SUBJECT: EP250704a / GRB250704B: Swift-XRT counterpart detection DATE: 25/07/04 14:06:18 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 Einstein Probe/WXT-detected source EP250704a (temporally coincident with the SVOM GRB 250704B, Wang et al., GCN 40940), collecting 1.7 ks of Photon Counting (PC) mode data between T0+1.8 ks and T0+3.7 ks after the trigger. A candidate counterpart has been found. The details of this source are: Source 2 (SWIFT J200329.1+120123): ================================== RA (J2000.0): 300.8716 = 20h 03m 29.18s Dec (J2000.0): +12.0233 = +12d 01' 23.9" Error: 3.6 (arcsec, radius, 90% confidence). Detect flag: REASONABLE Distance: 24 arcsec from the Einstein Probe/WXT position. Mean rate: 0.247 +/- 0.017 ct s^-1 Mean flux: (8.79 +/- 0.59)e-12 erg cm^-2 s^-1 Peak rate: 0.389 +/- 0.088 ct s^-1 Peak flux: (1.39 +/- 0.31)e-11 erg cm^-2 s^-1 ECF: 3.56e-11 erg cm^-2 ct^-1, assuming NH=1.47e+21 cm^-2, gamma=1.97; determined from a spectral fit. RASS UL: 2.9e-02 ct s^-1 (converted to XRT; 0.3-10 keV) so the source is not above this 3-sigma upper limit. The source may be fading, at the 1.7-sigma level. There is 1 2MASS object within the source's 3-sigma error radius. 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/EP. The detected afterglow position is consistent with the optical counterpart of the EP trigger, reported by Schneider et al. (GCN 40942). This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/40951. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...