TITLE: GCN CIRCULAR NUMBER: 40950 SUBJECT: GRB 250704A: Swift-XRT afterglow detection DATE: 25/07/04 14:00:05 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> J.P. Osborne (U. Leicester), B. Sbarufatti (INAF-OAB), C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), M.A. Williams (PSU), S. Dichiara (PSU), J.A. Kennea (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: We have analysed 2.0 ks of XRT data for GRB 250704A, from 2.9 ks to 8.7 ks after the SVOM/ECLAIRs trigger. The data are entirely in Photon Counting (PC) mode. A bright, fading source is detected inside the SVOM/ECLAIRs error region (Cao et al., GCN 40934), which we propose as the GRB afterglow. Using 2029 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 = 17.19414, -17.32397 which is equivalent to: RA (J2000): 01h 08m 46.59s Dec(J2000): -17d 19' 26.3" with an uncertainty of 2.8 arcsec (radius, 90% confidence). The light curve can be modelled with a power-law decay with a decay index of alpha=1.3 (+0.6, -0.5). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.03 (+0.31, -0.28). The best-fitting absorption column is 6.0 (+6.7, -4.5) x 10^20 cm^-2, consistent with the Galactic value of 1.6 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.2 x 10^-11 (3.8 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 6.0 (+6.7, -4.5) x 10^20 cm^-2 Galactic foreground: 1.6 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 2.03 (+0.31, -0.28) If the light curve continues to decay with a power-law decay index of 1.3, the count rate at T+24 hours will be 2.4 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.9 x 10^-14 (9.1 x 10^-14) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00019907. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/40950. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...