TITLE: GCN CIRCULAR NUMBER: 41645 SUBJECT: GRB 250901A: Swift-XRT afterglow detection DATE: 25/09/01 22:33:36 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> J.P. Osborne (U. Leicester), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), D.N. Burrows (PSU), S. Lanava (PSU), S. Dichiara (PSU), K.L. Page (U. Leicester) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of GRB 250901A. We searched for X-ray sources in 754 s of Photon Counting (PC) mode data. The total exposure at the position of the afterglow (see below) is 754 s, obtained between T0+2.6 ks and T0+3.3 ks. An uncatalogued X-ray source is detected and is above the LSXPS 3-sigma upper limit at this position, and is therefore likely the GRB afterglow. The position of this source is RA, Dec=356.1837, -15.7410 which is equivalent to: RA (J2000): 23h 44m 44.08s Dec(J2000): -15d 44' 27.7" with an uncertainty of 3.9 arcsec (radius, 90% confidence). The light curve can be modelled with a power-law decay with a decay index of alpha=3.2 (+1.8, -3.0). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.9 (+0.5, -0.4). The best-fitting absorption column is 6 (+12, -4) x 10^20 cm^-2, consistent with the Galactic value of 1.9 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 3.5 x 10^-11 (4.0 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 6 (+12, -4) x 10^20 cm^-2 Galactic foreground: 1.9 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.9 (+0.5, -0.4) If the light curve continues to decay with a power-law decay index of 3.2, the count rate at T+24 hours will be 2.9 x 10^-6 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.0 x 10^-16 (1.1 x 10^-16) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/03000066. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/SVOM_FIELD00033. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41645. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...