TITLE: GCN CIRCULAR NUMBER: 41909 SUBJECT: GRB 250920B: Swift-XRT refined Analysis DATE: 25/09/20 16:55:32 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> K.L. Page (U. Leicester), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB), S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester) and P.A. Evans report on behalf of the Swift-XRT team: We have analysed 6.4 ks of XRT data for GRB 250920B, from 87 s to 22.3 ks after the trigger. The data comprise 395 s in Windowed Timing (WT) mode (the first 2 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The refined XRT position is RA, Dec = 187.3175, +47.8376 which is equivalent to: RA (J2000): 12h 29m 16.21s Dec(J2000): +47d 50' 15.2" with an uncertainty of 3.5 arcsec (radius, 90% confidence). The late-time light curve (from T0+3.8 ks) can be modelled with a power-law decay with a decay index of alpha=1.20 (+/-0.05). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.490 (+0.035, -0.017). The best-fitting absorption column is consistent with the Galactic value of 1.1 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.89 (+/-0.07) and a best-fitting absorption column of 3.3 (+1.5, -1.4) x 10^20 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10^-11 (3.7 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 3.3 (+1.5, -1.4) x 10^20 cm^-2 Galactic foreground: 1.1 x 10^20 cm^-2 Excess significance: 2.5 sigma Photon index: 1.89 (+/-0.07) If the light curve continues to decay with a power-law decay index of 1.20, the count rate at T+24 hours will be 0.10 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.5 x 10^-12 (3.8 x 10^-12) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01351008. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41909. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...