TITLE: GCN CIRCULAR NUMBER: 41313 SUBJECT: GRB 250809A: Swift-XRT refined Analysis DATE: 25/08/10 09:44:16 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> J.P. Osborne (U. Leicester), A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB), A. Melandri (INAF-OAR), 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 report on behalf of the Swift-XRT team: We have analysed 6.7 ks of XRT data for GRB 250809A, from 113 s to 51.4 ks after the trigger. The data comprise 910 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The best available XRT position (using the promptly downlinked event data, the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue) is RA, Dec = 233.00043, -53.29626 which is equivalent to: RA (J2000): 01h 00m 00.00s Dec(J2000): -53d 00' 00.0" with an uncertainty of 3.0 arcsec (radius, 90% confidence). The light curve can be modelled with an initial rise, with a power-law index of alpha=-1.5 (NaN, NaN), followed by a break at T+281 s to an alpha of 8.0 (NaN, NaN). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.41 (+/-0.13). The best-fitting absorption column is 2.05 (+0.18, -0.17) x 10^22 cm^-2, in excess of the Galactic value of 8.0 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.9 x 10^-11 (1.6 x 10^-10) erg cm^-2 count^-1. A summary of the WT-mode spectrum is thus: Total column: 2.05 (+0.18, -0.17) x 10^22 cm^-2 Galactic foreground: 8.0 x 10^21 cm^-2 Excess significance: 11.9 sigma Photon index: 2.41 (+/-0.13) If the light curve continues to decay with a power-law decay index of 2.96, the count rate at T+24 hours will be 3.7 x 10^-8 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.8 x 10^-18 (6.1 x 10^-18) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01340954. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41313. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...