TITLE: GCN CIRCULAR NUMBER: 41531 SUBJECT: GRB 250823A: Swift-XRT refined Analysis DATE: 25/08/24 20:31:05 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> S. Dichiara (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), S. Campana (INAF-OAB), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU) and P.A. Evans report on behalf of the Swift-XRT team: We have analysed 4.8 ks of XRT data for GRB 250823A, from 111 s to 44.7 ks after the trigger. The data comprise 313 s in Windowed Timing (WT) mode (the first 7 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The light curve can be modelled with a series of power-law decays. The light curve initially rises, with an index alpha=-1.5 (+0.0, -0.6). At T+120 s it breaks to an alpha of 3.33 (+/-0.09). The light curve breaks again at T+349 s to a decay with alpha=2.31 (+0.23, -0.21), before a final break at T+1667 s s after which the decay index is 0.78 (+0.13, -0.12). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.91 (+/-0.04). The best-fitting absorption column is 1.99 (+0.16, -0.15) x 10^21 cm^-2, in excess of the Galactic value of 1.1 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.92 (+0.28, -0.26) and a best-fitting absorption column of 1.9 (+1.0, -0.8) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10^-11 (5.0 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.9 (+1.0, -0.8) x 10^21 cm^-2 Galactic foreground: 1.1 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.92 (+0.28, -0.26) If the light curve continues to decay with a power-law decay index of 0.78, the count rate at T+24 hours will be 9.6 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.6 x 10^-13 (4.8 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01344586. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41531. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...