TITLE: GCN CIRCULAR NUMBER: 41290 SUBJECT: GRB 250807A: Swift-XRT refined Analysis DATE: 25/08/08 15:31:42 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

J.P. Osborne (U. Leicester), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB), D.N. Burrows (PSU), M.A. Williams (PSU), S. Dichiara (PSU), K.L. Page (U. Leicester) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 4.6 ks of XRT data for GRB 250807A, from 93 s to 91.2 ks after the trigger. The data comprise 1.2 ks in Windowed Timing (WT) mode (the first 5 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.

The late-time light curve (from T0+6.7 ks) can be modelled with a power-law decay with a decay index of alpha=1.75 (+0.11, -0.10).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.96 (+/-0.03). The best-fitting absorption column is 1.90 (+/-0.11) x 10^21 cm^-2, in excess of the Galactic value of 1.4 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.85 (+0.11, -0.10) and a best-fitting absorption column of 1.8 (+0.4, -0.3) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.9 x 10^-11 (4.9 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 1.8 (+0.4, -0.3) x 10^21 cm^-2 Galactic foreground: 1.4 x 10^20 cm^-2 Excess significance: 8.0 sigma Photon index: 1.85 (+0.11, -0.10)

The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01340514.

This circular is an official product of the Swift-XRT team.

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