TITLE: GCN CIRCULAR NUMBER: 41425 SUBJECT: GRB 250818A: Swift-XRT refined Analysis DATE: 25/08/19 04:02:04 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk
D.N. Burrows (PSU), M.A. Williams (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB) and P.A. Evans report on behalf of the Swift-XRT team:
We have analysed 5.5 ks of XRT data for GRB 250818A, from 86 s to 83.2 ks after the trigger. The data comprise 247 s in Windowed Timing (WT) mode (the first 10 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.
The late-time light curve (from T0+4.4 ks) can be modelled with a power-law decay with a decay index of alpha=1.15 (+/-0.04).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.18 (+/-0.06). The best-fitting absorption column is 1.63 (+/-0.08) x 10^22 cm^-2, in excess of the Galactic value of 6.8 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.76 (+0.17, -0.16) and a best-fitting absorption column of 1.69 (+0.25, -0.23) x 10^22 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 6.6 x 10^-11 (1.2 x 10^-10) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus: Total column: 1.69 (+0.25, -0.23) x 10^22 cm^-2 Galactic foreground: 6.8 x 10^21 cm^-2 Excess significance: 7.2 sigma Photon index: 1.76 (+0.17, -0.16)
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01343270.
This circular is an official product of the Swift-XRT team.
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