TITLE: GCN CIRCULAR NUMBER: 38859 SUBJECT: GRB 250108B: Swift-XRT refined Analysis DATE: 25/01/09 01:41:47 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

M. Capalbi (INAF-IASFPA), D.N. Burrows (PSU), M. A. Williams (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M. Perri (SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 8.5 ks of XRT data for GRB 250108B, from 97 s to 33.7 ks after the BAT trigger. The data comprise 608 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.

The late-time light curve (from T0+5.5 ks) can be modelled with a power-law decay with a decay index of alpha=0.35 (+/-0.13).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.37 (+/-0.04). The best-fitting absorption column is 1.47 (+/-0.09) x 10^21 cm^-2, in excess of the Galactic value of 1.2 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.07 (+0.17, -0.16) and a best-fitting absorption column of 1.3 (+0.5, -0.4) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.3 x 10^-11 (4.4 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 1.3 (+0.5, -0.4) x 10^21 cm^-2 Galactic foreground: 1.2 x 10^20 cm^-2 Excess significance: 4.8 sigma Photon index: 2.07 (+0.17, -0.16)

If the light curve continues to decay with a power-law decay index of 0.35, the count rate at T+24 hours will be 0.045 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.5 x 10^-12 (2.0 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/01280056.

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

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