TITLE: GCN CIRCULAR NUMBER: 42362 SUBJECT: GRB 251018A: Swift-XRT refined Analysis DATE: 25/10/18 19:10:27 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

E. Ambrosi (INAF-IASFPA) , J.A. Kennea (PSU), D.N. Burrows (PSU), S. Lanava (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), M. Capalbi (INAF-OAR), M. Perri (SSDC & INAF-OAR) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 7.5 ks of XRT data for GRB 251018A, from 63 s to 50.2 ks after the trigger. The data comprise 259 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 light curve can be modelled with a power-law decay with a decay index of alpha=1.122 (+0.031, -0.029).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.68 (+0.15, -0.14). The best-fitting absorption column is 1.4 (+0.6, -0.5) x 10^21 cm^-2, consistent with the Galactic value of 9.4 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.81 (+0.25, -0.23) and a best-fitting absorption column of 2.1 (+1.1, -0.9) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.0 x 10^-11 (5.2 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 2.1 (+1.1, -0.9) x 10^21 cm^-2 Galactic foreground: 9.4 x 10^20 cm^-2 Excess significance: 2.0 sigma Photon index: 1.81 (+0.25, -0.23)

If the light curve continues to decay with a power-law decay index of 1.122, the count rate at T+24 hours will be 6.4 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.6 x 10^-13 (3.3 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/xrt_products/01405083.

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

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