TITLE: GCN CIRCULAR NUMBER: 41974 SUBJECT: GRB 250924A: Swift-XRT refined Analysis DATE: 25/09/24 21:44:35 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

S. Campana (INAF-OAB), A. Melandri (INAF-OAR), D.N. Burrows (PSU), S. Lanava (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), T. Sbarrato (INAF-OAB) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 8.3 ks of XRT data for GRB 250924A, from 181 s to 34.9 ks after the trigger. The data comprise 80 s in Windowed Timing (WT) mode (the first 1 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.

The light curve can be modelled with an initial power-law decay with an index of alpha=5.2 (+/-0.5), followed by a break at T+301 s to an alpha of 1.05 (+/-0.05).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 4.0 (+0.4, -0.3). The best-fitting absorption column is 5.1 (+/-0.8) x 10^21 cm^-2, in excess of the Galactic value of 2.8 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.06 (+0.20, -0.13) and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (5.4 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 2.8 (+0.7, -0.0) x 10^21 cm^-2 Galactic foreground: 2.8 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 2.06 (+0.20, -0.13)

If the light curve continues to decay with a power-law decay index of 1.05, the count rate at T+24 hours will be 8.0 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.9 x 10^-13 (4.4 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/01351790.

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

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