TITLE: GCN CIRCULAR NUMBER: 39089 SUBJECT: GRB 250129A: Swift-XRT refined Analysis DATE: 25/01/29 22:15:26 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

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

We have analysed 10 ks of XRT data for GRB 250129A, from 147 s to 46.4 ks after the BAT trigger. The data comprise 10 s in Windowed Timing (WT) mode (taken while Swift was slewing), with the remainder in Photon Counting (PC) mode.

The light curve can be modelled with a series of power-law decays. The initial decay index is alpha=3.64 (+1.35, -0.10). At T+1185 s the decay flattens to an alpha of 0.15 (+/-0.07) before breaking again at T+23.0 ks to a final decay with index alpha=2.2 (+0.7, -0.6).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.92 (+0.16, -0.10). The best-fitting absorption column is consistent with the Galactic value of 2.4 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.4 x 10^-11 (3.6 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Galactic foreground: 2.4 x 10^20 cm^-2 Intrinsic column: 2 (+/-38) x 10^20 cm^-2 at z=2.151 Photon index: 1.92 (+0.16, -0.10)

If the light curve continues to decay with a power-law decay index of 2.2, the count rate at T+24 hours will be 0.011 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.8 x 10^-13 (4.0 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/01285812.

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

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