TITLE: GCN CIRCULAR NUMBER: 40857 SUBJECT: GRB 250628A: Swift-XRT refined Analysis DATE: 25/06/28 10:41:22 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), M. Perri (SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), M.A. Williams (PSU) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 8.2 ks of XRT data for GRB 250628A, from 127 s to 27.6 ks after the trigger. The data comprise 47 s in Windowed Timing (WT) mode 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=5.0 (+0.6, -0.5). At T+358 s the decay flattens to an alpha of -0.01 (+0.11, -0.13) before breaking again at T+14.4 ks to a final decay with index alpha=2.6 (+/-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 (+0.17, -0.16). The best-fitting absorption column is 3.2 (+0.8, -0.7) x 10^21 cm^-2, in excess of the Galactic value of 2.3 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 4.0 x 10^-11 (5.8 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 3.2 (+0.8, -0.7) x 10^21 cm^-2 Galactic foreground: 2.3 x 10^20 cm^-2 Excess significance: 6.8 sigma Photon index: 1.92 (+0.17, -0.16)

If the light curve continues to decay with a power-law decay index of 2.6, the count rate at T+24 hours will be 1.6 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.5 x 10^-14 (9.3 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01328546.

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

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