TITLE: GCN CIRCULAR NUMBER: 39414 SUBJECT: GRB 250221A: Swift-XRT refined Analysis DATE: 25/02/22 00:09:17 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

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

We have analysed 7.0 ks of XRT data for GRB 250221A, from 107 s to 66.9 ks after the BAT trigger. The data comprise 9 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 power-law decay with a decay index of alpha=1.06 (+/-0.06).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.09 (+0.24, -0.23). The best-fitting absorption column is 1.4 (+0.7, -0.6) x 10^21 cm^-2, in excess of the Galactic value of 3.7 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 3.3 x 10^-11 (4.4 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 1.4 (+0.7, -0.6) x 10^21 cm^-2 Galactic foreground: 3.7 x 10^20 cm^-2 Excess significance: 2.7 sigma Photon index: 2.09 (+0.24, -0.23)

If the light curve continues to decay with a power-law decay index of 1.06, the count rate at T+24 hours will be 2.5 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.4 x 10^-14 (1.1 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/01290305.

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

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