TITLE: GCN CIRCULAR NUMBER: 40503 SUBJECT: GRB 250520A: Swift-XRT refined Analysis DATE: 25/05/20 14:29:54 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

M. Perri (SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR), 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), B. Sbarufatti (INAF-OAB) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.7 ks of XRT data for GRB 250520A, from 127 s to 34.5 ks after the trigger. The data comprise 16 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=2.35 (+0.23, -0.22).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.1 (+/-0.4). The best-fitting absorption column is 1.3 (+0.5, -0.4) x 10^22 cm^-2, in excess of the Galactic value of 3.7 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.0 x 10^-11 (1.1 x 10^-10) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus: Total column: 1.3 (+0.5, -0.4) x 10^22 cm^-2 Galactic foreground: 3.7 x 10^21 cm^-2 Excess significance: 3.6 sigma Photon index: 2.1 (+/-0.4)

If the light curve continues to decay with a power-law decay index of 2.35, the count rate at T+24 hours will be 10.0 x 10^-7 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.0 x 10^-17 (1.1 x 10^-16) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01315630.

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

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