GCN Circular 40503

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.