GCN Circular 41935

A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB), A. Melandri (INAF-OAR),
J.A. Kennea (PSU), D.N. Burrows (PSU), S. Lanava (PSU), A.P. Beardmore
(U. Leicester), J.P. Osborne (U. Leicester) and P.A. Evans report on
behalf of the Swift-XRT team:

We have analysed 5.1 ks of XRT data for GRB 250920C, from 110 s to
131.0 ks after the   trigger. The data comprise 64 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 late-time light curve (from T0+7.1 ks) can be modelled with a
power-law decay with a decay index of alpha=1.53 (+/-0.09).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.94 (+/-0.10). The
best-fitting absorption column is  4.8 (+2.1, -2.0) x 10^20 cm^-2, in
excess of the Galactic value of 1.4 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.89 (+0.16, -0.15)
and a best-fitting absorption column of 6.7 (+4.0, -3.5) x 10^20 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.6 x 10^-11 (4.1 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.7 (+4.0, -3.5) x 10^20 cm^-2
Galactic foreground: 1.4 x 10^20 cm^-2
Excess significance: 2.5 sigma
Photon index:	     1.89 (+0.16, -0.15)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01351039.

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