GCN Circular 41909

K.L. Page (U. Leicester), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA),
S. Campana (INAF-OAB), S. Dichiara (PSU), J.A. Kennea (PSU), D.N.
Burrows (PSU), A.P. Beardmore (U. Leicester) and P.A. Evans report on
behalf of the Swift-XRT team:

We have analysed 6.4 ks of XRT data for GRB 250920B, from 87 s to 22.3
ks after the   trigger. The data comprise 395 s in Windowed Timing (WT)
mode (the first 2 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. The refined XRT position is RA,
Dec = 187.3175, +47.8376 which is equivalent to:

RA (J2000): 12h 29m 16.21s
Dec(J2000): +47d 50' 15.2"

with an uncertainty of 3.5 arcsec (radius, 90% confidence).

The late-time light curve (from T0+3.8 ks) can be modelled with a
power-law decay with a decay index of alpha=1.20 (+/-0.05).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.490 (+0.035, -0.017). The
best-fitting absorption column is  consistent with the Galactic value
of 1.1 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 1.89 (+/-0.07) and a best-fitting absorption column
of 3.3 (+1.5, -1.4) x 10^20 cm^-2. The counts to observed (unabsorbed)
0.3-10 keV flux conversion factor deduced from this spectrum  is 3.5 x
10^-11 (3.7 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.3 (+1.5, -1.4) x 10^20 cm^-2
Galactic foreground: 1.1 x 10^20 cm^-2
Excess significance: 2.5 sigma
Photon index:	     1.89 (+/-0.07)

If the light curve continues to decay with a power-law decay index of
1.20, the count rate at T+24 hours will be 0.10 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.5 x
10^-12 (3.8 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01351008.

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