GCN Circular 41974

S. Campana (INAF-OAB), A. Melandri (INAF-OAR), D.N. Burrows (PSU), S.
Lanava (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page
(U. Leicester), T. Sbarrato (INAF-OAB) and P.A. Evans report on behalf
of the Swift-XRT team:

We have analysed 8.3 ks of XRT data for GRB 250924A, from 181 s to 34.9
ks after the   trigger. The data comprise 80 s in Windowed Timing (WT)
mode (the first 1 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The light curve can be modelled with an initial power-law decay with an
index of alpha=5.2 (+/-0.5), followed by a break at T+301 s to an alpha
of 1.05 (+/-0.05).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 4.0 (+0.4, -0.3). The
best-fitting absorption column is  5.1 (+/-0.8) x 10^21 cm^-2, in
excess of the Galactic value of 2.8 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.06 (+0.20, -0.13)
and a best-fitting absorption column consistent with the Galactic
value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.6 x 10^-11 (5.4 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.8 (+0.7, -0.0) x 10^21 cm^-2
Galactic foreground: 2.8 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.06 (+0.20, -0.13)

If the light curve continues to decay with a power-law decay index of
1.05, the count rate at T+24 hours will be 8.0 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.9 x
10^-13 (4.4 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/01351790.

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