GCN Circular 40233

C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB),
J.A. Kennea (PSU), D.N. Burrows (PSU), M. A. Williams (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 6.9 ks of XRT data for GRB 250424A, from 259 s to 18.9
ks after the  BAT trigger. The data are entirely in Photon Counting
(PC) mode. 

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=0.20 (+0.12, -0.13). At T+2382 s  the
decay steepens to an alpha of 0.53 (+0.16, -0.11) before breaking again
at T+17.4 ks to a final decay with index alpha=4.5 (+3.5, -2.8).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.05 (+/-0.11). The
best-fitting absorption column is  1.04 (+0.11, -0.10) x 10^22 cm^-2,
in excess of the Galactic value of 6.4 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 4.9 x 10^-11 (9.6 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.04 (+0.11, -0.10) x 10^22 cm^-2
Galactic foreground: 6.4 x 10^20 cm^-2
Excess significance: 15.7 sigma
Photon index:	     2.05 (+/-0.11)

If the light curve continues to decay with a power-law decay index of
4.5, the count rate at T+24 hours will be 5.3 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.6 x
10^-14 (5.1 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01306404.

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