GCN Circular 40354

J.A. Kennea (PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester),
J.P. Osborne (U. Leicester), A. Melandri (INAF-OAR), T. Sbarrato
(INAF-OAB), P. D'Avanzo (INAF-OAB), M. A. Williams (PSU) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 6.9 ks of XRT data for GRB 250504A, from 81 s to 56.7
ks after the  BAT trigger. The data comprise 47 s in Windowed Timing
(WT) mode 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=1.26 (+0.05, -0.04).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.60 (+0.24, -0.22). The
best-fitting absorption column is  2.9 (+1.4, -0.7) x 10^21 cm^-2,
consistent with the Galactic value of 2.1 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 4.8 x 10^-11 (6.0 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.9 (+1.4, -0.7) x 10^21 cm^-2
Galactic foreground: 2.1 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.60 (+0.24, -0.22)

If the light curve continues to decay with a power-law decay index of
1.26, the count rate at T+24 hours will be 1.7 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.3 x
10^-14 (1.0 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/01310284.

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