GCN Circular 40618

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

We have analysed 7.3 ks of XRT data for GRB 250603A, from 92 s to 34.7
ks after the   trigger. The data comprise 3 s in Windowed Timing (WT)
mode (taken while Swift was slewing), 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=1.17 (+0.09, -0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.48 (+0.32, -0.30). The
best-fitting absorption column is  1.6 (+0.5, -0.4) x 10^22 cm^-2, in
excess of the Galactic value of 6.9 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 7.6 x 10^-11 (1.1 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.6 (+0.5, -0.4) x 10^22 cm^-2
Galactic foreground: 6.9 x 10^21 cm^-2
Excess significance: 3.3 sigma
Photon index:	     1.48 (+0.32, -0.30)

If the light curve continues to decay with a power-law decay index of
1.17, the count rate at T+24 hours will be 8.8 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.7 x
10^-14 (9.8 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/01320335.

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