GCN Circular 40745

D.N. Burrows (PSU), M.A. Williams (PSU), S. Dichiara (PSU), J.P.
Osborne (U. Leicester), K.L. Page (U. Leicester), M.G. Bernardini
(INAF-OAB), E. Ambrosi	(INAF-IASFPA) , M. Capalbi (INAF-OAR) and P.A.
Evans report on behalf of the Swift-XRT team:

We have analysed 6.2 ks of XRT data for GRB 250615A, from 93 s to 40.3
ks after the   trigger. The data comprise 106 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=1.62 (+0.07, -0.06).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.62 (+0.27, -0.26). The
best-fitting absorption column is  2.4 (+0.6, -0.5) x 10^22 cm^-2, in
excess of the Galactic value of 5.5 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.0 (+0.4, -0.3) and
a best-fitting absorption column of 2.4 (+0.7, -0.6) x 10^22 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 6.7 x 10^-11 (1.5 x 10^-10) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.4 (+0.7, -0.6) x 10^22 cm^-2
Galactic foreground: 5.5 x 10^21 cm^-2
Excess significance: 5.3 sigma
Photon index:	     2.0 (+0.4, -0.3)

If the light curve continues to decay with a power-law decay index of
1.62, the count rate at T+24 hours will be 2.2 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.5 x
10^-14 (3.3 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/01324646.

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