GCN Circular 39997

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

We have analysed 3.9 ks of XRT data for GRB 250331C, from 137 s to 62.7
ks after the  BAT trigger. The data comprise 95 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=2.81 (+0.11, -0.10).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.11 (+0.11, -0.08). The
best-fitting absorption column is  consistent with the Galactic value
of 6.5 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 1.65 (+0.32, -0.30) and a best-fitting absorption
column of 9.0 (+3.4, -2.5) x 10^21 cm^-2. The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 5.9 x 10^-11 (8.6 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9.0 (+3.4, -2.5) x 10^21 cm^-2
Galactic foreground: 6.5 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.65 (+0.32, -0.30)

If the light curve continues to decay with a power-law decay index of
2.81, the count rate at T+24 hours will be 3.2 x 10^-7 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.9 x
10^-17 (2.8 x 10^-17) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01299967.

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