GCN Circular 39996

P. D'Avanzo (INAF-OAB), 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)  and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 5.5 ks of XRT data for GRB 250331B, from 321 s to 35.1
ks after the  BAT trigger. The data are entirely in Photon Counting
(PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.11 (+0.17, -0.14).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.2 (+0.9, -0.8). The
best-fitting absorption column is  2.1 (+1.5, -1.1) x 10^22 cm^-2, in
excess of the Galactic value of 6.6 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 5.7 x 10^-11 (1.6 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.1 (+1.5, -1.1) x 10^22 cm^-2
Galactic foreground: 6.6 x 10^21 cm^-2
Excess significance: 2.2 sigma
Photon index:	     2.2 (+0.9, -0.8)

If the light curve continues to decay with a power-law decay index of
1.11, the count rate at T+24 hours will be 7.9 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.5 x
10^-14 (1.2 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/01299965.

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