GCN Circular 39855

M. Perri (SSDC & INAF-OAR), J.A. Kennea (PSU), D.N. Burrows (PSU), M.
A. Williams (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U.
Leicester), V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti (INAF-OAB) and
P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 3.7 ks of XRT data for GRB 250322A, from 58 s to 22.2
ks after the  BAT trigger. The data comprise 7 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=2.28 (+0.18, -0.17).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.09 (+0.49, -0.26). The
best-fitting absorption column is  consistent with the Galactic value
of 2.7 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 3.6 x 10^-11 (5.4 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.7 (+/-2.3) x 10^21 cm^-2
Galactic foreground: 2.7 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.09 (+0.49, -0.26)

If the light curve continues to decay with a power-law decay index of
2.28, the count rate at T+24 hours will be 2.2 x 10^-7 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.9 x
10^-18 (1.2 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/01297832.

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