GCN Circular 39697

D.N. Burrows (PSU), M. A. Williams (PSU), J.P. Osborne (U. Leicester),
K.L. Page (U. Leicester), A. D'Ai (INAF-IASFPA), A. Melandri
(INAF-OAR), T. Sbarrato (INAF-OAB), S. Dichiara (PSU) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 8.3 ks of XRT data for GRB 250312A, from 385 s to 34.7
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=0.51 (+/-0.06).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.80 (+0.22, -0.21). The
best-fitting absorption column is  1.3 (+0.7, -0.6) x 10^21 cm^-2, in
excess of the Galactic value of 1.8 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.9 x 10^-11 (4.7 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.3 (+0.7, -0.6) x 10^21 cm^-2
Galactic foreground: 1.8 x 10^20 cm^-2
Excess significance: 3.0 sigma
Photon index:	     1.80 (+0.22, -0.21)

If the light curve continues to decay with a power-law decay index of
0.51, the count rate at T+24 hours will be 0.022 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.5 x
10^-13 (1.0 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01294765.

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