GCN Circular 38773

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

We have analysed 6.8 ks of XRT data for GRB 250101A, from 110 s to 39.2
ks after the  BAT trigger. The data comprise 10 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 series of power-law decays. The
initial decay index is alpha=2.8 (+/-0.7). At T+249 s  the decay
flattens to an alpha of 0.06 (+0.29, -0.74) before breaking again at
T+909 s to a final decay with index alpha=0.79 (+0.13, -0.09).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.02 (+0.18, -0.17). The
best-fitting absorption column is  1.7 (+1.3, -1.1) x 10^22 cm^-2, at a
redshift of 2.49, in addition to the Galactic value of 1.4 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 (5.1 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.4 x 10^21 cm^-2
Intrinsic column:    1.7 (+1.3, -1.1) x 10^22 cm^-2 at z=2.49
Photon index:	     2.02 (+0.18, -0.17)

If the light curve continues to decay with a power-law decay index of
0.79, the count rate at T+24 hours will be 0.018 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.4 x
10^-13 (9.1 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/01278305.

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