GCN Circular 42936

V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti (INAF-OAB), D.N. Burrows
(PSU), S. Lanava (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester),
K.L. Page (U. Leicester), C. Salvaggio (INAF-OAB) and P.A. Evans report
on behalf of the Swift-XRT team:

We have analysed 6.3 ks of XRT data for GRB 251201B, from 86 s to 23.3
ks after the   trigger. The data comprise 362 s in Windowed Timing (WT)
mode (the first 6 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+6.3 ks) can be modelled with a
power-law decay with a decay index of alpha=1.6 (+/-0.3).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.40 (+/-0.04). The
best-fitting absorption column is  2.30 (+/-0.12) x 10^21 cm^-2, in
excess of the Galactic value of 1.8 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.93 (+0.16, -0.15)
and a best-fitting absorption column of 10.0 (+4.5, -4.0) x 10^20
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.5 x 10^-11 (4.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     10.0 (+4.5, -4.0) x 10^20 cm^-2
Galactic foreground: 1.8 x 10^20 cm^-2
Excess significance: 3.4 sigma
Photon index:	     1.93 (+0.16, -0.15)

If the light curve continues to decay with a power-law decay index of
1.6, the count rate at T+24 hours will be 3.5 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-13 (1.5 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01418729.

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