GCN Circular 29108

A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), E. Ambrosi 
(INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR),
D.N. Burrows (PSU), J. D. Gropp (PSU), J.A. Kennea (PSU), J.P. Osborne
(U. Leicester) and K.L. Page report on behalf of the Swift-XRT team:

We have analysed 6.2 ks of XRT data for GRB 201221A (Page et al. GCN
Circ. 29096
Not found
 
 
), from 143 s to 23.8 ks after the  BAT trigger. The data
comprise 320 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Beardmore et al. (GCN Circ. 29097
Not found
 
 
).

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.71 (+/-0.04).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.39 (+0.16, -0.15). The
best-fitting absorption column is  6.3 (+8.3, -6.3) x 10^22 cm^-2, at a
redshift of 5.7, in addition to the Galactic value of 9.3 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 5.0 x 10^-11
(4.7 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 9.3 x 10^20 cm^-2
Intrinsic column:    6.3 (+8.3, -6.3) x 10^22 cm^-2 at z=5.7
Photon index:	     1.39 (+0.16, -0.15)

If the light curve continues to decay with a power-law decay index of
0.71, the count rate at T+24 hours will be 0.016 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.0 x
10^-13 (7.4 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/01013852.

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