GCN Circular 22260

P.A. Evans (U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A.
D'Ai (INAF-IASFPA), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU),
D.N. Burrows (PSU), S.L. Gibson (U. Leicester), J.P. Osborne (U.
Leicester) and F.E. Marshall report on behalf of the Swift-XRT team:

We have analysed 8.0 ks of XRT data for GRB 171216A (Marshall et al.
GCN Circ. 22256
Not found
 
 
), from 134 s to 64.4 ks after the  BAT trigger. The
data are entirely in Photon Counting (PC) mode. The enhanced XRT
position for this burst was given by Goad et al. (GCN Circ. 22257
Not found
 
 
).

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.9 (+0.6, -0.3), followed by a break at T+456 s to an
alpha of 0.86 (+0.10, -0.09).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.76 (+0.16, -0.09). The
best-fitting absorption column is  consistent with the Galactic value
of 2.8 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 4.2 x 10^-11 (5.3 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.8 (+/-0.3) x 10^21 cm^-2
Galactic foreground: 2.8 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.76 (+0.16, -0.09)

If the light curve continues to decay with a power-law decay index of
0.86, the count rate at T+24 hours will be 4.0 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x
10^-13 (2.1 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/00797441.

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