GCN Circular 22183

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

We have analysed 6.6 ks of XRT data for GRB 171205A (D'Elia et al. GCN
Circ. 22177
Not found
 
 
), from 134 s to 22.5 ks after the  BAT trigger. The data
comprise 238 s in Windowed Timing (WT) mode (the first 9 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Osborne et
al. (GCN Circ. 22179
Not found
 
 
).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=1.84 (+0.10, -0.18). At T+289 s  the decay
steepens to an alpha of 2.43 (+0.09, -0.08) before breaking again at
T+5714 s to a final decay with index alpha=-0.16 (+0.28, -0.69).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.717 (+0.035, -0.024). The
best-fitting absorption column is  consistent with the Galactic value
of 5.9 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 3.7 x 10^-11 (4.1 x 10^-11) erg cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Galactic foreground: 5.9 x 10^20 cm^-2
Intrinsic column:    5.89 (+/-0.22) x 10^20 cm^-2 at z=0.0368
Photon index:	     1.717 (+0.035, -0.024)

If the light curve continues to decay with a power-law decay index of
-0.16, the count rate at T+24 hours will be 0.025 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.1 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/00794972.

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