GCN Circular 22137

K.L. Page (U. Leicester), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), A.
Melandri (INAF-OAB), A. Tohuvavohu (PSU), S. J. LaPorte (PSU), J.A.
Kennea (PSU), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester)
and  report on behalf of the Swift-XRT team: report on behalf of the
Swift-XRT team:

We have analysed 8.2 ks of XRT data for GRB 171120A, from 78 s to 22.5
ks after the  BAT trigger. The data comprise 337 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 late-time light curve (from T0+3.9 ks) can be modelled with a
power-law decay with a decay index of alpha=0.41 (+/-0.09).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.78 (+0.15, -0.14). The
best-fitting absorption column is  2.34 (+0.28, -0.26) x 10^22 cm^-2,
in excess of the Galactic value of 1.2 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.92 (+/-0.18) and a
best-fitting absorption column of 2.4 (+/-0.3) x 10^22 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 6.6 x 10^-11 (1.4 x 10^-10) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.4 (+/-0.3) x 10^22 cm^-2
Galactic foreground: 1.2 x 10^20 cm^-2
Excess significance: 12.8 sigma
Photon index:	     1.92 (+/-0.18)

If the light curve continues to decay with a power-law decay index of
0.41, the count rate at T+24 hours will be 0.059 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.9 x
10^-12 (8.3 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/00791201.

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