GCN Circular 20852

A. D'Ai (INAF-IASFPA), D.N. Burrows (PSU), A. Cholden-Brown (PSU), S.
J. LaPorte (PSU), K.L. Page (U. Leicester), B. Mingo (U. Leicester),
A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAB), P. D'Avanzo
(INAF-OAB) and	report on behalf of the Swift-XRT team: report on
behalf of the Swift-XRT team:

We have analysed 6.8 ks of XRT data for GRB 170311A, from 108 s to 35.6
ks after the  BAT trigger. The data comprise 64 s in Windowed Timing
(WT) mode with the remainder in Photon Counting (PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.59 (+0.16, -0.17).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.52 (+0.28, -0.26). The
best-fitting absorption column is  2.2 (+1.5, -0.9) x 10^21 cm^-2,
consistent with the Galactic value of 1.3 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 4.8 x 10^-11 (5.6 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.2 (+1.5, -0.9) x 10^21 cm^-2
Galactic foreground: 1.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.52 (+0.28, -0.26)

If the light curve continues to decay with a power-law decay index of
1.59, the count rate at T+24 hours will be 6.1 x 10^-5 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.9 x
10^-15 (3.4 x 10^-15) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00741965.

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