TITLE:   GCN CIRCULAR
NUMBER:  20891
SUBJECT: GRB 170317A: Swift-XRT refined Analysis
DATE:    17/03/17 15:46:59 GMT
FROM:    Phil Evans at U of Leicester  <pae9@leicester.ac.uk>

A. Cholden-Brown (PSU), S. J. LaPorte (PSU), J.A. Kennea (PSU), B.
Mingo (U. Leicester), A.P. Beardmore (U. Leicester), P.A. Evans (U.
Leicester), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia
(ASDC) and  report on behalf of the Swift-XRT team: report on behalf of
the Swift-XRT team:

We have analysed 6.9 ks of XRT data for GRB 170317A, from 68 s to 13.5
ks after the  BAT trigger. The data comprise 8 s in Windowed Timing
(WT) mode (taken while Swift was slewing), with the remainder in Photon
Counting (PC) mode. 

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.8 (+4.2, -1.4). At T+105 s  the decay
flattens to an alpha of 0.58 (+0.07, -0.09) before breaking again at
T+3006 s to a final decay with index alpha=1.4 (+0.4, -0.3).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.05 (+0.10, -0.07). The
best-fitting absorption column is  consistent with the Galactic value
of 2.5 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 3.5 x 10^-11 (5.1 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.5 (+/-0.3) x 10^21 cm^-2
Galactic foreground: 2.5 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.05 (+0.10, -0.07)

If the light curve continues to decay with a power-law decay index of
1.4, the count rate at T+24 hours will be 2.2 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.9 x
10^-14 (1.1 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/00742866.

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