[vsnet-grb-info 19799] GRB 170906A: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  21836
SUBJECT: GRB 170906A: Swift-XRT refined Analysis
DATE:    17/09/06 19:39:09 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

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

We have analysed 6.4 ks of XRT data for GRB 170906A (Siegel et al. GCN
Circ. 21821), from 71 s to 62.4 ks after the  BAT trigger. The data
comprise 683 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Beardmore et al. (GCN Circ. 21824).

The late-time light curve (from T0+10.2 ks) can be modelled with a
power-law decay with a decay index of alpha=2.08 (+0.27, -0.25).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.64 (+/-0.03). The
best-fitting absorption column is  2.60 (+0.16, -0.15) x 10^21 cm^-2,
in excess of the Galactic value of 1.4 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.23 (+0.24, -0.23)
and a best-fitting absorption column of 4.7 (+1.3, -1.1) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.5 x 10^-11 (6.6 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.7 (+1.3, -1.1) x 10^21 cm^-2
Galactic foreground: 1.4 x 10^21 cm^-2
Excess significance: 4.9 sigma
Photon index:	     2.23 (+0.24, -0.23)

If the light curve continues to decay with a power-law decay index of
2.08, the count rate at T+24 hours will be 2.8 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.0 x
10^-13 (1.9 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/00770957.

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