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GCN Circular 28444

Subject
GRB 200917A: Swift-XRT refined Analysis
Date
2020-09-17T19:24:59Z (4 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), J.A. Kennea (PSU), A.
Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), A.
Melandri (INAF-OAB) and E. Sonbas report on behalf of the Swift-XRT
team:

We have analysed 7.1 ks of XRT data for GRB 200917A (Sonbas et al. GCN
Circ. 28435), from 112 s to 34.8 ks after the  BAT trigger. The data
are entirely in Photon Counting (PC) mode. The enhanced XRT position
for this burst was given by Goad et al. (GCN Circ. 28436).

The late-time light curve (from T0+5.1 ks) can be modelled with a
power-law decay with a decay index of alpha=1.31 (+0.17, -0.16).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.34 (+0.28, -0.26). The
best-fitting absorption column is  1.08 (+0.25, -0.22) x 10^22 cm^-2,
in excess of the Galactic value of 1.5 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 4.1 x 10^-11 (1.1 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.08 (+0.25, -0.22) x 10^22 cm^-2
Galactic foreground: 1.5 x 10^21 cm^-2
Excess significance: 7.0 sigma
Photon index:	     2.34 (+0.28, -0.26)

If the light curve continues to decay with a power-law decay index of
1.31, the count rate at T+24 hours will be 5.2 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.1 x
10^-13 (5.5 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/00996184.

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