[vsnet-grb-info 26376] GRB 200922A: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  28476
SUBJECT: GRB 200922A: Swift-XRT refined Analysis
DATE:    20/09/22 20:52:18 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

A. D'Ai (INAF-IASFPA), A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU),
D.N. Burrows (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester), P.A. Evans (U. Leicester), A. Melandri (INAF-OAB), T.
Sbarrato (INAF-OAB) and M.J. Moss report on behalf of the Swift-XRT
team:

We have analysed 6.8 ks of XRT data for GRB 200922A (Moss et al. GCN
Circ. 28471), from 81 s to 23.1 ks after the  BAT trigger. The data
comprise 123 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Osborne et al. (GCN Circ. 28473).

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.148 (+0.026, -0.025).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.33 (+0.18, -0.17). The
best-fitting absorption column is  1.6 (+0.5, -0.4) x 10^21 cm^-2, in
excess of the Galactic value of 4.7 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.18 (+0.24, -0.23)
and a best-fitting absorption column of 2.4 (+0.9, -0.8) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.3 x 10^-11 (5.0 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.4 (+0.9, -0.8) x 10^21 cm^-2
Galactic foreground: 4.7 x 10^20 cm^-2
Excess significance: 4.3 sigma
Photon index:	     2.18 (+0.24, -0.23)

If the light curve continues to decay with a power-law decay index of
1.148, the count rate at T+24 hours will be 4.8 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.6 x
10^-13 (2.4 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/00997024.

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