[vsnet-grb-info 28565] GRB 210820A: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30668
SUBJECT: GRB 210820A: Swift-XRT refined Analysis
DATE:    21/08/21 02:38:02 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

J.P. Osborne (U. Leicester), M. Capalbi (INAF-IASFPA), M. Perri (SSDC &
INAF-OAR), V. D'Elia (SSDC & INAF-OAR), J.A. Kennea (PSU), A.
Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), K.L. Page (U. Leicester)
and A.P. Beardmore report on behalf of the Swift-XRT team:

We have analysed 6.5 ks of XRT data for GRB 210820A (Beardmore et al.
GCN Circ. 30664), from 131 s to 18.2 ks after the  BAT trigger. The
data comprise 339 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. 30667).

The late-time light curve (from T0+5.9 ks) can be modelled with a
power-law decay with a decay index of alpha=1.9 (+0.6, -0.5).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.77 (+/-0.03). The
best-fitting absorption column is  1.20 (+/-0.11) x 10^21 cm^-2, in
excess of the Galactic value of 3.6 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.28 (+0.25, -0.13)
and a best-fitting absorption column consistent with the Galactic
value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 2.9 x 10^-11 (3.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.6 (+4.3, -0.0) x 10^20 cm^-2
Galactic foreground: 3.6 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.28 (+0.25, -0.13)

If the light curve continues to decay with a power-law decay index of
1.9, the count rate at T+24 hours will be 2.9 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.4 x
10^-15 (9.6 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/01069551.

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