GCN Circular 30668

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
Not found
 
 
), 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
Not found
 
 
).

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.