TITLE:   GCN CIRCULAR
NUMBER:  33667
SUBJECT: GRB 230420A: Swift-XRT refined Analysis
DATE:    23/04/21 12:21:34 GMT
FROM:    Phil Evans at U of Leicester <pae9@star.le.ac.uk>

E. Ambrosi  (INAF-IASFPA) , S. Dichiara (PSU), J.A. Kennea (PSU), A.
Tohuvavohu (U. Toronto), A.P. Beardmore (U. Leicester), P.A. Evans (U.
Leicester), J.P. Osborne (U. Leicester), M. Capalbi (INAF-IASFPA), M.
Perri (SSDC & INAF-OAR) and  report on behalf of the Swift-XRT team:

We have analysed 7.2 ks of XRT data for GRB 230420A, from 98 s to 84.8
ks after the  BAT trigger. The data comprise 102 s in Windowed Timing
(WT) mode with the remainder in Photon Counting (PC) mode. 

The light curve can be modelled with an initial power-law decay with an
index of alpha=4.13 (+0.26, -0.18), followed by a break at T+316 s to
an alpha of 0.83 (+0.10, -0.07).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.59 (+/-0.09). The
best-fitting absorption column is  4.6 (+2.5, -2.3) x 10^20 cm^-2, in
excess of the Galactic value of 1.7 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.82 (+0.21, -0.11)
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 3.5 x 10^-11 (3.6 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.7 (+4.0, -0.0) x 10^20 cm^-2
Galactic foreground: 1.7 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.82 (+0.21, -0.11)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01164980.

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