TITLE:   GCN CIRCULAR
NUMBER:  29876
SUBJECT: GRB 210420B: Swift-XRT refined Analysis
DATE:    21/04/21 22:44:06 GMT
FROM:    Phil Evans at U of Leicester  <pae9@leicester.ac.uk>

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

We have analysed 4.8 ks of XRT data for GRB 210420B (Moss et al. GCN
Circ. 29844), from 141 s to 87.5 ks after the  BAT trigger. The data
comprise 177 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The refined XRT position is RA, Dec =
254.3227, +42.5696 which is equivalent to:

RA (J2000): 16 57 17.45
Dec(J2000): +42 34 10.6

with an uncertainty of 3.5 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.23 (+0.11, -0.12), followed by a break at T+425 s to
an alpha of 1.13 (+/-0.07).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.72 (+0.08, -0.07). The
best-fitting absorption column is  9 (+11, -9) x 10^20 cm^-2, at a
redshift of 1.4, in addition to the Galactic value of 2.1 x 10^20 cm^-2
(Willingale et al. 2013). The PC mode spectrum has a photon index of
1.61 (+0.08, -0.07) 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 4.0 x 10^-11 (4.2
x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 2.1 x 10^20 cm^-2
Intrinsic column:    2.1 (+6.0, -0.0) x 10^20 cm^-2 at z=1.4
Photon index:	     1.61 (+0.08, -0.07)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01044382.

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