[vsnet-grb-info 28557] GRB 210818A: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30660
SUBJECT: GRB 210818A: Swift-XRT refined Analysis
DATE:    21/08/18 22:39:25 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

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

We have analysed 7.1 ks of XRT data for GRB 210818A (Beardmore et al.
GCN Circ. 30652), from 65 s to 69.5 ks after the  BAT trigger. The data
comprise 188 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. 30653).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=1.04 (+/-0.20). At T+111 s  the decay
steepens to an alpha of 6.8 (+0.7, -0.6). The light curve breaks again
at T+157 s to a decay with alpha=0.67 (+0.38, -0.29),  before a final
break at T+479 s s after which the decay index is 1.50 (+/-0.06).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 0.86 (+/-0.06). The
best-fitting absorption column is  2.5 (+0.5, -0.4) x 10^21 cm^-2, in
excess of the Galactic value of 1.7 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.64 (+0.14, -0.11)
and a best-fitting absorption column of 1.94 (+0.56, -0.20) x 10^21
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 4.4 x 10^-11 (5.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.94 (+0.56, -0.20) x 10^21 cm^-2
Galactic foreground: 1.7 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.64 (+0.14, -0.11)

If the light curve continues to decay with a power-law decay index of
1.50, the count rate at T+24 hours will be 3.1 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.3 x
10^-13 (1.6 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/01069169.

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