[vsnet-grb-info 20057] GRB 171123A: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22152
SUBJECT: GRB 171123A: Swift-XRT refined Analysis
DATE:    17/11/23 22:04:20 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), J.A. Kennea (PSU), B.
Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), S.L. Gibson (U.
Leicester), J.P. Osborne (U. Leicester), V. D'Elia (ASDC) and P.A.
Evans report on behalf of the Swift-XRT team:

We have analysed 6.5 ks of XRT data for GRB 171123A (Evans et al. GCN
Circ. 22148), from 98 s to 19.7 ks after the  BAT trigger. The data are
entirely in Photon Counting (PC) mode. The enhanced XRT position for
this burst was given by Goad et al. (GCN Circ. 22150).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.4 (+0.7, -0.6). At T+306 s  the decay
flattens to an alpha of -0.33 (+0.25, -0.45) before breaking again at
T+1892 s to a final decay with index alpha=0.68 (+0.13, -0.14).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.21 (+0.22, -0.21). The
best-fitting absorption column is  8.8 (+5.1, -4.4) x 10^20 cm^-2, in
excess of the Galactic value of 1.9 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 2.9 x 10^-11 (3.8 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     8.8 (+5.1, -4.4) x 10^20 cm^-2
Galactic foreground: 1.9 x 10^20 cm^-2
Excess significance: 2.6 sigma
Photon index:	     2.21 (+0.22, -0.21)

If the light curve continues to decay with a power-law decay index of
0.68, the count rate at T+24 hours will be 0.010 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.0 x
10^-13 (3.8 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/00791922.

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