[vsnet-grb-info 23858] GRB 191004A: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25957
SUBJECT: GRB 191004A: Swift-XRT refined Analysis
DATE:    19/10/05 18:31:05 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

J. D. Gropp (PSU), A. Tohuvavohu (U. Toronto), A.P. Beardmore (U.
Leicester), P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester),
M.G. Bernardini (INAF-OAB), E. Ambrosi	(INAF-IASFPA) , M. Capalbi
(INAF-IASFPA), J.A. Kennea (PSU) and V. Lipunov report on behalf of the
Swift-XRT team: report on behalf of the Swift-XRT team:

We have analysed 4.8 ks of XRT data for GRB 191004A (Lipunov et al. GCN
Circ. 25944), from 86 s to 12.4 ks after the  BAT trigger. The data
comprise 88 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. 25950).

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.74 (+0.07, -0.08), followed by a break at T+883 s to
an alpha of 1.36 (+0.34, -0.10).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.04 (+0.21, -0.13). The
best-fitting absorption column is  2.0 (+3.8, -0.5) x 10^20 cm^-2,
consistent with the Galactic value of 1.5 x 10^20 cm^-2 (Willingale et
al. 2013). The PC mode spectrum has a photon index of 1.81 (+0.14,
-0.13) and a best-fitting absorption column of 6.0 (+3.4, -3.1) x 10^20
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.6 x 10^-11 (4.0 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.0 (+3.4, -3.1) x 10^20 cm^-2
Galactic foreground: 1.5 x 10^20 cm^-2
Excess significance: 2.4 sigma
Photon index:	     1.81 (+0.14, -0.13)

If the light curve continues to decay with a power-law decay index of
1.36, the count rate at T+24 hours will be 2.9 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-13 (1.2 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/00927825.

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