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GCN Circular 24749

Subject
GRB 190604B: Swift-XRT refined Analysis
Date
2019-06-05T02:39:36Z (5 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU),
A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), J.P. Osborne
(U. Leicester), M.G. Bernadini (INAF-OAB), E. Ambrosi  (INAF-IASFPA) ,
V. D'Elia (ASDC) and K.L. Page report on behalf of the Swift-XRT team:

We have analysed 7.7 ks of XRT data for GRB 190604B (Page et al. GCN
Circ. 24742), from 120 s to 34.4 ks after the  BAT trigger. The data
comprise 285 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Evans et al. (GCN Circ. 24743).

The late-time light curve (from T0+4.9 ks) can be modelled with a
power-law decay with a decay index of alpha=0.94 (+/-0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.40 (+/-0.03). The
best-fitting absorption column is  1.49 (+0.16, -0.13) x 10^21 cm^-2,
consistent with the Galactic value of 1.4 x 10^21 cm^-2 (Willingale et
al. 2013). The PC mode spectrum has a photon index of 2.02 (+/-0.12)
and a best-fitting absorption column of 3.0 (+/-0.5) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.6 x 10^-11 (5.4 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.0 (+/-0.5) x 10^21 cm^-2
Galactic foreground: 1.4 x 10^21 cm^-2
Excess significance: 5.7 sigma
Photon index:	     2.02 (+/-0.12)

If the light curve continues to decay with a power-law decay index of
0.94, the count rate at T+24 hours will be 0.057 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.1 x
10^-12 (3.1 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00906654.

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