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GCN Circular 25568

Subject
GRB 190829A: Swift-XRT refined Analysis
Date
2019-08-30T09:06:05Z (5 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M. Capalbi
(INAF-IASFPA), M. Perri (ASDC), V. D'Elia (ASDC), D.N. Burrows (PSU),
J. D. Gropp (PSU), A. Tohuvavohu (PSU), A.P. Beardmore (U. Leicester)
and S. Dichiara report on behalf of the Swift-XRT team:

We have analysed 10 ks of XRT data for GRB 190829A (Dichiara et al. GCN
Circ. 25552), from 103 s to 39.7 ks after the  BAT trigger. The data
comprise 697 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. 25567).

The late-time light curve (from T0+4.8 ks) can be modelled with an
initial power-law decay with an index of alpha=2.6 (+0.7, -1.1),
followed by a break at T+144 s to an alpha of 0.33 (+0.09, -0.11).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.17 (+0.20, -0.19). The
best-fitting absorption column is  7.4 (+1.7, -1.5) x 10^21 cm^-2, in
excess of the Galactic value of 5.6 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.46 (+0.21, -0.20)
and a best-fitting absorption column of 1.53 (+0.23, -0.21) x 10^22
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 4.5 x 10^-11 (1.5 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.53 (+0.23, -0.21) x 10^22 cm^-2
Galactic foreground: 5.6 x 10^20 cm^-2
Excess significance: 11.5 sigma
Photon index:	     2.46 (+0.21, -0.20)

If the light curve continues to decay with a power-law decay index of
0.33, the count rate at T+24 hours will be 0.95 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.2 x
10^-11 (1.4 x 10^-10) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00922968.

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