GCN Circular 21647
Subject
GRB 170822A: Swift-XRT refined Analysis
Date
2017-08-22T17:25:14Z (7 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
P.A. Evans (U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC),
J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU),
S.L. Gibson (U. Leicester), J.P. Osborne (U. Leicester) and A. Melandri
report on behalf of the Swift-XRT team:
We have analysed 8.8 ks of XRT data for GRB 170822A (Melandri et al.
GCN Circ. 21640), from 153 s to 19.2 ks after the BAT trigger. The
data comprise 32 s in Windowed Timing (WT) mode (the first 7 s were
taken while Swift was slewing) with the remainder in Photon Counting
(PC) mode. The enhanced XRT position for this burst was given by
Beardmore et al. (GCN Circ. 21643).
The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=1.29 (+0.27, -0.23). At T+522 s the decay
flattens to an alpha of -0.77 (+0.28, -0.35). The light curve breaks
again at T+1393 s to a decay with alpha=0.81 (+0.09, -0.11), before a
final break at T+6591 s s after which the decay index is 1.89 (+0.37,
-0.14).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.84 (+/-0.08). The
best-fitting absorption column is 3.5 (+/-0.4) x 10^21 cm^-2, in
excess of the Galactic value of 1.9 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 4.2 x 10^-11 (5.9 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 3.5 (+/-0.4) x 10^21 cm^-2
Galactic foreground: 1.9 x 10^21 cm^-2
Excess significance: 6.6 sigma
Photon index: 1.84 (+/-0.08)
If the light curve continues to decay with a power-law decay index of
1.89, the count rate at T+24 hours will be 6.9 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.9 x
10^-13 (4.1 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/00768939.
This circular is an official product of the Swift-XRT team.