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GCN Circular 21414

GRB 170804A: Swift-XRT refined Analysis
2017-08-04T20:01:28Z (7 years ago)
Phil Evans at U of Leicester <>
D.N. Burrows (PSU), K.L. Page (U. Leicester), B. Mingo (U. Leicester),
A.P. Beardmore (U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia
(ASDC), A. D'Ai (INAF-IASFPA), S. J. LaPorte (PSU) and A. Cholden-Brown
report on behalf of the Swift-XRT team:

We have analysed 6.5 ks of XRT data for GRB 170804A (Cholden-Brown et
al. GCN Circ. 21410), from 112 s to 19.6 ks after the  BAT trigger. The
data comprise 95 s in Windowed Timing (WT) mode (the first 8 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 Evans
et al. (GCN Circ. 21413).

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.90 (+0.34, -0.22), followed by a break at T+519 s to
an alpha of 1.13 (+0.20, -0.13).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.37 (+0.16, -0.13). The
best-fitting absorption column is  consistent with the Galactic value
of 1.8 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 1.97 (+0.16, -0.14) and a best-fitting absorption
column consistent with the Galactic value. The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 3.2 x 10^-11 (3.4 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.8 (+2.4, -0.0) x 10^20 cm^-2
Galactic foreground: 1.8 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.97 (+0.16, -0.14)

If the light curve continues to decay with a power-law decay index of
1.13, the count rate at T+24 hours will be 8.3 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.7 x
10^-14 (2.8 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at

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