GCN Circular 21730

A. D'Ai (IASF-PA), V. D'Elia (ASDC), F. E. Marshall (NASA/GSFC) reports
on behalf of the Swift-XRT team:

We have analysed 1.1 ks of XRT data for GRB 170827A (Marshall et al.
GCN Circ. 21718
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 ), from 113 s to 17.3 ks after the  BAT trigger. The
data comprise 165 s in Windowed Timing (WT) mode (the first 9 s were
taken while Swift was slewing) with the remainder in Photon Counting
(PC) mode.

The initial light curve from T0+130s to T0+295s can be modelled with a
power-law decay with a decay index of alpha=1.19 (+/-0.08). In the
following observation at T0+16.1 ks the afterglow is not detected with
a 3-sigma upper limit of 1.6 x 10^-2 count^-1 s^-1. 

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.23 (+0.06, -0.05). The
best-fitting absorption column is  1.46 (+/-0.15) x 10^21 cm^-2, in
excess of the Galactic value of 1.2 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 3.0 x 10^-11 (4.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	  1.46 (+/-0.15) x 10^21 cm^-2
Galactic foreground: 1.2 x 10^21 cm^-2
Excess significance: 3.0 sigma
Photon index:	  2.23 (+0.06, -0.05)

Given the lack of detection in the second observation, no reliable
estimate of the X-ray count rate for the following days is possible.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00769567.

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