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GCN Circular 9723

Subject
GRB 090727: Swift-XRT refined analysis
Date
2009-07-28T11:30:11Z (15 years ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
P. A. Evans (U Leicester) reports on behalf of the Swift-XRT team:

We have analysed 11 ks of XRT data for GRB 090727 (Evans et al. GCN
Circ. 9718), from 91 s to 29.2 ks after the BAT trigger. The data
comprise 40 s in Windowed Timing (WT) mode from T+260 s to T+300 s,
with  the remainder in Photon Counting (PC) mode. The enhanced XRT
position for this burst was given by Goad et al. (GCN. Circ 9721).

The XRT initially observed the GRB for only 210 s before slewing away,
and the data in this interval are dominated by a flare, making it
impossible to determine the underlying decay rate. Fitting only the data
from the second snapshot onwards (beginning at T+3.9 ks) we find a
power-law decay with an index of alpha=0.73 (+/-0.19).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.60 (+0.20, -0.14). The
best-fitting absorption column is consistent with the Galactic value of
2.5 x 10^21 cm^-2  (Kalberla et al. 2005).

If the light curve continues to decay with a power-law decay index of
0.73, the count rate at T+24 hours will be 0.0043 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.3 x
10^-13 (2.9 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/00358520.

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