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GCN Circular 9528

Subject
GRB090618: Swift-XRT team refined analysis
Date
2009-06-18T19:06:26Z (15 years ago)
From
Andy Beardmore at U Leicester <apb@star.le.ac.uk>
A.P. Beardmore (U. Leicester) and P. Schady (MSSL-UCL) report on
behalf of the Swift-XRT team:

We have analysed 12.3 ks of Swift-XRT data for GRB 090618 (Schady et
al. GCN Circ. 9512), from 124 s to 25.7 ks after the BAT trigger. The
data span five orbits and comprise 2.3 ks in Windowed Timing (WT) mode
and 10.0 ks in Photon Counting (PC) mode. The UVOT-enhanced XRT
position for this burst was reported by Evans et al. (GCN 9521).

The burst was initially very bright in X-rays, observed at a rate 
of ~8000 count s^-1, and decayed rapidly with a slope of ~6, before
breaking at T+310 s to a shallower decay slope of 0.71 +/- 0.02.
A further break is seen at T+4700 s with the decay steepening to 
1.22 +/- 0.05.

The WT spectrum from T+250 s to T+1065 s can be fit by an absorbed
powerlaw with a photon index of 2.11 +/- 0.03 and intrinsic absorption
of (1.78 +/- 0.14) x 10^21 cm^-2 (at z=0.54, Cenko et al., GCN 9518),
in addition to the Galactic column of 5.8 x 10^20 cm^-2 in the
direction of the source (Kalberla et al. 2005). The observed 0.3-10
keV flux for this model is (8.2 +0.2 -0.1) x 10^-10 erg cm^-2
s^-1. The corresponding unabsorbed flux is (1.16 +/- 0.02) x 10^-9 
erg cm^-2 s^-1.

If the light curve continues to decay at the same rate we predict a
count rate of 0.13 count s^-1 at T+24 hours, which corresponds to an
observed (unabsorbed) 0.3-10 keV flux of 4.9 x 10^-12 (7.2 x 10^-12)
erg cm^-2 s^-1.

The results of the XRT team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00355083.

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