[vsnet-grb-info 8109] GRB 090709A: Swift XRT Refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9642
SUBJECT: GRB 090709A: Swift XRT Refined Analysis
DATE:    09/07/09 23:06:12 GMT
FROM:    Antonia Rowlinson at U.of Leicester  <bar7 at star.le.ac.uk>

A. Rowlinson (U Leicester) and D. C. Morris (PSU) report on behalf of
the Swift-XRT team:

We have analysed 9.4 ks of XRT data for GRB 090709A (Morris et al. GCN
Circ. 9625), from 74 s to 27.3 ks after the BAT trigger. The data
comprise 445 s in Windowed Timing (WT) mode with the remainder in Photon
Counting (PC) mode. The enhanced XRT position for this burst was given
by Osborne et al. (GCN. Circ 9636).

The light curve can be modelled with a broken power-law decay. The
initial decay index is alpha=0.77(+/-0.1), at T+7918 s the light curve
breaks to a decay with alpha=1.51(+/-0.06).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.78 (+/-0.04). The
best-fitting absorption column is 2.14 (+0.13, -0.12) x 10^21 cm^-2, in
excess of the Galactic value of 6.6 x 10^20 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.05 (+/-0.08) and a
best-fitting absorption column of 2.68 (+0.26, -0.25) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 4.2 x 10^-11 (6.7 x 10^-11) erg cm^-2
count^-1.

If the light curve continues to decay with a power-law decay index of
1.51, the count rate at T+24 hours will be 0.043 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of  1.8 x
10^-12 (2.9 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/00356890.

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