[vsnet-grb-info 7750] GRB 090429A: Swift-XRT refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9304
SUBJECT: GRB 090429A: Swift-XRT refined analysis
DATE:    09/04/30 10:39:40 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at star.le.ac.uk>

P.A. Evans (U. Leicester) and C. B. Markwardt (CRESST/GSFC/UMD) report
on behalf of the Swift-XRT team:

We have analysed 6 ks of XRT data for GRB 090429A (Markwardt et al. GCN
Circ. 9280). The first observation comprises a continuous observation
from T0+64 s to T0+255 s  with the data  entirely in Windowed Timing
(WT) mode. The second observation covers the range T0+35 to T0+64 ks,
and contains 5.8 ks of Photon Counting (PC) mode data. The enhanced XRT
position for this burst was given by Evans et al. (GCN. Circ 9297).

The first observation shows a number of flares superimposed on a short,
initial rise which breaks at T0+100s to a power-law decay with index 2.7
(+0.3, -0.2). The later PC mode data lie orders of magnitude above the
extrapolation of this decay; the addition of a break around T0+1 ks to a
decay with power-law index 0.56 (+0.12, -0.06) produces a good fit to
the data. Due to the large observing gap the behaviour of the light
curve in the interval T0+255 s to T0+35 ks is unknown; if it is more
complex than the simple behaviour we have modelled the final decay index
could differ significantly from the 0.56 (+0.12, -0.06) value we obtained.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.0 (+1.0, -0.7). The
best-fitting absorption column is 1.20 (+2.36, -0.77) x 10^21 cm^-2,
consistent with the Galactic value of 4.2 x 10^20 cm^-2 (Kalberla et al.
2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 3.8 x 10^-11 (5.0 x 10^-11) erg
cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of
0.56, the count rate at T+48 hours will be 5.9 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.2 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/00350853.

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