[vsnet-grb-info 7683] GRB 090424: Swift-XRT refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9237
SUBJECT: GRB 090424: Swift-XRT refined analysis
DATE:    09/04/24 23:15:47 GMT
FROM:    Raffaella Margutti at U. di Milano Bicocca  <raffaella.margutti at brera.inaf.it>

R. Margutti (Univ. Bicocca & INAF-OAB), C. Guidorzi (Univ. Ferrara),
J. Mao (INAF-OAB)  and J. K. Cannizzo (NASA/UMBC) report on behalf of
the Swift-XRT team:

We have analysed the first 2 orbits of Swift-XRT data obtained for
GRB 090424 (Cannizzo et al. GCN Circ. 9223), covering 1732 s of
Windowed Timing (WT) and 1329 s of Photon Counting (PC) mode data,
respectively, between 88 s and 8 ks after the trigger.
The UVOT-enhanced XRT position was given by Goad et al. in GCN Circ. 9232.

The light-curve can be modelled by a double broken power-law, with
a first decaying index alpha_1= 1.29 +/- 0.05 and a first break time
of about 260 s. The decay then flattens to alpha_2=  0.74  +/- 0.02.
After 1450 s the decay is best modelled by a power-law index
alpha_3= 1.15 +/- 0.05.

The very good statistics allows us to observe spectral evolution
in WT data before the first break, with the simple power law photon index
evolving from 2.6 to 2.1 . A spectrum extracted from WT mode data in the
time interval 0.4-1.4 ks can be modelled with an absorbed
power-law with a photon index Gamma= 2.13 +/- 0.05 and best-fitting
absorption column NH = (0.20 +/- 0.01) x10^22 cm^-2, in excess of the 
Galactic
value of 1.9 x 10^20 cm^-2 (Kalberla et al. 2005).

The spectrum extracted from PC data in the time interval 6.1-8 ks
can be modelled with an absorbed power-law, with Gamma = 2.06 +/- 0.09
and a column density of NH = (0.27 +/- 0.03)x10^22 cm^-2.
The observed (unabsorbed) 0.3 -10 keV flux over this time interval is
4.2x10^-11 (6.7x10^-11) erg cm^-2 count^-1.
Uncertainties are given at 90% confidence.

If the light-curve continues to decay with alpha ~1.15, the count rate 24
hours after the burst is estimated to be 0.10 count s^-1, which
corresponds to an observed (unabsorbed) flux of 4.2x10^-12
(6.7x10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00350311.

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