[AAVSO-HEN] GRB 090904A: Swift-XRT refined analysis

[grb at aavso.org]

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TITLE:   GCN CIRCULAR
NUMBER:  9894
SUBJECT: GRB 090904A: Swift-XRT refined analysis
DATE:    09/09/04 16:56:03 GMT
FROM:    Matteo Perri at ISAC/ASDC  <perri at asdc.asi.it>

M. Perri, V. D'Elia, G. Stratta (ASDC) report on behalf of the
Swift-XRT team:

We have analysed the first 12 ks of XRT data for GRB 090904A (Perri et
al.,
GCN Circ. 9879), from 134 s to 29.6 ks after the BAT trigger. The enhanced
XRT position for this burst was given by Evans (GCN. Circ. 9891).

During the first Swift orbit, the light-curve shows a steep decay
with a large superimposed flare peaking at about T+300 s. Starting
from T+1000 s, the curve is well described by a broken power-law model
with temporal decay indices alpha1=0.2+/-0.1, alpha2=0.9+/-0.3 and a
temporal break at around T+12 ks.

A spectrum extracted from the PC mode data from T+460 s to T+29.6 ks can
be fitted with an absorbed power-law model with a photon spectral index of
2.46 (+0.16,-0.26) and an absorption column density of
1.03 (+0.66,-0.64)e21 cm-2 in excess of the Galactic value of
9.3e20 cm-2 (Kalberla et al. 2005). The counts-to-observed (-unabsorbed)
0.3-10 keV flux conversion factor deduced from this spectrum is
3.3e-11 (6.2e-11) erg cm-2 count-1. Errors are given at the
90% confidence level.

If the light curve continues to decay with a power-law index of
0.9, the count rate at T+24 hours will be 6e-3 count s-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of
2.0e-13 (3.7e-13) erg cm-2 s-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00361830.

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