[vsnet-grb-info 7533] GRB 090404: Swift-XRT Team refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9091
SUBJECT: GRB 090404: Swift-XRT Team refined analysis
DATE:    09/04/05 00:01:05 GMT
FROM:    Kim Page at U.of Leicester  <kpa at star.le.ac.uk>

K.L. Page (U. Leicester) & H. Ziaeepour (UCL-MSSL) report on behalf of the 
Swift-XRT team:

We have analysed the first 3 orbits of XRT data obtained for GRB 090404 
(Ziaeepour et al. 2009, GCN Circ. 9086), comprising 129 s in Windowed 
Timing mode and 5.4 ks in Photon Counting (PC) mode. The UVOT-enhanced 
position was given by Beardmore et al. (GCN Circ. 9088).

The X-ray afterglow was initially very bright, starting at a count rate of 
~1200 count s^-1. The light-curve can be approximately modelled with a 
doubley broken power-law: alpha1 = 3.2 +/- 0.3 until around 120 s after 
the burst, at which point the decay steepened to a slope of alpha2 = 9.3 
+/- 0.3. After 265 s, the decay is much more gradual, with alpha3 = 0.17 
+/- 0.06.

The data show significant softening until at least 150 s after the 
trigger. A spectrum extracted from the first orbit of PC data can be 
fitted with a power-law of Gamma = 3.0 +/- 0.5 and a total absorbing 
column of 5.1x10^21 cm^-2, which is in excess of the Galactic value of 
2.0x10^20 cm^-2 (Kalberla et al. 2005). The counts to observed 
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum 
is 3.1x10^-11 (1.4x10^-10) erg cm^-2 count^-1.

If the light-curve continues to decay with alpha ~ 0.17, the predicted 
count rate at 24 hours is 0.14 count s^-1, corresponding to an observed 
(unabsorbed) flux of 4.3x10^-12 (2.0x10^-11) erg cm^-2 s^-1. However, we 
note that it is unlikely that the light-curve decay will still be this 
flat at 24 hours.

The results of the XRT-team automatic analysis are available at 
http://www.swift.ac.uk/xrt_products/00348428.

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