TITLE:   GCN CIRCULAR
NUMBER:  9734
SUBJECT: GRB 090728: Swift XRT refined analysis 
DATE:    09/07/29 15:26:40 GMT
FROM:    Vanessa Mangano at INAF-IASFPA  <vanessa@ifc.inaf.it>

V. Mangano, B. Sbarufatti (INAF IASF Pa)
report on beahalf the Swift XRT Team:


We have analysed 9.8 ks of XRT data for GRB 090728 (Mangano et al. GCN
Circ. 9725), from 116.9 s to 53.1 ks after the BAT trigger. The data
comprise 55.3 s in Windowed Timing (WT) mode from T+116.9 s to T+172.2
s, with  the remainder in Photon Counting (PC) mode. The enhanced XRT
position for this burst was given by Osborne et al. (GCN. Circ 9729).

The 0.3-10 keV XRT light curve is well modeled by a doubly broken
power law with initial decay slope -5.5 ± 0.5, first break at
T+204 ± 10 s, plateau phase slope -0.11 ± 0.14, second break
at T+1.8 ± 1.2 ks and final slope -1.7 ± 0.1.
Decaying at the present rate the source will reach a level of
2e-4 counts/s on July 30 at 14:45 UT (48 h after the trigger).

We extracted the average WT spectrum of the early decay phase (55.3 s
exposure), and the average PC spectra of the plateau phase (866 s
exposure, from T+174 s to T+1071 s) and the final decay phase (7.4 ks
exposure, from T+4383 s to T+18.4 ks). The three spectra are well fitted
by an absorbed power law model with NH consistent with the Galactic
value of 7.35e20 cm-2 (Kalberla et al. 2005) and photon indices of
2.4 ± 0.4, 1.8 ± 0.2, and 2.0 ± 0.3, respectively.
The average 0.3-10 keV observed [unabsorbed] fluxes are
1.6e-10 [2.9e-10] erg cm-2 s-1, 2.7e-11 [3.0e-11] erg cm-2 s-1,
and 1.5e-12 [2.3e-12] erg cm-2 s-1, respectively.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00358574.

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