TITLE:   GCN CIRCULAR
NUMBER:  8797
SUBJECT: GRB 090111: Swift-XRT refined analysis
DATE:    09/01/12 12:05:47 GMT
FROM:    Raffaella Margutti at U. di Milano Bicocca  <raffaella.margutti@brera.inaf.it>

R. Margutti (Univ. Bicocca & INAF-OAB), J. Mao (INAF-OAB),
T. Sakamoto (NASA/UMBC) report on behalf of the Swift XRT team:

We have analysed the first 2 orbits of Swift-XRT data obtained for
GRB 090111 (Sakamoto et al., GCN Circ. 8795), covering 70 s of
Windowed Timing (WT) and 4 ks of Photon Counting (PC) mode data,
respectively, between  77 s and 8 ks after the trigger.
The UVOT-enhanced XRT position was given by Evans et al. in
GCN Circ. 8796.

In the time interval 77-150 s, the light curve can be modelled by a
simple power law with a decay index alpha_1= 5.1 +/- 0.3 (90% confidence)
followed by a rebrightening around 0.5 ks.
After the rebrightnening the light curve decays with a powerlaw index
alpha_2= 3.6 +/- 0.4 and then flattens to alpha_3= 0.4 +/- 0.1
with a t_break of 1200 s.

The spectrum extracted from WT data can be modelled with an
absorbed simple power-law, with Gamma = 5.4 +/- 0.5  and
NH = (5.4 +/- 0.8)x10^21 cm^-2 in excess of the Galactic column in
this direction which is 6.5x10^20 cm^-2 (Kalberla et al. 2005).
The observed (unabsorbed) 0.3 -10 keV
flux over this time interval (77-150 s post-trigger) is
3.8x10^-10 (2.7x10^-8) erg cm^-2 s^-1.

The spectrum extracted from PC data in the time interval 150-8000 s
can be modelled with an absorbed power-law, with Gamma= 3.3 +/- 0.5
and an intrinsic column density of NH = (2.0 +/- 0.8)x10^21 cm^-2.
The observed (unabsorbed) 0.3 -10 keV flux over this time
interval is 3.0x10^-12 (1.2x10^-11) erg cm^-2 s^-1.
Uncertainties are given at 90% confidence.

If the light-curve continues to decay with alpha ~0.4, the count rate 24
hours after the burst is estimated to be 1.3x10^-2 count s^-1, which
corresponds to an observed flux of 4x10^-13 erg cm^-2 s^-1.

The results of the xrt automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00339696.

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