TITLE:   GCN CIRCULAR
NUMBER:  8847
SUBJECT: GRB 090123: Swift XRT refined analysis 
DATE:    09/01/23 19:21:36 GMT
FROM:    Dirk Grupe at PSU/Swift-XRT  <grupe@astro.psu.edu>

D. Grupe (PSU) reports on behalf of the Swift-XRT team:

We have analyzed 7.7 ks of XRT data for GRB090123 (Grupe et al. GCN 
Circ. 8842),
beginning 106 s after the BAT trigger. The data comprise 334 s in Windowed
Timing (WT) mode with the remainder in Photon Counting (PC) mode.
The enhanced XRT position for this burst was given by Osborne et al.
(GCN. Circ 8843).

The light curve can be modeled with an initial power-law decay with an
index of alpha=1.78 (+0.06, -0.08), followed by a break at T+1530 
(+720,-220) s to
an alpha of 1.20 (+0.08, -0.10).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.65+/-0.05 with an absorption 
column fixed to the Galactic
value of 1.79 x 1020 cm-2 (Kalberla et al. 2005). The PC mode spectrum 
has a
photon index of 1.81+/-0.08 and an absorption column density also 
consistent
with the Galactic value.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 4.2 x 10-11 (4.5 x 10-11) erg cm-2 count-1.


If the light curve continues to decay with a power-law decay index of 1.20,
the count rate at T+24 hours will be 0.0031 counts/s, corresponding to an
observed (unabsorbed) 0.3-10 keV flux of 1.3 x 10-13 (1.4 x 10-13) erg cm-2
s-1.