[vsnet-grb-info 2391] GRB051109A: XRT refined analysis

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4228
SUBJECT: GRB051109A: XRT refined analysis
DATE:    05/11/09 13:40:11 GMT
FROM:    Sergio Campana at INAF-OAB  <campana at merate.mi.astro.it>

S. Campana (INAF-OAB), T. Mineo (INAF-IASFPA), G. Tagliaferri (INAF-OAB),
V. Mangano (INAF-IASFPA), P. Romano (INAF-OAB), D.N. Burrows (PSU)
on behalf of the Swift/XRT team:

We have analysed 10 ks of XRT observations of GRB051109
(Tagliaferri et al., GCN 4213). The afterglow is bright and well
detected. The refined coordinates for this X-ray afterglow are:

RA(J2000):   22 01 15.3
Dec(J2000): +40 49 24

with an estimated uncertainty of 3.5 arcsec (90% containment) and
including the latest XRT boresight correction. The position is 3.2
arcsec from the on-board position reported by Tagliaferri et al.
(and 1.2 arcsec from the one by UVOT, ibid.) and 0.9 arcsec from
the optical afterglow first detected by ROTSE (Rykoff et al., GCN 4211).

The X-ray light-curve shows a clear break between the first and the second
Swift orbit (close to the end of the first orbit). In the first part the
decay is very steep with index alpha_1=-3.1 +/- 0.6 (90% containment)
whereas in the second part it flattens to alpha_2=-0.6 +/- 0.2.

The spectrum of the full data (WT and PC) can be modelled
with a power-law with photon index Gamma = 2.07 +/- 0.08 (90% containment).
There is evidence for an absorbing column higher than the Galactic value
(1.9e21 cm^-2) at a level of (8.2+/-5.2)e21 cm^-2 at the observed redshift
z=2.346 (Quimby et al., GCN 4221).

At 150 s the 0.5-10 keV unabsorbed flux was 3.2e-10 erg cm^-2 s^-1.
At 5000 s the afterglow had faded to flux level of 2.4e-11 erg cm^-2 s^-1.
Assuming the currect decay (which is very flat) we predict a flux
of ~5e-12 erg cm^-2 s^-1 at 1 d from the burst.