[vsnet-grb-info 8106] GRB090709A: High Energy BAT/XRT Analysis Suggests Low to Moderate Redshift

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9639
SUBJECT: GRB090709A: High Energy BAT/XRT Analysis Suggests Low to Moderate Redshift
DATE:    09/07/09 22:39:12 GMT
FROM:    S. Bradley Cenko at Caltech  <cenko at srl.caltech.edu>

Nat Butler (UC Berkeley) Reports:

We reduce the Swift XRT and BAT data using our automated pipeline (see,
http://astro.berkeley.edu/~nat/swift for more details of the fits and
summary plots for this and other Swift GRBs).

The intermediate time PC mode XRT data (t=4-10 ksec post trigger) are
well-fit (chi^2/nu=80.48/59) by an absorbed powerlaw with photon index
Gamma=1.9+/-0.1 and column density 1.7+/-0.5 x 10^21 cm^-2 in excess of
the anticipated Galactic contribution (6.6 x 10^20 cm^-2) at the 6.7-sigma
level.  This detection of an excess column is suggestive of a low to
moderate redshift (i.e., z<~2) for the GRB (e.g,. Grupe et al. 2007, ApJ,
133, 2216).

The time-integrated BAT spectrum is well-modeled by a powerlaw times
exponential function (chi^2/nu=29.67/55).  We derive Epeak,obs = 200
(176,298) keV and a rest-frame isotropic equivalent energy release Eiso ~
1.4 x 10^53 erg (z=1).  If the GRB were at very high z~8, Eiso ~ 4.5 x
10^54 erg, comparable to the intrinsically brightest few GRBs ever detected,
further suggestive of a low to moderate redshift for this event.