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GCN Circular 9639

GRB090709A: High Energy BAT/XRT Analysis Suggests Low to Moderate Redshift
2009-07-09T22:39:12Z (15 years ago)
S. Bradley Cenko at Caltech <>
Nat Butler (UC Berkeley) Reports:

We reduce the Swift XRT and BAT data using our automated pipeline (see, 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.
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