Discovery of Radio Afterglow from the Most Distant Cosmic Explosion
  We report the discovery of radio afterglow emission from the gamma-ray burst
GRB 090423, which exploded at a redshift of 8.3, making it the object with the
highest known redshift in the Universe. By combining our radio measurements
with existing X-ray and infrared observations, we estimate the kinetic energy
of the afterglow, the geometry of the outflow and the density of the
circumburst medium. Our best fit model is a quasi-spherical, high-energy
explosion in a low, constant-density medium. \event had a similar energy
release to the other well-studied high redshift GRB 050904 ($z=6.26$), but
their circumburst densities differ by two orders of magnitude. We compare the
properties of \event with a sample of GRBs at moderate redshifts. We find that
the high energy and afterglow properties of \event are not sufficiently
different from other GRBs to suggest a different kind of progenitor, such as a
Population III star. However, we argue that it is not clear that the afterglow
properties alone can provide convincing identification of Population III
progenitors. We suggest that the millimeter and centimeter radio detections of
\event at early times contained emission from a reverse shock component. This
has important implications for the detection of high redshift GRBs by the next
generation of radio facilities.
