The most distant cosmological explosion

Abstract

At a redshift of 8.3 GRB 090423 marks the highest known redshift object in the Universe. By combining our radio measurements with existing X‐ray and infrared observations, we estimated 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. We compare the properties of GRB 090423 with a sample of GRBs at moderate redshifts. We find that the high energy and afterglow properties of GRB 090423 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 GRB 090423 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.