Public Release: 

Binary neutron star merger produced a compact jet of material detectible From Earth

American Association for the Advancement of Science

Using a global network of radio telescopes, researchers have found that the binary neutron star merger event GW170817 produced a jet of material expanding at close to the speed of light, a new study reports. In a galaxy 130 million light years from Earth, a pair of neutron stars merged together. The collision emitted gravitational waves and electromagnetic radiation, which were detected on Earth by telescopes and gravitational wave detectors in August of 2017. The first few days of emissions suggested it was produced by a kilonova, a radioactive-decay-powered emission originating from the material ejected during and after the merger. However, in the weeks following the initial detection, increasing X-ray and radio emissions were detected, which continued to be observed for several months. These long-lasting emissions have been interpreted as the "afterglow" of the merger and suggest interaction of the expanding ejected material with surrounding interstellar gas. However, how this afterglow emission was created remains poorly understood, because previous data did not have the necessary resolution to determine the size of the source, according to the authors. Giancarlo Ghirlanda and colleagues used an array of 32 radio telescopes - spread over five continents - to observe the radio afterglow 207.4 days after the merger. Using the technique of Very Long Baseline Interferometry (VLBI), Ghirlanda et al. combined the data from all the telescopes to constrain the source's angular size. The results indicate that the size and position of the radio source are not compatible with models of a "choked-jet" or "cocoon" scenario as some have suggested. Rather, the data indicates that GW170817 produced a structured jet expanding nearly as fast as the speed of light, which was able to punch through the merger's surrounding ejecta into interstellar space beyond.

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