The binary star merger event GW170817, which took place in a galaxy 130 million light years away, produced a jet expanding at nearly light speed, according to new measurements from a global network of radio telescopes. The collision emitted gravitational waves and electromagnetic radiation that was detected on Earth in August 2017.

Jet breaking out

Artist’s impression of a jet that breaks out of the material ejected by the neutron stars merger. The jet is produced by the black hole, surrounded by a hot disc, which was formed after the merger. Credit: O.S. Salafia, G. Ghirlanda, NASA/CXC/GSFC/B. Williams et al.

Although initial measurements of the emission suggested that the radiation was produced by a kilonova, a radioactive-decay-powered emission coming from the ejected material before and after the emission, follow-up measurements detected X-ray and radio emissions that continued to be observed for several months. This “afterglow” of the merger, as it has been interpreted, suggest the expanding ejected material interacted with surrounding interstellar gas.

The way this emission was created is not very well understood because of the low resolution of previous data, which prevented researchers from determining the size of the source.

The team reporting its work in Science, has now used an array of 32 radiotelescopes (spread over five continents) to observe the radio afterglow 207.4 days after the merger. Using Very Long Baseline Interferometry (VLBI), the scientists combined data from all the telescopes to constrain the source’s angular size.

The results indicate that GW170817 produced a structured jet that was able to punch through the merger’s surrounding ejecta into interstellar space and beyond.

The French laboratories involved in this research are: The Laboratoire Univers et Particules de Montpellier, Universitè de Montpellier, Centre National de la Recherche Scientifique/Institute National de Physique Nucleaire et Physique des Particules (CNRS/IN2P3); AstroParticule et Cosmologie (APC), Université Paris Diderot, CNRS/IN2P3, Commissariat à l’Énergie Atomique et aux Énergies Alternatives/ Institute for Research on the Fundamental Laws of the Universe (CEA/IRFU), Observatoire de Paris, Sorbonne Paris Cité; Galaxies, Etoiles, Physique et Instrumentation (GEPI) Observatoire de Paris, CNRS UMR 8111;

Reference: Compact radio emission indicates a structured jet was produced by a binary neutron star merger,” by G. Ghirlanda et al., Science DOI: 10.1126/science.aau8815