Science & Technology

Merging neutron stars produce a jet of material visible from Earth.

Artist’s impression of the merger of two neutron stars with all the material expelled into space and the observed jet after breaking through this shell (Beabudai design)
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Using a global network of telescopes, astronomers have observed a jet of material moving at near-light speed ejected from two merging neutron stars. 

Astronomers have used a global network of telescopes to spot a compact jet of material moving at near-light speed after being ejected from merging neutron stars – known as the neutron star merger event GW170817 – in a galaxy 130 million-light-years away.

The collision has already been detected on Earth as a result of gravitational waves and electromagnetic radiation it also emitted being detected back in August 2017.

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

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-term emissions have been determined to be the afterglow of the merger and it suggests the interaction of a jet of the expanding material interacting with surrounding interstellar gas.

Representation of all radio telescopes that participated in this observation. (Paul Boven)
Representation of all radio telescopes that participated in the observation of material ejected by merging neutron stars (Paul Boven)

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 and the team 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.

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.

Rather, the data indicate 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.

Featured image caption: Artist’s impression of the merger of two neutron stars with all the material expelled into space and the observed jet after breaking through this shell (Beabudei design)

This research appears in the 22 February 2019 issue of Science.

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