Astronomers expose the aftermath of a violent merger of neutron stars!

Astronomers from around the world have combined radio telescopes on five continents, including at Jodrell Bank, to prove the existence of  jets (narrow streams of material that emerge from the aftermath of a violent collision of neutron stars).

Neutron stars are ultra-dense stars, roughly the same mass as the Sun, but similar in size to a city like Manchester.

In August 2017, two neutron stars were observed colliding and producing gravitational waves that were detected by the American LIGO and European VIRGO detectors. This event is the first and only one of its type that has been observed so far, and happened in a galaxy 130 million light years away from Earth.

Astronomers observed the event and its subsequent evolution across the entire electromagnetic spectrum, from gamma-rays, X-rays to visible light and radio waves. Two hundred days after the merger, observations combining radio telescopes in Europe, Africa, Asia, Oceania, and North America proved the existence of a jet emerging from this violent collision.

These findings have been published in the scientific journal Science by an international team of astronomers, led by Giancarlo Ghirlanda from the National Institute for Astrophysics (INAF, Italy).

This neutron star merger is the first time it has been possible to associate a detection of gravitational waves to an object emitting light. The event has confirmed scientific theories that have been discussed for decades, and the association of neutron star mergers with one of the most powerful explosions in the Universe: gamma-ray bursts.

Dr Robert Beswick, co-author from Jodrell Bank, said: “This is a really exciting discovery. The team has determined that this event produced a powerful jet able to pierce through the material ejected from the neutron stars as they merged. These types of observations will allow us to unveil the processes that take place during and after some of the most powerful events in the Universe.”

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.

After the merger, a huge amount of material and energy was ejected into space, forming a shell around the object. Astronomers have been tracing its evolution at different wavelengths. However, there were still some remaining questions concerning this event that could not be clarified by any previous observations.

“We expected part of the material to be ejected through a collimated jet, but it was unclear whether this material could successfully pierce through the surrounding shell.” explains Ghirlanda. “There were two competing scenarios: In one case, the jet cannot break through the shell, instead generating an expanding bubble around the object. In the other, the jet is successful in penetrating the shell and then propagates further into space”, expands Tiziana Venturi (INAF).

Only acquiring very sensitive radio images with very high resolution would discard one scenario or the other. This required the use of a technique known as Very Long Baseline Interferometry (VLBI) that allows astronomers to combine radio telescopes all around the Earth.

The international team conducted global observations in the direction of the merger last year using 33 radio telescopes from around the world. The UK’s e-MERLIN array, which comprises 6 large radio telescopes spread across the southern half of the UK and which is operated from Jodrell Bank, joined in this campaign allowing the team to verify and confirm the interpretation of these results.

Dr Javier Moldon, co-author from Jodrell Bank added: “It is only with the extremely high resolution imaging power of the e-MERLIN, the EVN and the VLBA arrays combined that we were able to image and hence confirm the presence of a jet.” In the coming years, many more of these neutron star binary mergers will be discovered. “The obtained results also suggest that more than 10% of all these mergers should exhibit a successful jet.”, explains Benito Marcote (JIVE).