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Using a vast system of telescopes, astronomers from all over the globe, including Ralph Wijers, Lex Kaper and Antonia Rowlinson from the University of Amsterdam, have been able for the first time to gather observational data on electromagnetic radiation from a source of gravitational waves. Emitted during the merger of two neutron stars, the waves were registered by LIGO detectors in the United States and the Virgo detector in Europe on 17 August 2017.

Never before have scientists observed both gravitational waves and electromagnetic radiation (gamma, X-ray, ultraviolet, infrared, optical and radio) from one and the same event. This is thanks to a unique worldwide collaboration between seventy research teams and the deployment of a large number of telescopes, including the Dutch LOFAR radio telescope and telescopes of the European Southern Observatory (ESO) in northern Chile.

Unravelling the gamma-ray burst mystery

Researchers involved in the project include Ralph Wijers, Lex Kaper and Antonia Rowlinson from the UvA's Anton Pannekoek Institute for Astronomy. Wijers, who contributed to unravelling the mystery of gamma-ray bursts twenty years ago: ‘This is hugely exciting. It really reminds me of that period. And the great thing is that it's thanks in part to what we learned back then that we were so well prepared to make this discovery.’

Decades ago, theoretical scientists predicted that a clash of neutron stars was bound to produce gravitational waves and what is known as a gamma-ray burst. In such an event, part of the material of which the neutron stars are made up is hurled into space, producing a signal in optical and infrared wavelengths (a ‘kilonova’) and, at a later stage, X-ray and radio emissions. 

Artist's impression of colliding neutron stars (Copyright: NASA/Dana Berry)

Formation of heavy metals

The observed kilonova and the extraodinary short  gamma-ray burst registered by NASA’s Fermi telescope and ESA’s Integral telescope two seconds after the detection of GW170817  spectacularly confirm the theory on the radiation and matter that are generated when two neutron stars merge. A neutron star is the extremely compact, collapsed core of a heavy star that remains after a supernova explosion.

One important astronomical result of the follow-up campaign is the observation of the kilonova, the cataclysmic aftermath of the merger of two neutron stars, that scientists had for so long been looking for. In a kilonova explosion, which is about 1000 times as bright as a typical nova, material that remained after the clash  is expelled into space. Heavy elements such as platinum, lead and gold are created in the process and spread across the universe.

‘The clash probably resulted in a black hole,’ Peter Jonker (SRON/Radboud University Nijmegen) adds. ‘Just before this black hole was created, material was hurled into space from which various rare earth metals were formed. So in fact we've discovered a space lab where all sorts of exotic materials are created. It's truly spectacular.’

Nearest gravational wave event

According to estimates based on both gravitational wave data and other observations, GW170817 is at about the same distance from Earth as NGC 4993 , which is roughly 130 million light years. This means it is not only the nearest gravitational wave event to date, but also by far the nearest gamma-ray burst ever observed. So while one mystery seems to have been solved, others have appeared on the horizon. Despite the fact that the short gamma-ray burst was much nearer than any that had been observed previously, the signal was surprisingly weak relative to the distance. To explain this, new models are required.

Virgo spokesperson Jo van den Brand (Nikhef/VU Free University Amsterdam) was equally excited: ‘Discovering all these new things right at the first detection of a double neutron star collision really is extraordinary.’ The telescopes will stay focused on the after-glow of the neutron star collision over the next few weeks and months in order to collect as much data as possible about the individual stages in the merger, interaction with the environment and the processes that generate the rare earth metals in the universe.

‘Once the LIGO and Virgo detectors start making new measurements following a series of technical improvements one year from now, we're in for a whole lot of new detections,’ says Radboud University astronomer Gijs Nelemans. ‘That's when we'll really be able to conduct statistical studies that will tell us how these systems  came into being.’

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