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UCPH scientists contributed to sensational neutron stars discovery

Astrophysics — By combining measurements of gravitational waves and observations of light, the scientists have discovered and described a clash between two neutron stars. Analyses of light from the explosion - called a kilonova - show the creation of heavy elements, solving an older scientific problem.

It is no more than two years ago that scientists first observed gravitational  waves– i.e. ripples in space-time. This was at the LIGO observatory  in the United States, and the discovery led to this year’s Nobel Prize in physics.

Now Danish scientists have helped take the next step, according to a series of press releases from the Niels Bohr Institute at the University of Copenhagen (UCPH).

They have, for the first time, used a combination of gravitational wave measurements and observations of electromagnetic radiation with telescopes to describe a clash between two neutron stars merging and setting off a kilonova, a giant explosion which set off a powerful burst of gamma rays in two opposite directions.

Scientists from the Dark Cosmology Center (DARK) at UCPH had hoped to contribute to the discovery.

»This is exactly what we wanted to do, but we had not expected it to happen so fast. We thought we would have had to wait for several years. With this, it all came together,« says Professor Jens Hjorth, head of DARK, according to the Institute’s Danish press release.

Radioactive fireball

That the research environment was well-prepared for the discovery of the neutron star collision is also evident in the fact that Jens Hjorth told news site in 2016 that he hoped to be able to observe a gamma burst after a gravitational wave.

The discovery of the kilonova took place after the gravitational wave monitors in Virgo (Italy) and LIGO recorded minor fluctuations in space-time on 17th August, 2017. Shortly afterwards, the predicted gamma burst of electromagnetic radiation hit the space observatories Fermi and Integral.

By linking the measurements from the different instruments, astronomers around the world could search in a demarcated area of ​​the universe for the origin of the waves and the radiation.

It took 11 hours. Then a research team which included UCPH Professor Enrico Ramirez-Ruiz found a bright spot in space, a radioactive fireball that was expanding at one fifth the speed of light.

In the fusion of the two neutron stars, matter corresponding to three and five per cent of the mass of the Sun was flung out into space.

After one and a half days, the radius of the ball was eight billion kilometres.

Astronomers used to be able to see it, now they can both see and hear it

According to PhD student Jonatan Selsing of UCPH, the use of gravitational observations means that astronomers in effect have gained an extra sense:

»Imagine you’re out for a walk in the forest. You hear something rustling on the forest floor, then you know approximately where to look for an animal. With a little luck you will be able to see it. This is what it is like in astronomy now, because we – with gravitational wave detectors – can ‘listen’ to the universe and find out where to look closer.«

Solves heavy elements puzzle

A neutron star is what is left over when a dead  giant star explodes into a supernova; it is a small, hot, heavy core, just 20 kilometers in diameter, of hard, compressed neutrons – a matchbox sized neutron star weighs three billion tons, corresponding to a 800 by 800 metres cube of the Earth.

The neutron star collision has already made it into a series of scientific articles with new discoveries.

These include studies of the radiation from the kilonova, showing that inside the hot matter a number of heavy elements are created. Some of the neutrons from the two stars decayed to protons and electrons in the process, accumulating thereby into new atoms, including heavy substances like cesium and tellurium. This can be seen in the spectrum of light emitted by the kilonova, an analysis that PhD student Jonathan Selsing contributed to.

He says that each morning after the kilonova detection at 7am, he would receive data from observatories in Chile, analysing it with software, he developed himself.

»I was lucky to have the right skill-set at the right time,« he says.

The formation of heavy elements has been one of the big enigmas of science, because at the Big Bang only hydrogen, helium and lithium were created. And the lighter elements – up to iron – are formed by fusion processes in the interior of stars.

Astrophysics have put forward the theory that heavy elements such as gold and platinum are formed in the collision between neutron stars, and the new discovery of the star collision seems to support the idea.

The researchers at the Niels Bohr Institute will continue to analyze their data and possibly identify more elements created in the explosion.