17. oktober 2017

Neutron star smash shakes universe


Scientists have, for the first time, captured gravitational waves as well as light, from two neutron stars that orbited each other ever closer to eventually collide. The groundbreaking discovery begins a new era in astronomy, and is important for understanding how heavy elements like gold, platinum and uranium are formed.

Artistic creation of clashes between two neutron stars

Artistic creation of clashes between two neutron stars. Picture: ESO

Researchers from the Niels Bohr Institute at the University of Copenhagen were ready when the two neutron stars collided on August 17th, 14.41 Danish time. Specifically, the Dark Cosmology Centre's (DARK) astrophysicists hoped they would experience such an event and have the opportunity to direct their telescopes at the so-called kilonova, which appeared when the neutron stars merged.

"We could see that the kilonova went from blue to red. This would only happen if heavy elements were formed. We therefore believe that it's in the merger of neutron stars that the heaviest of the elements we have on Earth today were originally made. It's fundamental science and very exciting," says Professor Jens Hjorth, who leads DARK.

He and several other researchers from the Niels Bohr Institute have been heavily involved in the analysis of the cosmic clash. Their work has already resulted in 20 scientific articles published today in magazines such as Nature, Science and Astrophysical Journal Letters.

The discovery can not only help explain how heavy elements are formed and spread in galaxies. It also points the way to a new method of measuring the distances to remote celestial bodies, because when the astronomers have both gravitational waves and light to work with, they can measure remote distances more precisely. Based on the distance measured, they can calculate how fast the universe expands.

Ultra-sensitive detectors captured gravitational waves

Neutron stars are the extremely compact remnants of large stars that have burned out. About 140 million light years from here in the galaxy NGC 4993, two neutron stars circled each other at increasingly rapidly speed, eventually reaching hundreds of orbits every second. They came so close to each other that they collided in a gigantic explosion.

In the collision, the two neutron stars merged 2.8 times the mass of the sun together to form a very large neutron star or a black hole. A small part of the mass, equivalent to between three and five percent of the sun's mass, escaped this destiny and instead was thrown into space.

Here, the ejected material created a giant radioactive sphere that expanded at one fifth of the speed of light - approximately ​​60,000 kilometers per second, a speed equivalent to travelling one and a half times around the Earth in a second. In this fireball large amounts of heavy elements were created from a million degree plasma made of neutrons.

"We have confirmed our belief that the majority of the heaviest elements will be formed when two neutron stars merge," explains Jonathan Selsing, a PhD student at DARK, who ensured the maximum information possible was obtained from the light from the kilonova.

The radioactive sphere expanding as a result of neutron star collision

The neutron star clash resulted in a giant radioactive sphere that expands by one fifth of the speed of light. Graphics: NASA

In the last 100 seconds before the collision, the two neutron stars emitted gravitational waves that could be captured by extremely sensitive detectors. Gravitational waves are small ripples in space itself, and the most powerful gravitational waves come when the heaviest and most compact objects of the universe - black holes and neutron stars - orbit each other tightly.

It is the first time that gravitational waves have been detected from colliding neutron stars. Earlier detections have been from black holes that merged. More importantly, for the first time, it has been possible to detect both gravitational waves and electromagnetic radiation as light from the same event.

Gravitational waves, gamma-ray bursts and kilonova confirm theory

After the gravitational wave detectors had detected the collision between the neutron stars, less than two seconds passed before a short gamma-ray burst was detected by the orbiting telescopes, Fermi and Integral. A gamma-ray burst is a powerful pulse of electromagnetic radiation at very high energies (gamma-rays). Now we know that short gamma-ray bursts can be emitted when neutron stars collide.

With data from the gravitational wave detectors and space telescopes, astronomers could locate the area of the sky where the neutron stars had collided. A large number of telescopes started mapping this area of the sky, and after 11 hours of hunting, a new, luminous source was discovered - the kilonova.

It is most easily observed from the southern hemisphere; telescopes in Chile were therefore first to discover the source. Enrico Ramirez-Ruiz, Niels Bohr Professor at the Niels Bohr Institute with a grant of 30 million DKK from the Danish National Research Foundation, was on the team that first saw the kilonova. He also participated in the further work of interpreting the large quantities of data that flowed in.

"It's a huge discovery, perhaps the biggest in my lifetime. Finally, we managed to observe one of the most exotic phenomena in the universe, and we caught both electromagnetic waves and gravity waves. We were lucky, but luck favours the prepared and we were ready," he says.

The combination of gravity waves from colliding neutron stars, a gamma-ray burst and a kilonova was precisely what the researchers at the Niels Bohr Institute had hoped to observe. Now it is clear that the three phenomena are connected. It is no longer just a theory, but confirmed with observations.

"It's amazing that we both caught gravitational waves, gamma-ray bursts and optical light from the collision. That's exactly what we wanted to do, but we did not expect it to be so fast. We thought we might have to wait several years. But it all happened at once, "says Jens Hjorth, who received a grant of 40 million DKK from the Villum Foundation earlier this year, so that he, as a Villum Investigator, could research how elements are created in cosmic explosions.

Daniele Malesani, who is a postdoc at DARK, and who also played a major role in the discovery, completes the picture, "This is the discovery of a brand new astronomical object. Now we are sure that kilonova exist and that they are formed when neutron stars collide. Our predictions have come true. It is hugely satisfying that for years we have prepared ourselves for this event and that it happened as we expected. We could hardly believe it -- it's almost too good to be true.


Jens Hjorth, professor and head of Dark Cosmology Centre at the Niels Bohr Institute at the University of Copenhagen, Phone: +45 3532-5928, Email: jens@dark-cosmology.dk