Gravitational waves from colliding neutron stars usher in new era of astronomy

The lenticular galaxy NGC 4993, with an arrow pointing to the afterglow of the kilonova. Image: ESO

For the first time, gravitational waves have been detected coming from the violent collision of two neutron star potentially solving the mystery of where heavy elements like gold come from as well as producing a visible afterglow detected by over 70 telescopes around the world.

On 17 August 2017 the LIGO (Laser Interferometer Gravitational-wave Observatory) detector in the United States, working with the Virgo experiment in Italy, detected gravitational waves rippling through our planet. These waves are undulations in the fabric of space-time, generated by the close interactions of massive bodies.

This was only the fifth time that gravitational waves have been detected, but the previous four events originated from the mergers of black holes with masses dozens of times that of our Sun. In each of those events the gravitational wave signal – what scientists call a ‘chirp’ – lasted just fractions of a second. In the case of this latest event, however, the chirp lasted for more than 60 seconds, chronicling the in-spiral not of two massive black holes colliding, but instead two neutron stars.

“This was the strongest gravitational wave signal that we’ve ever measured,” says Samaya Nissanke of Radboud University in the Netherlands. “The gravitational waves came from an entirely new type of source, a pair of neutron stars that were essentially losing energy due to the in-spiral from gravitational wave emission.”

A neutron star is formed when a massive star many times the size of our Sun explodes as a supernova, and what remains of the star’s core collapses under its own gravity. Neutron stars are only 20 or 30 kilometres across, but they pack the mass of an entire star inside them, with matter crushed so densely that all the protons and electrons merge to form neutron particles, hence their name. It’s often said that a teaspoon of neutron star material would weigh the same as Mount Everest. The estimated masses for the two neutron stars that merged in GW 170817 range between 0.86 and 2.26 times the mass of the Sun.

An artist’s impression of a kilonova – the merger two neutron stars during which gravitational waves are released. Image: ESO/L. Calçada/M. Kornmesser

So despite their small size, neutron stars are pretty hefty objects. When two stars orbiting each other in a binary system explode as supernovae, they leave behind a binary neutron star system. Over time the orbits of the neutron stars degrade and they spiral closer and closer together until they collide in an explosive event called a kilonova, releasing a powerful gamma-ray burst (GRB) that lasts just fractions of a second.

Two seconds after LIGO and Virgo had detected GW 170817, a short-duration GRB tripped the detectors on two orbiting spacecraft, the European Space Agency’s INTErnational Gamma-Ray Astrophysics Laboratory (known as INTEGRAL for short) and NASA’s Fermi gamma-ray space telescope. The GRB came from the same region of sky as the gravitational waves, near the Southern Hemisphere constellation of Hydra, and the timing was surely too much of a coincidence for the two events to be unrelated.

Observatories around the world were immediately alerted, with dozens of telescopes turning towards Hydra as a huge observing campaign took place to search for the afterglow of the kilonova. The Carnegie Institution’s new one-metre Swope Telescope, located at Cerro Las Campanas in Chile, was the first to spot the faint smudge of light, powered by the radioactive decay or elements, on the outskirts of the dusty galaxy NGC 4993, which is 130 million light years away.
“[The kilonova] is possibly the most unusual flash in the sky we have even seen,” says Stephen Smartt of Queen’s University Belfast.
It is the first time that an afterglow of light has been connected to a gravitational wave event, heralding a new era in astronomy that connects traditional observations in the electromagnetic spectrum with gravitational wave measurements, allowing scientists to fully characterise these events that are so extreme that they literally cause space to shake.

Merging neutron stars had for a long time been thought to be responsible for short GRBs and kilonovae, and the gravitational wave measurements now confirm that theory. Colliding neutron stars are not merely an astronomical curiosity, but they are also the source of many of the elements in the periodic table more massive than iron. Indeed, a study of the kilonova’s spectrum revealed the presence of heavy elements including caesium and tellurium, created in the intense energy of the explosion. The gold that you wear in your jewellery may well have been forged in an ancient kilonova.

Although the findings have answered many questions, some mysteries remain. At present it is unknown what type of object, if any, the kilonova left behind. Did the two merging neutron stars form a black hole, or did they combine into an even more massive neutron star? And do kilonovae produce all of the elements heavier than iron in the Universe, or just a fraction of them, in which case what are the other sources of these elements? Perhaps the next LIGO–Virgo observing run will provide new examples of kilonovae that will help solve these mysteries.