Light captured alongside a gravitational wave for the first time ever

University of Bath astrophysicists have been closely involved in the first ever combined detection of both light and gravitational waves from the merging of two neutron stars, a cataclysmic cosmic event.

The findings, involving an international team of thousands using a global collection of gravitational wave detectors and ground- and space-based astronomical telescopes, heralds a new era in modern astrophysics and help us understand the most powerful and violent events in the Universe.

Neutron stars are the collapsed remnants produced in the violent death throes of massive stars; they are only about 20 kilometres in diameter, but contain more mass than our Sun – making them the smallest and densest stars in the universe. They are so dense that one sugar cube of neutron star material would weigh one billion tons.

Doomed neutron stars whirl toward their demise in this illustration. Gravitational waves (pale arcs) bleed away orbital energy, causing the stars to move closer together and merge. Credit: NASA's Goddard Space Flight Center/CI Lab

Einstein’s theory of general relativity predicted that ripples in space-time would be produced if dense cosmic objects spin around one another and finally coalesce. The first detection of gravitational waves from pairs of massive black holes were reported in the 2015 and resulted in the award of the 2017 Nobel Prize in Physics.

For the mergers of the black hole pairs, no light was detected, nor was this expected due to the light-trapping nature of black holes. Therefore scientists remained hopeful that they would eventually discover signatures of other dense objects merging to produce gravitational waves. In particular, a collision between two neutron stars held the promise of shedding light on a wide range of astrophysical theories about the nature of such events, their associated explosions and the material ejected into space at vast speeds during the blast. The expectation was that, as detections accumulated, the scientific community would slowly be able to bring the pieces of the puzzle together.

However on August 17 a single merging event was detected that delivered all the puzzle pieces in one swoop, when for the first time a range of sophisticated equipment picked up a series of signals from two neutron stars colliding. These included a gravity wave, a ripple in the fabric of space time, detected by the famous LIGO and VIRGO observatories. The gravity wave was quickly found to nearly coincide with a burst of gamma rays from detected by the Fermi satellite. The observatories swiftly alerted a world-wide association of astronomers with access to a vast suite of telescopes and satellites capable of detecting cosmic light spanning the entire electromagnetic spectrum, from radio waves to high-energy gamma rays.

This equipment captured the light from a kilo-nova, a type of radioactive explosion produced in the collision and predicted by theory.  Two weeks later, the Chandra satellite detected high-energy X-rays consistent with a gamma ray burst, the most powerful kind of explosion in the universe, a signature of the birth of a black hole and the launch of a beam of material at speeds close to that of light.

Taken together this series of measurements not only confirm Einstein’s predictions, but greatly advance our understanding of these massively energetic events. With one cosmic event, scientists have solved four breakthrough questions in modern astrophysics that will revolutionise the field of high-energy astrophysics: proof that gravitational waves are produced by merging neutron stars, confirmation of the origin and nature of short gamma-ray bursts, constraints on the violent production of heavy elements, such as gold, via the kilonova process, and the role of magnetic fields in this kind of extreme environment.

The results are published in a series of papers in Nature and Nature Astrophysics, Science, Physical Review Letters and Astrophysical Journal Letters.

NASA's Chandra X-ray Observatory first detected X-rays from the source of the neutron star collision known as GW170817. Credit: NASA/CXC/E. Troja

Dr Hendrik van Eerten, a computational astrophysicist from the Department of Physics at the University of Bath who led the theoretical input to one of the studies said: “This is a momentous event - the first ever detection from neutron stars colliding, and we have an unexpectedly rich dataset. In one way it’s serendipitous because we could have been waiting for decades, but getting everything ready to make these observations was 50 years in the preparation. It’s massive triumph for engineering and a revolution in high-energy astrophysics.”

Dr Van Eerten is one of the lead authors on a study published in Nature today, describing the detection of the X-rays associated with the Gamma Ray Burst. By using his models scientists could trace the source to the Gamma Ray Burst, and confirm the prediction from theory that X-rays would eventually be detectable even when the gas was being expelled at near light-speed in a direction other than our line of sight, as well as the more basic prediction that the gas actually had a direction to begin with instead of being expelled to all directions at once.

Professor Carole Mundell, who leads the Bath Astrophysics group and is a co-author on a related paper in Nature Astrophysics probing the symmetry of the kilo-nova and the presence of magnetic fields, said: “We have been pioneering the technology to search for light from systems producing gravitational waves for some time but we really didn’t expect to succeed for many years.  Suddenly we have data that validate long-standing predictions. It is a beautiful example of how the science and engineering together advance the frontiers of knowledge.

“This is the birth of a new area of physics and a new window on our universe. It’s very exciting as we now have a unique physical insight into what’s happening.”

Dr Van Eerten added: “Because we have measurements from several different phenomena, we have several different perspectives from the same event. This means using the data collectively we can make advances where otherwise we would have be left to speculate.”

When neutron stars collide, the explosion blasts some of the debris away in particle jets moving at nearly the speed of light, as shown in this illustration. Credit: NASA's Goddard Space Flight Center/CI Lab

The papers “The X-ray counterpart to the gravitational-wave event GW170817” (DOI: 10.1038/nature24290) and “The unpolarized macronova associated with the gravitational wave event GW 170817” (DOI: 10.1038/s41550-017-0285-z) are published in Nature and Nature Astrophysics respectively.

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