Detection of colliding neutron stars is just the beginning of a new era of gravitational wave astronomy

The new observations confirm that at least some short gamma-ray bursts are generated by the merging of neutron stars.

In yet another interstellar feat, scientists at the LIGO detectors and the world over detected the first "ripples in space" or gravitational waves produced by the merger of two ancient remnants of stars known as neutron stars. The neutron stars merged in a galaxy called NGC 4993, located about 130 million light years from Earth in the constellation Hydra. Seven new papers describe the first-ever detection of light from a gravitational wave source.

The neutron stars just after the collision. Image: CFA.

The neutron stars just after the collision. Image: CFA.

The event, caused by two neutron stars colliding and merging together, was dubbed "GW170817" because it sent ripples through space-time that reached Earth on 17 August. Around the world, hundreds of excited astronomers mobilised quickly and were able to observe the event using numerous telescopes, providing a wealth of new data. Previous detections of gravitational waves have all involved the merger of two black holes, a feat that won the 2017 Nobel Prize in Physics earlier in October.

However, black hole mergers are not expected to produce any electromagnetic radiation (light), meaning they cannot be detected by conventional telescopes. In contrast, binary neutron star (NS-NS) mergers have long been expected to produce an energetic explosion and a plume of radioactive material, generating light, but have never previously been detected. These mergers provide important clues as to how matter behaves under these extreme conditions.

The papers published online in Science describe how light from the NS-NS merger was precisely located, subsequent observations at X-ray, ultraviolet (UV), optical, infrared (IR) and radio wavelengths, and theoretical analysis of the event. "This is a milestone in the growing effort by scientists worldwide to unlock the mysteries of the universe and of earth," said Professor Ehud Nakar of Tel Aviv University's Raymond and Beverly Sackler School of Physics and Astronomy.

A neutron star forms when a star much bigger and brighter than the sun exhausts its thermonuclear fuel supply and explodes into a violent supernova. The explosion of neutron stars, which are made almost entirely of neutrons, was detected by multiple telescopes across the electromagnetic spectrum, from gamma rays and visible light to radio waves. The merger is the first cosmological event observed in both gravitational waves--ripples in the fabric of space-time--and the entire spectrum of light, from gamma rays to radio waves.

Gravitational waves from the event arrived first at the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Hanford, Washington, and Livingston, Louisiana. Because of the orientation of the neutron star pair, the newly operational Virgo detector, located near Pisa, Italy, observed a weaker signal. Less than two seconds later, the Gamma-ray Burst Monitor on NASA's Fermi Gamma-ray Space Telescope detected a short burst of gamma rays.

A rapid analysis of these signals enabled the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration to locate the signal in a region covering less than 0.1 per cent of the total sky area as viewed from Earth. Fermi independently identified a larger area consistent with that identified by LIGO and Virgo. Astronomers around the globe then directed more than 70 space- and ground-based telescopes toward the event for follow-up observations.

"This is the most intensely observed astronomical event in history," said Peter Shawhan, a professor of physics at University of Maryland (UMD) and an LSC principal investigator. "For the binary black hole mergers LIGO has already observed, the signals were much shorter--just a fraction of a second," said Alessandra Buonanno, a UMD College Park Professor of Physics and LSC principal investigator.

"We were able to track the evolution of GW170817 over time in a way we could not achieve for two black holes. As we continue to examine the signal, this wealth of data will likely allow us to pursue new and more stringent gravitational tests, which could put Einstein's theory of general relativity in question." Theorists have predicted that when neutron stars collide, they should give off gravitational waves and gamma rays, along with powerful jets that emit light across the electromagnetic spectrum.

The new observations confirm that at least some short gamma-ray bursts are generated by the merging of neutron stars--something that was only theorized before. But while one mystery appears to be solved, new mysteries have emerged. The observed short gamma-ray burst was one of the closest to Earth seen so far, yet it was surprisingly weak for its distance. Scientists are beginning to propose models for why this might be the case.

"While the results from this event are beyond my wildest dreams, the most exciting part is that this is really just the beginning," said Brad Cenko, principal investigator of the Swift Gamma-ray Burst Mission at NASA's Goddard Space Flight Centre.

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