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Gravitational waves produced by colliding neutron stars detected for the first time

Published: 16 October 2017
Impression of merging neutron stars
Artists impression of merging neutron stars. Credit: ESO/L Calçada/M Kornmesser

Scientists have directly detected gravitational waves — ripples in space and time — in addition to light from the spectacular collision of two neutron stars. This marks the first time that a cosmic event has been viewed in both gravitational waves and light.

The observation was made by a large, international team of scientists, including researchers from Mathematical Sciences and Physics and Astronomy at the University of Southampton.

The detection was made using the US-based Laser Interferometer Gravitational-Wave Observatory (LIGO); the Europe-based Virgo detector; and some 70 ground and space-based observatories.

Neutron stars are the smallest, densest stars known to exist and are formed when massive stars explode in supernovae. As these particular neutron stars spiralled together, they emitted gravitational waves that were detectable for about 100 seconds; when they collided, a flash of light in the form of gamma rays was emitted and seen on Earth about two seconds after the gravitational waves. In the days and weeks following the collision, other forms of light, or electromagnetic radiation — including X-ray, ultraviolet, optical, infrared, and radio waves — were detected.

Confirmation of the existence of gravitational waves was first announced in February 2016, following the detection of two colliding black holes in 2015.

Professor Ian Jones, a mathematician at the University of Southampton, has spent 14 years working on the international gravitational wave detection project, providing colleagues with models for what the gravitational wave signals from neutron stars might look like and advising how best to search for these signals amongst other ‘noisy’ data.

Professor Jones comments: “For thousands of years we have used light to study the heavens.  In 2015 we learnt how to use gravity itself to do astronomy, when the LIGO detectors picked up a signal from two colliding black holes.  Now, for the first time, we have used both gravity and light together, to see two neutron stars, each with a mass greater than the Sun, crashing into one another at a tremendous speed.

“The ripples in gravity we detected, along with the electromagnetic observations made by our colleagues of the accompanying explosion and glowing fireball, show that the era of multi-messenger astronomy has truly arrived.”

Artist's impression of two merging neutron stars.
Artist's impression. Credit: NSF / LIGO / Sonoma State University

The University of Southampton had scientists working on both elements of the observation – to detect the neutron star event using gravitational waves and to detect the event using light. Professor Jones, Dr Wynn Ho and PhD student Emma Osborne, are part of the 1,200 strong LIGO team, made up of scientists from 16 different countries around the world. Meanwhile, Professor Mark Sullivan and Postdoctoral Fellow Dr Cosimo Inserra, of Physics and Astronomy, are part of the ePESSTO collaboration which led a Nature paper on the electromagnetic observation of this new neutron star event – revealing a transient that has physical parameters that broadly match the theoretical predictions from neutron-star mergers (a ‘kilonova’), as well as the first direct evidence that such events are a major source for the synthesis of elements heavier then iron.

Dr. Inserra comments “The optical observations we made of this gravitational wave source revealed an astronomical event unlike any other previously observed. Our data show that events like this can be a major source for creating the very heaviest elements in the universe.”

The LIGO-Virgo results are published today (16 October 2017) in the journal Physical Review Letters, with the ePESSTO observations of the electromagnetic counterpart published in Nature. Additional papers from the LIGO and Virgo collaborations and the astronomical community have been either submitted or accepted for publication in various journals.

Approximately 130 million years ago, the two neutron stars detected were in their final moments of orbiting each other, separated only by about 300 kilometres, or 200 miles – gathering speed while closing the distance between them. As the stars spiralled faster and closer together, they stretched and distorted the surrounding space-time, giving off energy in the form of powerful gravitational waves, before smashing into each other.

At the moment of collision, the bulk of the two neutron stars merged into one ultra-dense object, emitting a ‘fireball’ of gamma rays. The initial gamma-ray measurements, combined with the gravitational-wave detection, also provide confirmation for Einstein’s general theory of relativity, which predicts that gravitational waves should travel at the speed of light.

In the weeks and months ahead, telescopes around the world will continue to observe the afterglow of the neutron star merger and gather further evidence about its various stages, its interaction with its surroundings, and the processes that produce the heaviest elements in the universe.

LIGO is funded by the NSF, and operated by Caltech and MIT, which conceived of LIGO and led the Initial and Advanced LIGO projects. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the UK (Science and Technology Facilities Council) and Australia (Australian Research Council) making significant commitments and contributions to the project. More than 1,200 scientists and some 100 institutions from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration and the Australian collaboration OzGrav. Additional partners are listed at http://ligo.org/partners.php

The Virgo collaboration consists of more than 280 physicists and engineers belonging to 20 different European research groups: six from Centre National de la Recherche Scientifique (CNRS) in France; eight from the Istituto Nazionale di Fisica Nucleare (INFN) in Italy; two in the Netherlands with Nikhef; the MTA Wigner RCP in Hungary; the POLGRAW group in Poland; Spain with the University of Valencia; and the European Gravitational Observatory, EGO, the laboratory hosting the Virgo detector near Pisa in Italy, funded by CNRS, INFN, and Nikhef.

Notes for editors

i) The gravitational signal, named GW170817, was first detected on 17 August 2017 at 8:41 a.m. Eastern Daylight Time; the detection was made by the two identical LIGO detectors, located in Hanford, Washington, and Livingston, Louisiana. The information provided by the third detector, Virgo, situated near Pisa, Italy, enabled an improvement in localising the cosmic event. At the time, LIGO was nearing the end of its second observing run since being upgraded in a program called Advanced LIGO, while Virgo had begun its first run after recently completing an upgrade known as Advanced Virgo.

ii) Each detector consists of two long tunnels arranged in an L shape, at the joint of which a laser beam is split in two. Light is sent down the length of each tunnel, then reflected back in the direction it came from by a suspended mirror. In the absence of gravitational waves, the laser light in each tunnel should return to the location where the beams were split at precisely the same time. If a gravitational wave passes through the observatory, it will alter each laser beam’s arrival time, creating an almost imperceptible change in the observatory’s output signal. https://www.ligo.caltech.edu/page/ligo-detectors

iii) The Gravity Group at the University of Southampton is a leading centre in the modelling of relativistic systems and their gravitational wave emission. For more information visit:  http://www.southampton.ac.uk/stag/about/research-gravity.page

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