The 2015 detection of gravitational waves – ripples in the very fabric of space and time – was one of the biggest scientific breakthroughs in a century. But because it was caused by two black holes merging, the event was all but invisible, detectable indirectly via the LIGO facility. Now a team of scientists has announced the fifth detection of gravitational waves, but there's a groundbreaking difference this time around: the ripples were caused by the collision of two neutron stars, meaning the event was accompanied by light, radio, and other electromagnetic signals for the first time.
First predicted by Albert Einstein over 100 years ago, gravitational waves are caused by cosmic cataclysms like the collision of two black holes, but because of the immense distance, by the time they reach us here on Earth the distortions are occurring on the subatomic scale. To observe waves that tiny, LIGO (Laser Interferometer Gravitational-wave Observatory) beams lasers down a 4-km (2.5-mi) long tunnel and measures how gravitational waves might warp the beam as they wash over our local corner of spacetime. That delicate process is effective at confirming the phenomenon, but still somewhat indirect.
This latest detection marks the fifth time gravitational waves have been observed, but it's by far the most important case since the initial discovery in September 2015. That's because this time black holes weren't responsible: instead, the source was a pair of neutron stars colliding, meaning their merger was far more visible.
"This is the first time that the collision of two neutron stars has been detected, and this is the closest and most precisely located gravitational wave signal we've received," says Susan Scott, the Leader of the General Relativity Theory and Data Analysis Group at Australian National University (ANU), which played a key role in the observation. "It is also the loudest gravitational wave signal we've detected."
The collision occurred in a galaxy called NGC 4993, which lies about 130 million light-years away – that might sound far, but it's much closer than previous observations, which occurred at distances of billions of light-years.
As well as producing gravitational waves, the neutron stars' collision sent a host of electromagnetic signals sweeping across the universe, including a short gamma ray burst, X-rays, light and radio waves. These were picked up by observatories all over the world, helping pinpoint the source. ANU was among those, using SkyMapper and the Siding Spring Observatory in New South Wales, Australia, to observe the brightness and color of the light signals given off.
"We saw the light from a fireball blasting out from the neutron star collision into space in the hours and days afterwards," says Christian Wolf, ANU astronomer. "SkyMapper was the first telescope to report the color of the fireball, which indicates the temperature of the fireball was about 6,000º C (10,800º F) – roughly the surface temperature of the Sun."
Along with learning more about gravitational waves, the discovery can teach astronomers about neutron stars. Created when larger stars collapse, neutron stars are relatively tiny – only about 10 km (6.2 mi) wide – and incredibly dense, with very strong magnetic fields. Other than that, not a whole lot is known about them.
"With this discovery we have the opportunity to learn so much more about neutron stars, which have been quite a mystery to us," says Scott. "Unlike black holes, neutron star collisions emit other signals such as gamma rays, light and radio waves so astronomers around the world were able to observe the event through telescopes. This is an amazing time to be a scientist."
The research was published in the journals Physical Review Letters, Nature and Astrophysical Journal Letters.
Source: Australian National University
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