Researchers have detected gravitational waves reverberating from a pair of colliding neutron stars and light produced by the subsequent fireball. The feat of detecting both types of signals has allowed astronomers to gaze at the colliding neutron stars and confirm long-standing theories about how such a cosmic cataclysm would unfold.
The achievement was announced by astronomers from the US-based Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo detector in Italy at simultaneous press conferences in Washington, DC, and Garching, Germany.
“We have talked about multi-messenger astronomy for a long time,” says David Blair, a physicist at the University of Western Australia who is a member of the LIGO collaboration, “Suddenly it’s an actual reality.”
The dual broadcast of gravitational waves – tiny ripples in spacetime predicted by Einstein – and light waves reached Earth on 17 August after a journey of some 130 million light-years. A paper describing the scientific results is published in the journal Physical Review Letters, with a flurry of other papers to appear soon in Nature, Science and other journals.
This is the first time that gravitational waves given off by a pair of colliding neutron stars have been detected. The event – dubbed GW170817 – also marks the first time a gravitational wave signal has been matched to observations of gamma rays and visible light, and may provide an answer to a long-standing mystery about the origin of heavy elements such as gold and platinum.
There have been four previous detections of gravitational waves, all of which came from colliding black holes. But black holes stay black even when they collide: they don’t produce a flash or give off any other radiation.
Neutron stars are different. Though small – only 10 or 20 kilometres across – they comprise such a dense neutron soup that they weigh more than the Sun. Intense beams of radiation fire from their magnetic poles as they spin, which makes them appear to pulse on and off. Another difference from black holes is that their radiation isn’t trapped by gravity. So optical astronomers have long predicted that the collision of neutron stars would give them plenty to see, if only they knew where and when to look.
Enter gravitational wave detectors like LIGO. By detecting the gravitational ripples from colliding neutrons stars, they would give optical astronomers the heads up for where to point their telescopes. “Gravitational wave detectors were all designed for neutron star binaries,” explains Blair.
The downside to detecting gravitational ripples from colliding neutron stars is that, being less massive than black holes, the gravitational waves they produce are weaker. On the other hand, neutron star collisions are much more common. “There are maybe 10,000 or 100,000 black hole mergers in the universe every year, but neutron stars might be more like one every second,” says Blair.
At 12:41 GMT on 17 August, optical astronomers got the heads up they were waiting for. The LIGO detectors at opposite ends of the US – one in Hanford, Washington and the other in Livingston, Louisiana – both detected a distortion in spacetime, one ten-thousandth the diameter of a proton, caused by gravitational waves. Two seconds later, NASA’s Fermi space telescope (which always keeps an eye out for gamma ray bursts) spied one coming from the same part of the sky.
The featherweight masses of the objects involved – between 1.1 and 1.6 times that of the Sun – identified them as neutron stars. Combining the LIGO and Fermi results with a much weaker signal from Italy’s Virgo gravitational wave detector allowed astronomers to triangulate the coordinates. Almost at once, email bulletins went out around the world and optical telescopes soon traced the source to a new bright spot near the elliptical galaxy NGC 4993 in the southern skies.
Just as they had predicted, the optical astronomers saw that the extremely bright flash of the gamma ray burst (GRB) was followed by a massive longer-lived fireball called a kilonova. The gamma ray bursts, shining a million trillion times more brightly than the Sun, was an immediate result of the collision, while the subsequent kilonova was due to the radioactive decay of heavy atoms formed when the neutron-rich guts of the compressed star was liberated from the overwhelming pressure of the interior.
Astronomers are gobsmacked that their long-standing predictions of universe-shattering events have been confirmed in the blink of an eye. “We didn’t expect to see this so soon,” says Eric Howell of the University of Western Australia, who studies neutron stars, gravitational waves and gamma ray bursts. The LIGO detector is not yet at full strength. That’s expected around 2020. “Even at [full] design sensitivity and at its greatest astronomical reach, we thought there was only around a 50 per cent chance to see a gravitational wave associated with gamma-ray burst.”
“It was clear as can be,” says Blair. “The gravitational waves told us about the neutron stars coalescing, we confirmed it was the source of the gamma ray burst, and then we saw the light from the kilonova.”
The combination of the three confirms that neutron star collisions are at least one source of short gamma ray bursts, which have until now puzzled astronomers. The simultaneous appearance also proves they produce kilonovas, which are believed to be the source of the majority of heavy elements in the universe.
As a bonus, the near-simultaneous arrival of the LIGO signal and the gamma ray burst confirms that gravitational waves travel at the speed of light, as predicted by the theory of relativity, and provides a new method to determine the Hubble constant, which measures the rate of expansion of the universe.
Like all gravitational wave discoveries to date, this one has been shrouded in secrecy. The announcement confirms rumours that have circulated since sharp-eyed observers noticed a large number of telescopes all turned to look in the direction of NGC 4993, and an astronomer or two made indiscreet comments on Twitter.
“I think we are converging towards a time when LIGO/Virgo alerts will become public and will be distributed within minutes,” Howell says.
Eventually there may even be advance publicity for cosmic spectacles like this. The next-generation Laser Interferometer Space Antenna (LISA), a space-based gravitational wave detector planned to begin operation in 2028, will be able to detect the signs of an approaching collision a month or two in advance.
“It could actually tell you OK, there’s going to be a binary black hole merger about now, somewhere about there,” says Howell. “People will be tuning in to Youtube to watch it live.”