Cosmic fireball sheds light on source of gravitational waves

Colliding neutron stars have confirmed theories about gamma-ray bursts and the origin of gold and other heavy elements in the universe. Michael Lucy reports.

Artist’s illustration of two merging neutron stars.
Artist’s illustration of two merging neutron stars. The rippling space-time grid represents gravitational waves that travel out from the collision, while the narrow beams show the bursts of gamma rays that are shot out just seconds after the gravitational waves. Swirling clouds of material ejected from the merging stars are also depicted. The clouds glow with visible and other wavelengths of light.
NSF/LIGO/Sonoma State University/A. Simonnet

“I can’t think of a similar situation in the field of science in my lifetime where a single event provides so many staggering insights about our universe.”

Astronomer Daniel Holz at the University of Chicago is referring to a collision that took place 130 million light-years away between two neutron stars. It was detected on 17 August 2017, marking the first time scientists have ever witnessed such a cataclysm. They could do it because the two gravitational wave detectors of the US-based Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo detector in Italy alerted them to the event. Astronomers could then point their telescopes to watch.

Together the observations confirm long-standing theories about how such an event would unfold. Papers about the findings have been published in Physical Review Letters, Science and Nature. A final compendium submitted to the Astrophysical Journal Letters has 3,500 authors – close to a third of the world’s astronomers.

The arrows show the optical counterpart to the gravitational-wave source.
The arrows show the optical counterpart to the gravitational-wave source. (The bright spot in the centre is the host galaxy NGC 4993.) The left image is from August 17, 2017, 11 hours after the detection of the gravitational-wave source. The right image is from four days later, when the source has faded significantly and its color became much redder.
1M2H/UC Santa Cruz and Carnegie Observatories/Ryan Foley

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 burst, 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 were 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.”

A periodic table of the elements showing the astronomical sources of each one.
A periodic table of the elements showing the astronomical sources of each one. A significant fraction of the heavy elements, shown in yellow, are produced by merging neutron stars.

Columbia University’s Brian Metzger had predicted just such an object in 2010, and coined the name kilonova, expecting it would be as bright as a thousand novae. Over the next few days the kilonova radiated light waves across the spectrum, from X-rays to blue to infrared.

The gamma-ray burst was an immediate result of the collision; the subsequent kilonova light was released by the shroud of material ejected during and after. Neutrons combined with surrounding elements to create heavy elements. From their spectral signatures we now know the heaviest elements are created in such cataclysms, solving a 60-year-old mystery since E. Margaret Burbidge, Fred Hoyle and colleagues showed elements heavier than iron were too unstable to form within stars.

The grandaddy finding of them all is the near-simultaneous arrival of the gravitational chirp and the gamma-ray burst (just two seconds apart). That confirms Einstein’s 100-year-old prediction that gravitational waves travel at the speed of light.

Astronomers are gobsmacked that so many long-standing predictions 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.

Michael Lucy is features editor of Cosmos.
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