A neutron star collision led to a rumble of gravitational waves and a worldwide race to spot the resulting kilonova. The dozens of studies coming out of the effort will “go down in the history of astronomy.”
Quanta Magazine said:On Aug. 17, the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detected something new. Some 130 million light-years away, two super-dense neutron stars, each as small as a city but heavier than the sun, had crashed into each other, producing a colossal convulsion called a kilonova and sending a telltale ripple through space-time to Earth.
When LIGO picked up the signal, the astronomer Edo Berger was in his office at Harvard University suffering through a committee meeting. Berger leads an effort to search for the afterglow of collisions detected by LIGO. But when his office phone rang, he ignored it. Shortly afterward, his cellphone rang. He glanced at the display to discover a flurry of missed text messages:
Edo, check your email!
Pick up your phone!
“I kicked everybo*dy out that very moment and jumped into action,” Berger said. “I had not expected this.”
LIGO’s pair of ultrasensitive detectors in Louisiana and Washington state made history two years ago by recording the gravitational waves coming from the collision of two black holes — a discovery that earned the experiment’s architects the Nobel Prize in Physics this month. Three more signals from black hole collisions followed the initial discovery.
Yet black holes don’t give off light, so making any observations of these faraway cataclysms beyond the gravitational waves themselves was unlikely. Colliding neutron stars, on the other hand, produce fireworks. Astronomers had never seen such a show before, but now LIGO was telling them where to look, which sent teams of researchers like Berger’s scurrying to capture the immediate aftermath of the collision across the full range of electromagnetic signals. In total, more than 70 telescopes swiveled toward the same location in the sky.
They struck the motherlode. In the days after Aug. 17, astronomers made successful observations of the colliding neutron stars with optical, radio, X-ray, gamma-ray, infrared and ultraviolet telescopes. The enormous collaborative effort, detailed today in dozens of papers appearing simultaneously in Physical Review Letters, Nature, Science, Astrophysical Journal Letters and other journals, has not only allowed astrophysicists to piece together a coherent account of the event, but also to answer longstanding questions in astrophysics.
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The merger also solved another mystery that has vexed astrophysicists for the past five decades.
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A neutron star merger should trigger a very strong gamma-ray burst, with most of the energy released in a fairly narrow beam called a jet. The researchers believe that the GRB signal hitting Earth was weak only because the jet was pointing at an angle away from us. Proof arrived about two weeks later, when observatories detected the X-ray and radio emissions that accompany a GRB. “This provides smoking-gun proof that normal short gamma-ray bursts are produced by neutron-star mergers,” Berger said. “It’s really the first direct compelling connection between these two phenomena.”
Hughes said that the observations were the first in which “we have definitively associated any short gamma-ray burst with a progenitor.” The findings indicate that at least some GRBs come from colliding neutron stars, though it’s too soon to say whether they all do.
Striking Gold
Optical and infrared data captured after the neutron-star merger also help clarify the formation of the heaviest elements in the universe, like uranium, platinum and gold, in what’s called r-process nucleosynthesis. Scientists long believed that these rare, heavy elements, like most other elements, are made during high-energy events such as supernovas. A competing theory that has gained prominence in recent years argues that neutron-star mergers could forge the majority of these elements. According to that thinking, the crash of neutron stars ejects matter in what’s called a kilonova. “Once released from the neutron stars’ gravitational field,” the matter “would transmute into a cloud full of the heavy elements we see on rocky planets like Earth,” Dent explained.
Optical telescopes picked up the radioactive glow of these heavy elements — strong evidence, scientists say, that neutron-star collisions produce much of the universe’s supply of heavy elements like gold.
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