Quanta Magazine

https://www.quantamagazine.org/neutron-star-collision-shakes-space-time-and-lights-up-the-sky-20171016/ October 16, 2017

Neutron-Star Collision Shakes Space-Time andLights Up the SkyA neutron star collision led to a rumble of gravitational waves and a worldwide race to spot theresulting kilonova. The dozens of studies coming out of the effort will “go down in the history ofastronomy.”

By Katia Moskvitch

Ana Kova for Quanta Magazine

On Aug. 17, the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detectedsomething new. Some 130 million light-years away, two super-dense neutron stars, each as small asa city but heavier than the sun, had crashed into each other, producing a colossal convulsion called akilonova 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 Universitysuffering through a committee meeting. Berger leads an effort to search for the afterglow ofcollisions detected by LIGO. But when his office phone rang, he ignored it. Shortly afterward, hiscellphone rang. He glanced at the display to discover a flurry of missed text messages:

Edo, check your email!

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Pick up your phone!

“I kicked everybody out that very moment and jumped into action,” Berger said. “I had not expectedthis.”

LIGO’s pair of ultrasensitive detectors in Louisiana and Washington state made history two yearsago by recording the gravitational waves coming from the collision of two black holes — a discoverythat earned the experiment’s architects the Nobel Prize in Physics this month. Three more signalsfrom black hole collisions followed the initial discovery.

Yet black holes don’t give off light, so making any observations of these faraway cataclysms beyondthe gravitational waves themselves was unlikely. Colliding neutron stars, on the other hand, producefireworks. Astronomers had never seen such a show before, but now LIGO was telling them where tolook, which sent teams of researchers like Berger’s scurrying to capture the immediate aftermath ofthe collision across the full range of electromagnetic signals. In total, more than 70 telescopesswiveled toward the same location in the sky.

They struck the motherlode. In the days after Aug. 17, astronomers made successful observations ofthe 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 inPhysical Review Letters, Nature, Science, Astrophysical Journal Letters and other journals, has notonly allowed astrophysicists to piece together a coherent account of the event, but also to answerlongstanding questions in astrophysics.

“In one fell swoop, gravitational wave measurements” have opened “a window onto nuclearastrophysics, neutron star demographics and physics and precise astronomical distances,” said ScottHughes, an astrophysicist at the Massachusetts Institute of Technology’s Kavli Institute forAstrophysics and Space Research. “I can’t describe in family-friendly words how exciting that is.”

Today, Berger said, “will go down in the history of astronomy.”

X Marks the SpotWhen Berger got the calls, emails, and the automated official LIGO alert with the probablecoordinates of what appeared to be a neutron-star merger, he knew that he and his team had to actquickly to see its aftermath using optical telescopes.

The timing was fortuitous. Virgo, a new gravitational-wave observatory similar to LIGO’s twodetectors, had just come online in Europe. The three gravitational-wave detectors together wereable to triangulate the signal. Had the neutron-star merger occurred a month or two earlier, beforeVirgo started taking data, the “error box,” or area in the sky that the signal could have come from,would have been so large that follow-up observers would have had little chance of finding anything.

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https://www.quantamagazine.org/neutron-star-collision-shakes-space-time-and-lights-up-the-sky-20171016/ October 16, 2017

LIGO Lab/Caltech/MIT

The LIGO gravitational-wave detectors in Hanford, Washington (left), and Livingston, Louisiana.

The LIGO and Virgo scientists had another stroke of luck. Gravitational waves produced by mergingneutron stars are fainter than those from black holes and harder to detect. According to ThomasDent, an astrophysicist at the Albert Einstein Institute in Hannover, Germany, and a member ofLIGO, the experiment can only sense neutron-star mergers that occur within 300 million light-years.This event was far closer — at a comfortable distance for both LIGO and the full range ofelectromagnetic telescopes to observe it.

But at the time, Berger and his colleagues didn’t know any of that. They had an agonizing wait untilsunset in Chile, when they could use an instrument called the Dark Energy Camera mounted on theVictor M. Blanco telescope there. The camera is great when you don’t know precisely where you’relooking, astronomers said, because it can quickly scan a very large area of the sky. Berger alsosecured use of the Very Large Array (VLA) in central New Mexico, the Atacama Large MillimeterArray (ALMA) in Chile and the space-based Chandra X-ray Observatory. (Other teams that receivedthe LIGO alert asked to use VLA and ALMA as well.)

A few hours later, data from the Dark Energy Camera started coming in. It took Berger’s team 45minutes to spot a new bright light source. The light appeared to come from a galaxy called NGC4993 in the constellation Hydra that had been pointed out in the LIGO alert, and at approximatelythe distance where LIGO had suggested they look.

“That got us really excited, and I still have the email from a colleague saying ‘Holy [smokes], look atthat bright source near this galaxy!’” Berger said. “All of us were kind of shocked,” since “we didn’tthink we would succeed right away.” The team had expected a long slog, maybe having to wadethrough multiple searches after LIGO detections for a couple of years until eventually spottingsomething. “But this just stood out,” he said, “like when an X marks the spot.”

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Fermilab

The Dark Energy Camera, mounted on the Victor M. Blanco Telescope (center), was one of at least six instrumentsthat spotted the new bright light source from the neutron-star merger soon after it was detected by LIGO.

Meanwhile, at least five other teams discovered the new bright light source independently, andhundreds of researchers made various follow-up observations. David Coulter, an astronomer atUniversity of California, Santa Cruz, and colleagues used the Swope telescope in Chile to pinpointthe event’s exact location, while Las Cumbres Observatory astronomers did so with the help of arobotic network of 20 telescopes around the globe.

For Berger and the rest of the Dark Energy Camera follow-up team, it was time to call in the HubbleSpace Telescope. Securing time on the veteran instrument usually takes weeks, if not months. Butfor extraordinary circumstances, there’s a way to jump ahead in line, by using “director’sdiscretionary time.” Matt Nicholl, an astronomer at the Harvard-Smithsonian Center forAstrophysics, submitted a proposal on behalf of the team to take ultraviolet measurements withHubble — possibly the shortest proposal ever written. “It was two paragraphs long — that’s all wecould do in the middle of the night,” Berger said. “It just said that we’ve found the first counterpartof a binary neutron star merger, and we need to get UV spectra. And it got approved.”

As the data trickled in from the various instruments, the collected data set was becoming more andmore astounding. In total, the original LIGO/Virgo discovery and the various follow-up observationsby scientists have yielded dozens of papers, each describing astrophysical processes that occurredduring and after the merger.

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Mystery BurstsNeutron stars are compact neutron-packed cores left over when massive stars die in supernovaexplosions. A teaspoon of neutron star would weigh as much as one billion tons. Their internalstructure is not completely understood. Neither is their occasional aggregation into close-knit binarypairs of stars that orbit each other. The astronomers Joe Taylor and Russell Hulse found the firstsuch pair in 1974, a discovery that earned them the 1993 Nobel Prize in Physics. They concludedthat those two neutron stars were destined to crash into each other in about 300 million years. Thetwo stars newly discovered by LIGO took far longer to do so.

The analysis by Berger and his team suggests that the newly discovered pair was born 11 billionyears ago, when two massive stars went supernova a few million years apart. Between these twoexplosions, something brought the stars closer together, and they went on circling each other formost of the history of the universe. The findings are “in excellent agreement with the models ofbinary-neutron-star formation,” Berger said.

The merger also solved another mystery that has vexed astrophysicists for the past five decades.

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NSF/LIGO/Sonoma State University/A. Simonnet

Artist’s rendering of the neutron-star merger depicting a gamma-ray burst and ejected material swirling around the

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merging stars.

On July 2, 1967, two United States satellites, Vela 3 and 4, spotted a flash of gamma radiation.Researchers first suspected a secret nuclear test conducted by the Soviet Union. They soon realizedthis flash was something else: the first example of what is now known as a gamma ray burst (GRB),an event lasting anywhere from milliseconds to hours that “emits some of the most intense andviolent radiation of any astrophysical object,” Dent said. The origin of GRBs has been an enigma,although some people have suggested that so-called “short” gamma-ray bursts (lasting less than twoseconds) could be the result of neutron-star mergers. There was no way to directly check until now.

In yet another nod of good fortune, it so happened that on Aug. 17, the Fermi Gamma-Ray SpaceTelescope and the International Gamma-Ray Astrophysics Laboratory (Integral) were pointing in thedirection of the constellation Hydra. Just as LIGO and Virgo detected gravitational waves, thegamma-ray space telescopes picked up a weak GRB, and, like LIGO and Virgo, issued an alert.

A neutron star merger should trigger a very strong gamma-ray burst, with most of the energyreleased in a fairly narrow beam called a jet. The researchers believe that the GRB signal hittingEarth was weak only because the jet was pointing at an angle away from us. Proof arrived about twoweeks 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-starmergers,” Berger said. “It’s really the first direct compelling connection between these twophenomena.”

Hughes said that the observations were the first in which “we have definitively associated any shortgamma-ray burst with a progenitor.” The findings indicate that at least some GRBs come fromcolliding neutron stars, though it’s too soon to say whether they all do.

Striking GoldOptical and infrared data captured after the neutron-star merger also help clarify the formation ofthe heaviest elements in the universe, like uranium, platinum and gold, in what’s called r-processnucleosynthesis. 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 gainedprominence in recent years argues that neutron-star mergers could forge the majority of theseelements. According to that thinking, the crash of neutron stars ejects matter in what’s called akilonova. “Once released from the neutron stars’ gravitational field,” the matter “would transmuteinto 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 elementslike gold.

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Kris Snibbe/Harvard Staff Photographer

Edo Berger, an astronomer at Harvard University and principal investigator of a team that uses the Dark EnergyCamera to follow up on LIGO detections.

“With this merger,” Berger said, “we can see all the expected signatures of the formation of theseelements, so we are solving this big open question in astrophysics of how these elements form. Wehad hints of this before, but here we have a really nearby object with exquisite data, and there is noambiguity.” According to Daniel Holz, an astrophysicist at the University of Chicago, “back-of-the-envelope calculations indicate that this single collision produced an amount of gold greater than theweight of the Earth.”

The scientists also inferred a sequence of events that may have followed the neutron-star collision,providing insight into the stars’ internal structure. Experts knew that the collision outcome “dependsvery much on how large the stars are and how ‘soft’ or ‘springy’ — in other words, how much theyresist being deformed by super-strong gravitational forces,” Dent said. If the stars are extra soft,they may immediately be swallowed up inside a newly formed black hole, but this would not leaveany matter outside to produce a gamma-ray burst. “At the other end of the scale, he said, “the twoneutron stars would merge and form an unstable, rapidly spinning super-massive neutron star,which could produce a gamma-ray burst after a holdup of tens or hundreds of seconds.”

The most plausible case may lie somewhere in the middle: The two neutron stars may have mergedinto a doughnut-shaped unstable neutron star that launched a jet of super-energetic hot matterbefore finally collapsing as a black hole, Dent said.

Future observations of neutron-star mergers will settle these questions. And as the signals roll in,

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experts say the mergers will also serve as a precision tool for cosmologists. Comparing thegravitational-wave signal with the redshift, or stretching, of the electromagnetic signals offers a newway of measuring the so-called Hubble constant, which gives the age and expansion rate of theuniverse. Already, with this one merger, researchers were able to make an initial measurement ofthe Hubble constant “in a remarkably fundamental way, without requiring the multitude ofassumptions” that go into estimating the constant by other methods, said Matthew Bailes, a memberof the LIGO collaboration and a professor at the Swinburne University of Technology in Australia.Holz described the neutron star merger as a “standard siren” (in a nod to the term “standardcandles” used for supernovas) and said that initial calculations suggest the universe is expanding ata rate of 70 kilometers per second per megaparsec, which puts LIGO’s Hubble constant “smack inthe middle of [previous] estimates.”

To improve the measurement, scientists will have to spot many more neutron-star mergers. Giventhat LIGO and Virgo are still being fine-tuned to increase their sensitivity, Berger is optimistic. “It isclear that the rate of occurrence is somewhat higher than expected,” he said. “By 2020 I expect atleast one to two of these every month. It will be tremendously exciting.”