2017-10-16 10:16:02
LIGO Detects Fierce Collision of Neutron Stars for the First Time

10:16, October 16 541 0

This is the story of a gold rush in the sky.

Astronomers have now seen and heard a pair of dead stars collide, giving them the first glimpse of what they call a “cosmic forge,” where the world’s jewels were minted billions of years ago.

The collision rattled space-time and sent a wave of fireworks across the universe, setting off sensors in space and on Earth on Aug. 17 as well as producing a long loud chirp in antennas designed to study the Einsteinian ripples in the cosmic fabric known as gravitational waves. It set off a stampede around the world as astronomers scrambled to turn their telescopes in search of a mysterious and long-sought kind of explosion called a kilonova.

After two months of underground and social media rumblings, the first wave of news is being reported Monday about one of the least studied of cosmic phenomena: the merger of dense remnants known as neutron stars, the shrunken cores of stars that have collapsed and burst.

Such collisions are thought to have profoundly influenced the chemistry of the universe, creating many of the heavier elements in the universe, including almost all the precious metals like gold, silver, platinum and uranium. Which is to say that the atoms in your wedding band, in the pharaoh’s jewels and the bombs that destroyed Hiroshima and still threaten us all were formed in a cosmic gong show that reverberated across the heavens billions of years ago.

As astronomers gather for news conferences in several cities around the world, a blizzard of papers are being published, including one in The Astrophysical Journal Letters that has 4,500 authors — a third of all the professional astronomers in the world — from 910 institutions. “That paper almost killed the paperwriting team,” said Vicky Kalogera, a Northwestern University astrophysicist who was one of 10 people who did the actual writing.

More papers are appearing in Nature and in Science, on topics including nuclear physics and cosmology.

“It’s the greatest fireworks show in the universe,” said David Reitze of the California Institute of Technology and the executive director of the Laser Interferometer Gravitational-Wave Observatory, or LIGO.

Daniel Holz, an astrophysicist at the University of Chicago and a member of the LIGO Scientific Collaboration, a larger group that studies gravitational waves, said, “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.”

It was a century ago that Albert Einstein predicted that space and time could shake like a bowl of jelly when massive things like black holes moved around. But such waves were finally confirmed only in 2016, when LIGO recorded the sound of two giant black holes colliding, causing a sensation that eventually led this month to a Nobel Prize.

For the LIGO researchers, this is in some ways an even bigger bonanza than the original discovery. This is the first time they have discovered anything that regular astronomers could see and study. All of LIGO’s previous discoveries have involved colliding black holes, which are composed of empty tortured space-time — there is nothing for the eye or the telescope to see.

But neutron stars are full of stuff, matter packed at the density of Mount Everest in a teacup. When neutron stars slam together, all kinds of things burst out: gamma rays, X-rays, radio waves. Something for everyone who has a window on the sky.

“Joy for all,” said David Shoemaker, a physicist at the Massachusetts Institute of Technology who is the spokesman for the LIGO Scientific Collaboration.

It began on the morning of Aug. 17, Eastern time. Dr. Shoemaker was on a Skype call when alarms went off. One of the LIGO antennas, in Hanford, Wash., had recorded an auspicious signal and sent out an automatic alert. Twin antennas, in Washington and Louisiana, monitor the distance between a pair of mirrors to detect the submicroscopic stretching and squeezing of space caused by a passing gravitational wave. Transformed into sound, the Hanford signal was a long 100-second chirp, that ended in a sudden whoop to 1000 cycles per second, two octaves above middle C. Such a high frequency indicated that whatever was zooming around was lighter than a black hole.

Checking the data from Livingston to find out why it had not also phoned in an alert, Dr. Shoemaker and his colleagues found a big glitch partly obscuring the same chirp.

Meanwhile, the Fermi Gamma-Ray Space Telescope, which orbits Earth looking at the highest-energy radiation in the universe, recorded a brief flash of gamma rays just two seconds after the LIGO chirp. Fermi sent out its own alert. The gamma-ray burst lasted about two seconds, which put it in a category of short gamma ray bursts associated with the formation of black holes perhaps as a result of neutron stars colliding.

“When we saw that,” Dr. Shoemaker said, “the adrenaline hit.”

Dr. Kalogera, who was in Utah hiking and getting ready for August’s total solar eclipse when she got the alarm, recalled thinking: “Oh my God, this is it. This 50-year-old mystery, the holy grail, is solved.”

Together the two signals told a tale of a pair of neutron stars — dense balls about as massive as the sun but only about the size of Manhattan — spiraling around each other like the blades of a kitchen blender.

The stars were each the battered survivors of cosmic violence: All that was left of a pair of stars whose explosions had once lit up their galaxy, circling each other and merging in a cataclysm never before seen by human eyes.

And it was loud, meaning that it was relatively close to Earth, said Zsuzsa Marka, a Columbia astrophysicist, showing off the chirp on a laptop in her office recently. But where?

Luckily the European Virgo antenna had joined the gravitational wave network only two weeks before, and it also showed a faint chirp at the same time. The fact that it was so weak allowed the group to localize the signal to a small region of the sky in the southern constellation Hydra that was in Virgo’s blind spot.

The hunt was on. By then Hydra was setting in the southern sky. It would be 11 hours before astronomers in Chile could take up the chase.

One of them was Ryan Foley, who was working with a team on the Swope telescope run by the Carnegie Institution on Cerro Las Campanas in Chile. Figuring the burst had come from a galaxy, they made a list of the biggest galaxies in that region and set off to photograph them all systematically, the biggest ones first. “Just mow the lawn,” as Dr. Foley, a professor at the University of California, Santa Cruz, put it in a phone interview.

The fireball showed up in the ninth galaxy photographed, as a new bluish pinprick of light in the outer regions of NGC 4993, a swirl of stars about 130 million light years from here. “These are the first optical photons from a kilonova humankind has ever collected,” Dr. Foley said.

Within 10 minutes, another group of astronomers, led by Marcelle Soares-Santos of Brandeis University and using the Dark Energy Camera, which could photograph large parts of the sky with a telescope at the nearby Cerro Tololo Interamerican Observatory, had also spotted the same speck of light.

Emails went flying about in the Chilean night. Within hours four more groups had found the fireball.

When the Hubble Space Telescope swung over to the galaxy, it was inadvertently announced on Twitter, which led J. Craig Wheeler, an astronomer at the University of Texas, to respond with his own tweet: “New LIGO. Source with optical counterpart. Blow your sox off!”

Dr. Wheeler quickly deleted his tweet, but the discovery was creating a social media buzz among astronomers and stargazers.

When it was first identified, the fireball of 8,000-degree gas was about the size of Neptune’s orbit and radiating about 100 million times as much energy as the sun.

Within a few days, the orbiting Chandra X-ray Observatory and the Swift satellite both detected X-rays coming from the location of the burst, and radio telescopes like the Very Large Array in New Mexico recorded radio emissions. Over the course of a few days, meanwhile, the visible fireball faded and went from blue to red.

From all this, scientists have begun patching together a tentative story of what happened in the NGC 4993 galaxy.

“It’s actually surprising how well we were able to anticipate what we’re seeing,” said Brian David Metzger, a theorist at Columbia University who coined the term kilonova back in 2010. But Dr. Kalogera cautioned that this was not a “vanilla gamma ray burst, being way fainter and closer than any previously observed.”

As they tell it, the merging objects were probably survivors of stars that had been orbiting each other and had each puffed up and then died in the spectacular supernova explosions in which massive stars end their luminous lives. Making reasonable assumptions about their spins, these neutron stars were about 1.1 and 1.6 times as massive as the sun, smack in the known range of neutron stars.

As they approached each other swirling a thousand times a second, tidal forces bulged their surfaces outward. Quite a bit of what Dr. Metzger called “neutron star guts” were ejected and formed a fat doughnut around the merging stars.

At the moment they touched each other, a shock wave squeezed more material out of their polar regions, but the doughnut and extreme magnetic fields confined the material into an ultra-high-speed jet emitting a blitzkrieg of radiation. Those were the gamma rays, carrying news of the catastrophe to the outside universe.

As the jet slowed down and widened, encountering interstellar gas in the galaxy, it began to glow in X-rays and then radio waves.

The subatomic nuggets known as neutrons meanwhile were working their cosmic alchemy. The atoms in normal matter are mostly empty space: a teeny tiny nucleus of positively charged protons and electrically neutral neutrons enveloped in a fluffy ephemeral cloud of negatively charged electrons. Under the enormous pressures of a supernova explosion, however, the electrons get squeezed back into the protons turning them into neutrons packed into a ball as dense as an atomic nucleus.

The big splat liberates these neutrons into space where they inundate the surrounding atoms, transmuting them into heavy elements. The radioactivity of these newly created elements keeps the fireball hot and glowing.

Dr. Metzger estimated that an amount of gold equal to 40 to 100 times the mass of the Earth could have been produced over a few days and blown into space. For uranium the number is 10 to 30 times the mass of the Earth. In the coming eons, those metals could be incorporated into new stars and planets and in some far, far day become the material for an alien generation’s jewels or weapons.

The discovery filled a long-known chink in the accepted explanation of how the chemistry of the universe evolved from pure hydrogen and helium into the diverse place it is today. Stars and supernovas could manufacture the elements up to iron or so, according to classic papers in the 1950s by Margaret and Geoffrey Burbidge, Fred Hoyle and William A. Fowler, but heavier elements required a different thermonuclear chemistry called r-process and lots of free neutrons floating around. Where would they have come from?

One idea was neutron star collisions, or kilonovas, which now seem destined to take their place on the laundry list of cosmic catastrophes along with the supernova explosions and black hole collisions that have shaped the history of the universe.

Until now there was only indirect evidence of kilonovas. Astronomers found a fireball from a gamma-ray burst in 2013, but there was no proof that neutron stars were involved. At least some of the mysterious flashes in the sky known as short gamma-ray bursts, astronomers now know, are caused by mating neutron stars. Dr. Kalogera said this had been expected for decades: “For the first time ever, we have proof.”

One burning question is what happened to the remnant of this collision. According to the LIGO measurements, it was about as massive as 2.6 suns. Scientists say that for now they are unable to tell whether it collapsed straight into a black hole, formed a fat neutron star that hung around in this universe for a few seconds before vanishing, or remained as a neutron star. They may never know, Dr. Kalogera said.

Neutron stars are the densest form of stable matter known. Adding any more mass over a certain limit will cause one to collapse into a black hole, but nobody knows what that limit is.

Future observations of more kilonovas could help physicists understand where the line of no return actually is.

Back when LIGO was being designed, in the 1970s, few astronomers knew there would be black hole collisions to see, but everybody knew there were binary star systems containing neutron stars, that should collide. And so LIGO was designed and sold to see these. And now it has.

Dr. Holz, the University of Chicago astrophysicist, said, “I still can’t believe how lucky we all are,” reciting a list of fortuitous circumstances. They had three detectors running for only a few weeks, and it was the closest gamma-ray burst ever recorded and the loudest gravitational wave yet recorded. “It’s all just too good to be true. But as far as we can tell it’s really true. We’re living the dream.”