Neutron stars collide, gravitational wave hunters strike gold

Collide

Photo courtesy National Science Foundation/LIGO/Sonoma State University/A. Simonnet

An artist’s conception shows two neutron stars colliding, producing gravitational waves and a burst of light.

BOZEMAN — A long time ago in a galaxy far, far away, two neutron stars collided, producing an unprecedented light show and a raft of gravitational waves that traveled the cosmos for 130 million years until MSU scientists played a role in detecting them this summer.

That detection is the most significant discovery in astronomy since the Nobel Prize-winning first detection of gravitational waves in September 2015 — the result of the collision of two black holes that time — and solved a mystery involving some of Earth’s most precious metals.

At 6:41 a.m. Mountain Daylight Time on Aug. 17, the U.S.-based Laser Interferometer Gravitational Observatory, known as LIGO, and the Europe-based Virgo gravitational wave detectors detected gravitational waves from the collision of two neutron stars in a galaxy 130 million light years from Earth, said Neil Cornish, professor of physics in MSU’s College of Letters and Science.

Neutron stars are the smallest, densest stars known to exist and are formed when massive stars explode in supernovas. Neutron stars contain roughly one-and-a-half times more mass than the sun, “crammed into a sphere just 15 miles across,” Cornish said.

“Unlike the previous gravitational wave detections, which had all been from the collisions of black holes, this event also produced many forms of light, including high-energy gamma rays and X-rays, visible light, infrared light and radio waves,” he said.

In addition to the LIGO and VIRGO detectors, there were around 70 ground- and space-based observatories used to detect the merger.

“What is great is that this event was both measured in gravitational waves and with traditional electromagnetic telescopes,” said Nicolas Yunes, associate professor of physics and a founding member of MSU’s eXtreme Gravity Institute. “Combining this information, we therefore learn that when two neutron stars collide, they indeed emit a very powerful burst of gamma rays, an association we could not make before with near absolute certainty.”

NASA’s Fermi satellite picked up gamma rays from the cosmic event just two seconds after the gravitational wave signal ended, and, hours later, optical telescopes discovered a short-lived, bright new star at the same location.

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The event also provided new insight into where gold and platinum come from.

Scientists have known that lighter elements, such as carbon and oxygen, are produced inside stars like the sun, while many of the heavier elements such as iron, cobalt and titanium are produced by supernovas.

It had been predicted, Cornish said, that if two neutron stars collided, most of the material in the neutron stars would get turned into a black hole, but some material would get thrown into space and fuse to form gold and platinum.

“It was estimated that each collision would produce roughly a Jupiter-size mass of gold — worth 100 million trillion trillion dollars at today’s prices,” he said.

In the days following the collision, the U.S. Gemini Observatory, the European Very Large Telescope and NASA’s Hubble Space Telescope picked up telltale signatures of the radioactive decay associated with the production of gold, thus solving the mystery of where gold, platinum and about half of all elements heavier than iron are produced.

Neil Cornish

Neil Cornish

Cornish, who is also director of MSU’s XGI, and Margaret Millhouse, a physics graduate student, were part of the LIGO team that detected the gravitational wave signals from the neutron star merger.

Cornish said that while the detection was never in doubt, there was a problem with the data from the LIGO detector in Livingston, La. The data from Louisiana was marred by a loud burst of “noise” that overlapped the signal from the neutron star merger. Referred to as a glitch, the burst of noise was 20 times louder than the signal itself, making it difficult for scientists to decipher the properties of the merger.

Using a technique developed at MSU by Cornish and former graduate student Tyson Littenberg (now at the NASA Marshall Space Flight Center), the glitch was “surgically” removed from the data, and the details of the event were revealed.

The paper describing the discovery of the neutron star merger was published today in the journal Physical Review Letters. In the images from the paper, Figure 1 shows the signals in the cleaned data, while Figure 2 shows how the glitch was removed by the MSU-developed algorithm.

“This discovery is truly revolutionary, not just because of it being ‘a first,’ but more importantly because of all of the physics and astronomy we can learn from it,” Yunes said.

Coincidentally, on the morning of the discovery, XGI was hosting the first day of its workshop “eXtreme matter meets eXtreme gravity” at the MSU Alumni Foundation.

“The theme of the workshop was what we could learn about nuclear physics and the behavior of matter at extreme densities by observing neutron star mergers, using both gravitational waves and light,” Cornish said.

“Many of the world experts on this topic had gathered in Bozeman for the meeting,” he said. “It is almost unbelievable that the first-ever detection of a neutron star merger happened on the opening day of our workshop.”

LIGO is funded by the National Science Foundation and operated by Caltech and MIT, which conceived of LIGO and led the Initial and Advanced LIGO projects. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council) making significant commitments and contributions to the project.

“It is tremendously exciting to experience a rare event that transforms our understanding of the workings of the universe,” said France A. Córdova, director of the National Science Foundation. “This discovery realizes a long-standing goal many of us have had, that is, to simultaneously observe rare cosmic events using both traditional as well as gravitational-wave observatories. Only through NSF’s four-decade investment in gravitational-wave observatories, coupled with telescopes that observe from radio to gamma-ray wavelengths, are we able to expand our opportunities to detect new cosmic phenomena and piece together a fresh narrative of the physics of stars in their death throes.”

More than 1,200 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration and the Australian collaboration OzGrav. Additional partners are listed at http://ligo.org/partners.php.

The Virgo collaboration consists of more than 280 physicists and engineers belonging to 20 different European research groups: six from Centre National de la Recherche Scientifique (CNRS) in France; eight from the Istituto Nazionale di Fisica Nucleare (INFN) in Italy; two in the Netherlands with Nikhef; the MTA Wigner RCP in Hungary; the POLGRAW group in Poland; Spain with the University of Valencia; and the European Gravitational Observatory, EGO, the laboratory hosting the Virgo detector near Pisa in Italy, funded by CNRS, INFN, and Nikhef.

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