Stellar explosion rocks the universe

The merger of two equal mass neutron stars is simulated using the 3-D code SNSPH. As the two stars merge, their outer edge ejects a spiral of neutron-rich material. The radioactivity in this ejected material is the primary power source for the optical and infrared light observed in the kilonova. A single hyper-massive neutron star remains at the center in a wide field of ejecta material. This hyper-massive neutron star will quickly collapse to a black hole. Los Alamos National Laboratory ISTI/ASC Co-Design Summer School

Astrophysicists at Los Alamos National Laboratory were enjoying a typical Friday evening with friends and family on Aug. 25, 2017, when they began hearing excited chatter about a major new astronomical observation pouring in over the phone and social media. Breaking news doesn’t happen that often in astronomy, and this was big. LIGO, the Laser Interferometer Gravitational-wave Observatory, had detected another gravitational-wave signal, just the fifth announced by the LIGO team since the observatory began operating two years ago.

The signal appeared to be coming from two neutron stars merging in a galaxy 130 million light-years away. The resulting cataclysm was still going on and giving off not just gravitational waves, but light and other electromagnetic radiation across the spectrum, a combination of signals that earned it the moniker of “multi-messenger” event.

Astronomers at observatories around the world (and in space) and theoreticians at research institutions were leaping into action to watch this event unfold over the next few days in x-rays, gamma rays, radio waves, ultraviolet light waves and plain old visible light. This scientific feeding frenzy was a once-in-a-generation opportunity for astrophysicists.

Neutron-star mergers are a specialty for the Center for Theoretical Astrophysics at Los Alamos National Laboratory, a team that develops theoretical models, runs them as simulations on the lab’s unique supercomputers, then tests those models and simulations against evidence from astronomical observations as they come in.

When LIGO began operating in late 2015, it quickly accomplished the first-ever detection of gravitational waves, the subtle jiggles in the fabric of space-time produced by massive, accelerating bodies, like neutron stars and black holes. That detection not only confirmed part of Einstein’s Theory of General Relativity, but it opened a vast new multi-messenger field of observational astronomy: looking at gravitational waves while also studying the corresponding signals of electromagnetic radiation.

Neutron stars are super-dense stellar cores that remain after the stars explode as supernovae. As the name suggests, they are made almost exclusively of neutrons, and as a result they are super tiny by stellar standards: only about 12 kilometers across, and held together by the enormous forces of Einsteinian gravity. Roughly the mass of the sun and compressed into spherical shapes no bigger than Los Alamos, the neutron stars of the August detection whirled around each other in a tighter and tighter orbit, accelerated by the emission of gravitational waves and the forces of mutual gravitational attraction.

Finally, like colliding snowballs, they smashed them together, ejecting huge streams of energy and matter, emitting a tremendous flash of gamma rays, and releasing the unique gravitational-wave signal. Theorists had long thought a neutron-star merger would emit a short gamma-ray burst, but this observation provided the first direct and definitive evidence.

Understanding all of the implications would require input from a broad, multi-disciplinary set of scientists — and major supercomputing horsepower. In a flurry of work at the lab, the team outlined a series of calculations about how radiation moves in the extreme environment of a neutron-star merger. An urgent phone call to the lab’s Metropolis Center for High Performance Computing freed up crucial supercomputing time. Within a few hours, the team was up and running on the “big iron” at the computing center.

The team soon realized the LIGO data showed more ejected mass from the merger than the simulations accounted for. Turning back to their models, they tweaked their calculations to accommodate the real-world observations, which is exactly how theory and observation work together. The results confirmed one of the model’s important predictions: Heavy elements beyond iron on the periodic table were formed by what’s known as the r-process, or rapid process, in the neutron-star merger.

Beyond that confirmation, the August gravitational wave observation is having a major impact on theory and helping to settle long-standing conundrums in physics and astrophysics. Scientists across Los Alamos — this is a classic cross-disciplinary, multi-science effort at the lab — have contributed to several peer-reviewed papers on theory and observation related to the LIGO observation.

These results are the tip of the iceberg. The LIGO results also indicated that neutron star mergers happen between 50 and 100 times a year, which is far more than expected. At that rate, gravitational-wave observations will open up a whole new field of science around neutron-star mergers. These events have much to tell about how heavy elements such as plutonium and gold are created and about how nuclear reactions occur.

Because the Laboratory’s primary mission centers on the nation’s nuclear stockpile, Los Alamos maintains deep expertise in nuclear physics, astrophysics, the physics of radiation transport, data analysis and the computer codes that run massive nuclear simulations on world-leading supercomputers. That makes the laboratory a natural resource for confirming the conclusions about what the observatory discovers and for extending LIGO discoveries into theories and models. It also makes for pretty exciting science. Astrophysics has gained a new messenger, gravitational waves, and the world of astrophysics is poised and ready for the next message.

Chris Fryer, Oleg Korobkin, and Ryan Wollaeger are astrophysicists in the Center for Theoretical Astrophysics at Los Alamos National Laboratory. Several Los Alamos researchers across a range of disciplines also worked on the neutron-star merger and published papers of their findings. Read more about their work here:

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