Revealing the Role of Neutron Stars in Gravitational Wave Discoveries

Published February 19, 2019

Written by Kimberly Mann Bruch, SDSC

Worldwide researchers recently used the San Diego Supercomputer’s (SDSC) Comet supercomputer, the Texas Advanced Computing Center’s (TACC) Stampede2 supercomputer,  and the Pittsburgh Supercomputing Center’s (PSC) Bridges supercomputer to identify ways that neutron stars reveal groundbreaking insight into gravitational waves, or invisible space ripples.

During a supernova, a star explodes – some remnants die and form black holes while others survive. Some of these supernova survivors are stars whose centers collapse and their protons and electrons form into a neutron star, which has an average gravitational pull that is two billion times the gravity on Earth. Here are highlights from two separate recently published studies that relied on the power of supercomputers to create detailed simulations.

Advancing Models for the Detection of Gravitational Waves from Oscillating Binary Neutron Stars

Researchers from Canada, the U.S., and Brazil have been focusing on the construction of a gravitational wave model for the detection of eccentric binary neutron stars. Using Comet and Stampede2, the scientists performed simulations of oscillating binary neutron stars to develop a novel model to predict the timing of various pericenter passages, which are the points of closest approach for revolving space objects. Their study, Evolution of Highly Eccentric Binary Neutron Stars Including Tidal Effects was published in Physical Review D.

“Our study’s findings provide insight into binary neutron stars and their role in detecting gravitational waves,” explained co-author Huan Yang of the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, Canada. “We can see that the oscillation of the stars significantly alters the trajectory and it is important to mention the evolution of the modes. For this case, during some of the later close encounters, the frequency of the orbit is larger when this evolution is tracked – compared to when it is not – as energy and angular momentum are taken out of the neutron star oscillations and put back into orbit.”

In other words, probing gravitational waves from eccentric binary neutron stars provides a unique opportunity to observe neutron star oscillations. Through these measurements, researchers can infer the internal structure of neutron stars.

“This is analogous to the sample of hearing the shape on a drum, where the shape of a drumhead can be determined by measuring frequencies of its modes,” said Yang. “Here, by hearing the modes of neutron stars with gravitational waves, the star’s size and internal structure will be similarly determined, or at least constrained.”

What was the role of supercomputers in this discovery? According to co-author Vasileios Paschalidis of the University of Arizona’s Theoretical Astrophysics Program, Comet and Stampede2 were used to perform parallel simulations of eccentric binary neutron stars where the relevant physics is self-consistently incorporated.

“In particular, our dynamical space-time simulations solve the equations of Einstein’s theory of general relativity coupled to perfect fluids,” said Paschalidis. “Neutron star matter can be described as a perfect fluid, therefore the simulations contain the necessary physics to understand how neutron stars oscillate due to tidal interactions after every pericenter passage, and how the orbit changes due to the excited neutron star oscillations. Such simulations are computationally very expensive and can be performed only in high-performance computing centers.”

But even many of today’s supercomputers are not fast enough to allow the researchers to either simulate multiple pericenter passages or multiple different scenarios involving such eccentric encounters, which is what makes this theoretical model important.

“Since the model necessarily makes some approximations, supercomputer simulations, which contain all the relevant physics with far fewer approximations, are necessary to calibrate such theoretical models,” said Paschalidis.

A portion of the research for this study was made possible by support from National Science Foundation (NSF) grant PHY-1607449, the Simons Foundation, Natural Sciences and Engineering Research Council of Canada (NSERC), and the Canadian Institute for Advanced Research (CIFAR). Additional support was from the National Aeronautics and Space Administration (NASA) grant NNX16AR67G and the Perimeter Institute for Theoretical Physics. Research at Perimeter Institute is supported by the Government of Canada through the Department of Innovation, Science and Economic Development Canada and by the Province of Ontario through the Ministry of Research, Innovation and Science.

Computational resources were provided by the Extreme Science and Engineering Discovery Environment (XSEDE) under grant TG-PHY100053 and the Perseus cluster at Princeton University.

Neutron Star Mergers Form the Cauldron that Brews Gravitational Waves

The merger of two neutron stars produces a hot (up to one trillion degrees Kelvin), rapidly rotating massive neutron star. This remnant is expected to collapse to form a black hole within a timescale that could be as short as one millisecond, or as long as many hours to form a black hole, depending on the sum of the masses of the two neutron stars.

Featured in a recent issue of the Monthly Notices of the Royal Astronomical Society, Princeton University Computational and Theoretical Astrophysicist David Radice and his colleagues presented results from their simulations of the formation of neutron star merger remnants surviving for at least one tenth of a second.

It has been long thought that this type of merger product would be driven toward solid-body rotation by turbulent angular momentum transport, which acts as an effective viscosity. However, Radice and his collaborators discovered that the evolution of these objects is actually more complex.

“We found that long-lived neutron star merger remnants are born with so much angular momentum that they are unable to reach solid body rotation,” explained Radice. “Instead, they are viscously unstable. We expect that this instability will result in the launching of massive neutron rich winds. These winds, in turn, will be extremely bright in the UV/optical/infrared bands. The observation of such transients, in combination with gravitational-wave events or short gamma-ray bursts, would be ‘smoking gun’ evidence for the formation of long-lived neutron star merger remnants.”

If detected, the bright transients predicted in this study could allow astronomers to measure the threshold mass below which neutron star mergers do not result in rapid black hole formation. This insight would be key in the quest to understand the properties of matter at extreme densities found in the hearts of neutron stars.

The winds produced in these events are also a site of production for heavy rapid neutron-capture process, or r-process, elements such as gold. This study suggests that a large fraction of, if not the majority, of these heavy elements found in nature could originate in these winds.

This research used 35 high-resolution, general-relativistic neutron star merger simulations, which calculated the geometry of space-time as predicted by Einstein's equations and simulated the neutron star matter using sophisticated microphysical models. On average, one of these simulations required about 300,000 CPUs per hour.

“The cost can be up to a factor of three times higher for the selected models that were run at even higher resolution and depending on the detail level in the microphysics,” said Radice. “Because of the unique requirements of this study, which included a large number of intermediate-size simulations and few larger calculations, a key enabler was the availability of a combination of capability and capacity supercomputers in the U.S. and Europe - including Comet at SDSC, and Bridges at PSC. This study also used the Blue Waters supercomputer at the National Center for Supercomputing Applications.”

Support was partially provided by a Frank and Peggy Taplin Membership at the Institute for Advanced Study and the Max- Planck/Princeton Center (MPPC) for Plasma Physics (NSF PHY- 1523261), the INFN initiative ‘High Performance Data Network’ funded by CIPE, the Institute for Nuclear Theory (17-2b program), the EU H2020 under ERC Starting Grant BinGraSp-714626, and NASA grant number NNX15AK85G. Computations were performed on Bridges, Comet, and Stampede2 via NSF XSEDE allocation TG-PHY160025.

About SDSC

As an Organized Research Unit of UC San Diego, SDSC is considered a leader in data-intensive computing and cyberinfrastructure, providing resources, services, and expertise to the national research community, including industry and academia. Cyberinfrastructure refers to an accessible, integrated network of computer-based resources and expertise, focused on accelerating scientific inquiry and discovery. SDSC supports hundreds of multidisciplinary programs spanning a wide variety of domains, from earth sciences and biology to astrophysics, bioinformatics, and health IT. SDSC’s petascale Comet supercomputer is a key resource within the National Science Foundation’s XSEDE (eXtreme Science and Engineering Discovery Environment) program.

Media Contacts:
Jan Zverina, SDSC Communications, 858 534-5111 or jzverina@sdsc.edu