CITA astrophysicists awarded major grants to study how magnetic energy from neutron stars is converted to radiation

Canadian Institute for Theoretical Astrophysics (CITA) faculty member Bart Ripperda and postdoctoral fellow Gibwa Musoke have been awarded 250,000 computing hours at the Oak Ridge Leadership Computing Facility (OLCF) for research on processes that power the extremely bright emissions that we see from neutron stars.

The research project, which is a collaboration between CITA, University of Maryland, Columbia University and Caltech is also supported by a NASA Astrophysics Theory Program (ATP) grant. Both awarding institutions have categorised it as “fundamental research” and allocated substantial financial and computational resources to its completion.

The question the team is set to answer will impact our understanding of the energy transformation processes causing some of the brightest short bursts of radiation observed in the universe. Such “bright transients” are associated with the activity of neutron stars and are likely powered by magnetic energy dissipation. But what causes magnetic energy to transform into kinetic energy and then power radiation?

Neutron stars, compact endpoints of stellar evolution, are some of the densest bodies in the universe, with a mass comparable to the sun packed into a radius of roughly 10 kilometers and with strong magnetic fields up to a trillion times stronger than that of the Earth. Magnetars are neutron stars with exceptionally strong magnetic fields whose existence was proposed by Christopher Thompson, another CITA faculty, in 1992, and later observationally confirmed.

Both magnetars and merging neutron stars can power bright transients. Researchers have speculated that they can be the result of a star quake on a magnetar, or a cataclysmic event such as the merging of two neutron stars. In such events, the magnetic field of the stars is suddenly disrupted and finds a way to quickly convert its energy to heat, radiating it away in a fraction of a second. The energy in such bursts can be enormous, shining up to a million trillion times as bright as the sun.

As Ripperda explains, “The mechanism behind extremely bright transients from magnetars and neutron star mergers is based on very similar physics, where the basic unsolved question is how magnetic energy is converted to the observed radiation. Many processes such as shocks, turbulence or magnetic field lines breaking open and reconnecting, have been seen as the probable cause of the radiation. Our challenge is to, for the first time, reliably show whether and how these processes occur. Doing that requires large-scale three-dimensional simulations capturing both the environment of the neutron stars as well as the small-scale plasma physics.”

The simulations Ripperda, Musoke and their collaborators at Caltech, Columbia, and Maryland intend to run to answer the above question are extremely demanding and can only be run on supercomputers. Therefore, the large grant of 250,000 node hours they have won for the Summit supercomputer at Oak Ridge National Laboratory in Tennessee will be instrumental for their success. In total, their project will take millions of node hours and result in the highest-resolution neutron star simulations carried out thus far, giving an unprecedented picture of the mechanisms leading to the emission.

Commenting on the importance of the project, CITA Director Juna Kollmeier added: “CITA has a long history of using state-of-the art computers to obtain deeper insight into complex physics that you can’t always work out with a pencil and paper — even if you are as brilliant as CITA Professor Chris Thompson! Bart and Gibwa’s research represents the next generation vanguard of this work and, in collaboration with senior faculty and researchers abroad, they run with the torch of knowledge even further and faster into the frontiers of human understanding.”