Superbursts: Multi-dimensional Simulations of Deep Carbon Explosions on Neutron Stars

Michigan State University, East Lansing MI

Investigators

Abstract

Neutron stars (NS) are the densest objects in the universe, other than black holes, and have fascinated theorists and observers since they were first conjectured. Despite a wealth of observations across the electromagnetic spectrum over the last four decades, much of what happens in the deep interior of NS remains inscrutable. Knowledge of neutron star physics is growing rapidly, due to X-ray telescopes in space, laboratory nuclear experiments, and gravitational wave detectors such as the Laser Interferometer Gravitational Wave Observatory (LIGO). A research group at Michigan State University will help astronomers to better understand neutron stars by modeling the thermally unstable ignition of carbon on the NS surface and the resulting explosion (a "superburst") using a state-of-the-art multidimensional hydrodynamics computer code. The models can reveal conditions in the outer layer of neutron stars, and help answer some critical science questions identified in National Academy reports: what is the nature of matter at exceedingly high density and temperature?, and what controls the mass, radius, and spin of compact stellar remnants? The planned work includes activities to recruit the next generation of scientists and to inform the public, who ultimately support the scientific endeavor. The group will develop educational resources to make it easier to use open-source computer simulations to explore stellar physics and incorporate discoveries made during the award into public talks at Michigan State's Abrams Planetarium. Neutron stars are unique natural laboratories to study quantum chromodynamics at finite density and low temperature. A number of recent observational, theoretical, and experimental developments have raised new questions about the internal constitution of neutron stars. For low-mass X-ray binaries, in which the neutron star accretes hydrogen- or helium-rich material, X-ray observations of accretion-induced phenomena over timescales ranging from seconds to decades provide important information about different layers of the neutron star. During accretion, runaway thermonuclear burning of the accreted material is observable as a Type I X-ray burst. The thousands of X-ray bursts and the rarer and more energetic superbursts encode information about the physics of dense matter and the workings of the accretion disk and boundary layer. The researchers will use ignition models that account for relevant heating and cooling processes in the neutron star crust. The simulations will yield testable predictions for both the precursor burst observed from some sources and the evolution of luminosity with time during the superburst. The models will inform astronomers about conditions in the outer layer of neutron stars, and may give information about the explosion's effect on the accretion flow and the nature of nuclear flames in dense matter. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.