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| Astrophysicists at the State University of New York, Stony Brook, and University of California, Berkeley created 3D simulations of X-ray bursts on the surfaces of neutron stars. Two views of these X-ray bursts are shown: the left column is viewed from above while the right column shows it from a shallow angle above the surface. The panels (from top to bottom) show the X-ray burst structure at 10 milliseconds, 20 milliseconds and 40 milliseconds of simulation time. Image Credit: Michael Zingale/Department of Physics and Astronomy at SUNY Stony Brook. |
Understanding how a thermonuclear flame spreads across the surface of a neutron star — and what that spreading can tell us about the relationship between the neutron star’s mass and its radius — can also reveal much about the star’s composition.
Neutron stars — the compact remnants of supernova explosions — are found throughout the universe. Because most stars are in binary systems, it is possible for a neutron star to have a stellar companion. X-ray bursts occur when matter accretes on the surface of the neutron star from its companion and is compressed by the intense gravity of the neutron star, resulting in a thermonuclear explosion.
Astrophysicists at the State University of New York, Stony Brook, and University of California, Berkeley, used the Oak Ridge Leadership Computing Facility’s Summit supercomputer, located at the Department of Energy’s Oak Ridge National Laboratory, to compare models of X-ray bursts in 2D and 3D.
“We can see these events happen in finer detail with a simulation. One of the things we want to do is understand the properties of the neutron star because we want to understand how matter behaves at the extreme densities you would find in a neutron star,” said Michael Zingale, a professor in the Department of Physics and Astronomy at SUNY Stony Brook who led the project.
By comparing computer models of the thermonuclear flames with observed X-ray burst radiation, researchers can put constraints on the size of the source to calculate the neutron star’s radius. Neutron stars have around 1.4 to 2 times the mass of the sun despite averaging only 12 miles in diameter. Mass and radii are important factors in understanding neutron stars’ interiors based on how matter behaves under extreme conditions. This behavior is determined by the equation of state that describes how the pressure and internal energy in a neutron star respond to changes in its density, temperature, and composition.
The study generated a 3D simulation based on insights from a previous 2D simulation that the team had performed to model an X-ray burst flame moving across the neutron star’s surface. The 2D study centered on the flame’s propagation under different conditions like surface temperature and rotation rate. The 2D simulation indicated that different physical conditions lead to different flame spread rates.
Extending those results, the 3D simulation used the Castro code and its underlying exascale AMReX library on Summit. The AMReX library was developed by the Exascale Computing Project to help science applications run on DOE’s exascale systems, including the OLCF’s HPE Cray EX supercomputer, Frontier. The simulation results were published in The Astrophysical Journal.
“The big goal is always to connect the simulations of these events to what we’ve observed,” Zingale said. “We’re aiming to understand what the underlying star looks like, and exploring what these models can do across dimensions is vital.”
The team’s 3D simulation used a neutron star crust temperature several million times hotter than the sun and a rotation rate of 1,000 hertz. Because time and size constraints greatly increase the computing expense for full-star flame propagation, the simulation focused on the flame’s early evolution.
The 3D flame does not stay perfectly circular as it propagates around the neutron star, so the team used the mass of the ash material produced by the flame to determine how rapidly the burning occurred compared with the burning of the 2D flame.
Although the burning was slightly faster in the 2D model, the growth trends in both simulations were extremely similar. The agreement between the models indicated that the 2D simulation remains a good model for modeling the flame spreading on the neutron star’s surface.
The results also suggested that initial explorations of full star bursts can be done in 2D as well.
Other facilities are used to study these astrophysical systems but are tackling other parts of the problem. The Facility for Rare Isotope Beams, or FRIB, at Michigan State University has launched the world’s most powerful heavy ion accelerator. FRIB will explore the proton-rich nuclei that are created by X-ray bursts, and Zingale’s team will be able to use those data to improve its own simulations.
“We’re close to modeling the flame spread across the whole star from pole to pole. It’s exciting,” Zingale said.
Published in journal: The Astrophysical Journal
Source/Credit: Oak Ridge National Laboratory
Reference Number: phy022024_02