New insights into behavior of ultra-dense star core

Scattered throughout the universe are unimaginably dense remnants of stellar death, cold cores of large stars that have burned through their fuel, collapsed, and blown off their outer layers in supernova explosions. Known as neutron stars, these exotic remnants are often gravitationally locked with another star and over time siphon off some of the other star’s outermost surfaces.

Now, a scientist at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) has helped explain two phenomena associated with this process that have long baffled researchers. “This research started with abstract questions,” said PPPL physicist Russell Kulsrud, lead author of a paper reporting the results in the Journal of Plasma Physics. “How can matter from a companion star break through a neutron star’s powerful magnetic field to produce X-rays, and what causes the observed changes in those fields?”

Kulsrud was trying to explain how neutron stars could emit large amounts of X-ray light, which had been observed by telescopes. Scientists know that if plasma from the companion star could fall through a neutron star’s atmosphere, the plasma would slow down and emit the powerful X-ray radiation. But how could it pass through the neutron star’s powerful magnetic field?

The researchers found that once the plasma accumulates, its bulk puts gravitational pressure on the magnetic field lines, creating an instability that allows the plasma to flow onto the neutron star. The plasma then follows the field lines to the star’s poles and eventually settles across the star’s entire surface while emitting X-rays.

“Understanding exactly how neutron stars accrete matter and produce X-ray radiation is an unsolved problem in astrophysics,” said PPPL director Steven Cowley. “Kulsrud has now clarified part of this problem and produced yet another fundamental finding.”

The findings also explain observed deformations of neutron star magnetic fields. “The added mass on the neutron star’s surface can distort the outer region of the star’s magnetic field,” Kulsrud said. “If you’re observing the star, you should see that the radiation emitted by the magnetic field will gradually change. And in fact this is what we see.”

These new results came about in part because of COVID-19 quarantining. “When the pandemic started and everyone was confined to their homes, I decided to take up the model of a neutron star and work out a few things,” Kulsrud said.

The findings also pertain to fusion, which scientists are seeking to replicate on Earth for a virtually inexhaustible supply of power to generate electricity. “Though there aren’t any direct applications of this research to the development of fusion energy, the physics is parallel,” he said. “The diffusion of energy through tokamaks, doughnut-shaped fusion facilities used around the word, resembles the diffusion of matter across a neutron star’s magnetic field.”

Kulsrud and Rashid Sunyaev, a physicist at the Max Planck Institute for Plasma Physics in Garching, Germany, gathered their data from two spacecraft: the Nuclear Spectroscopic Telescope Array (NuSTAR), an orbiting telescope launched in 2012 by the National Aeronautics and Space Administration (NASA) to study high-energy X-ray light; and NASA’s Neil Gehrels Swift Observatory, which was launched in 2004 to study gamma-ray bursts, extremely powerful explosions that occur throughout the universe.

Neutron stars are one of the wonders of the natural world. Produced when stars larger than the sun undergo supernova explosions, these leftover cores have such strong gravitational fields that the electrons in the remaining atoms are squeezed into the protons, producing neutrons. Neutron stars pack more material than exists in the sun into a sphere about the size of New York City. They are so dense that a tablespoon of neutron star material would weigh as much as Mt. Everest.

Kulsrud is an institution at PPPL. He joined the lab in 1954 when it was still known as Project Matterhorn, begun by Princeton University astrophysics professor Lyman Spitzer as a base for the study of controlled thermonuclear reactions. Kulsrud’s research topics have ranged from the movement of particles in twisty fusion devices known as stellarators, which Spitzer invented, to magnetic turbulence, black holes, and the origin of the magnetic fields that permeate the universe.

“Almost 70 years after making fundamental theoretical contributions to theory describing the plasma equilibrium in fusion reactors, Russell is developing new theories that explain recent spacecraft observations of ultra-luminous X-ray sources in nearby galaxies,” said Stuart Hudson, interim head of PPPL’s Theory department. “Russell continues to make important contributions to plasma physics.”

PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://energy.gov/science