Scientists know how plasma blasts through neutron stars' crazy magnetic fields

New calculations by physicists give us a better understanding of how matter falls on a neutron star to fire off a powerful blast of X-rays.

If enough plasma is gravitationally drawn from the binary companion to the dead star, it's massive enough to force it through the barrier created by the neutron star's strong magnetic field to reach the neutron star's atmosphere.

This is an important part of the long-unsolved mystery of neutron star accumulation and X-ray flares. The discovery could help us better understand how plasmas behave in magnetic fields -- which may apply to the development of plasma fusion here on Earth.

"This research started with abstract questions," said plasma physicist Russell Kulsrud of the Princeton Plasma Physics Laboratory.

"How does material from the companion star break through the neutron star's strong magnetic field to produce X-rays, and what causes the observed changes in these fields?"

Neutron stars are among the densest objects in the universe. This happens when a star of a certain mass (between 8 and 30 times the mass of the Sun) reaches the end of its main sequence lifetime and dies.

The material of the outer star is blown away in a supernova explosion, and the star's core collapses under the force of gravity, forming a compact, ultra-dense sphere that will cease to glow for millions of years -- keeping its The only element of luminescence is residual heat.

When we talk about the density of celestial bodies, the density of black holes is comparable to that of neutron stars (if the mass of the precursor star exceeds 30 solar masses, the black hole will collapse). A neutron star, about 1.5 times the mass of the sun, is packed within about 10 kilometers (6.2 miles).

These extreme celestial bodies hang in space, often with magnetic fields a trillion times stronger than Earth's. Sometimes they are accompanied by binary companions so close together that the neutron star can capture and accumulate material from the companion's atmosphere.

When this happens, the matter forms a disk, feeds down into the neutron star, and gains energy as it accelerates under the force of gravity. This energy escapes in the form of X-radiation, usually concentrated in cylinders or hot spots at the neutron star's poles. We know this will happen; we have observed it. However, the question of how the plasma moves through the magnetic field remains.

"When the Covid-19 pandemic started and everyone was stuck at home, I decided to take the model of neutron stars and look at some questions," Kursrud explained.

He and his colleague Rashid Sunyaev, an astrophysicist at the Max Planck Institute for Astrophysics in Germany, performed mathematical modeling to see that the plasma is fixed in a magnetic field up and drag the magnetic field, or manage to slip through the magnetic field, leaving it intact.

According to their calculations, it is the latter. If the mass of the falling plasma is high enough, it will exert a gravitational force on the magnetic field. This creates cascading fluctuations in the magnetic field strength that cause instability so that the plasma cannot pass through.

Once the plasma is on the other side, it is transported along the neutron star's funnel-shaped magnetic field lines to the pole, where it accumulates on the neutron star.

According to the model, the plasma that builds up on the magnetic poles becomes too heavy to support on the surface and sinks into the interior of the neutron star. Additional internal pressure on the poles distorts the magnetic field. Over time, the pressure causes the incoming plasma to spread over the entire surface of the neutron star, producing global X-rays.

"Additional mass on the surface of a neutron star deforms the outer regions of the star's magnetic field," Kursrud said. "If you're looking at a star, you should see a gradual change in the radiation emitted by the magnetic field. In fact, that's what we see. arrived."

The team noted that their speculations are unlikely to hold for all neutron stars because their treatment of instability is approximate. However, the findings do predict the shape of the magnetic field over time and what the end result will be.

Over the course of tens of thousands of years, the neutron star will gradually increase its mass and radius by about a millimeter per year, eventually reaching a steady state for its magnetic field.

Mathematics could be applied in the development of tokamak fusion reactors, which use magnetic fields to confine plasma.

"Although this research has no direct application to the development of fusion energy, the physics is parallel," Kursrud said, "through the tokamak, the diffusion of energy by the doughnut-shaped fusion facilities used around the world, Similar to the diffusion of matter in the magnetic field of a neutron star."