NASA scientists are gaining a more detailed understanding of the complex, high-energy environment around spinning neutron stars, known as pulsars.
By studying what amounts to a computer-simulated "pulsar in a box", the scientists were able to trace the paths of charged particles in magnetic and electric fields near a neutron star, revealing behaviours that may help explain how pulsars emit gamma-ray and radio pulses with ultraprecise timing.
"Efforts to understand how pulsars do what they do began as soon as they were discovered in 1967, and we're still working on it," said Gabriele Brambilla, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
"Even with the computational power available today, tracking the physics of particles in the extreme environment of a pulsar is a considerable challenge."
A pulsar is the crushed core of a massive star that ran out of fuel, collapsed under its own weight and exploded as a supernova. Gravity forces more mass than the Sun's into a ball while also revving up its rotation and strengthening its magnetic field. Pulsars can also spin thousands of times a second and wield the strongest magnetic fields known to man.
These characteristics also make pulsars powerful dynamos, with super strong electric fields that can rip particles out of the surface and accelerate them into space.
NASA's observations show that the high-energy emission occurs farther away from the neutron star than the radio pulses. And various physical processes ensure that most of the particles around a pulsar are either electrons or their antimatter counterparts, positrons.
Just a few hundred yards above a pulsar's magnetic pole, electrons pulled from the surface may have energies comparable to those reached by the most powerful particle accelerators on Earth," said Goddard astronomer Alice Harding.
To trace the behavior and energies of these particles, the astronomers used a new type of pulsar model called a "particle in cell" (PIC) simulation. In the last five years, the PIC method has been applied to similar astrophysical settings by research teams.
"The PIC technique lets us explore the pulsar from first principles. We start with a spinning, magnetized pulsar, inject electrons and positrons at the surface, and track how they interact with the fields and where they go," said assistant research scientist Konstantinos Kalapotharakos.
"The process is computationally intensive because the particle motions affect the electric and magnetic fields and the fields affect the particles, and everything is moving near the speed of light."
The simulation shows that most of the electrons tend to race outward from the magnetic poles. The positrons, on the other hand, mostly flow out at lower latitudes, forming a relatively thin structure called the current sheet. In fact, the highest-energy positrons here are capable of producing gamma rays similar to those Fermi detects, confirming the results of earlier studies.
The simulation ran on the Discover supercomputer at NASA's Center for Climate Simulation. The model actually tracks 'macroparticles', each of which represents many trillions of electrons or positrons.
"So far, we lack a comprehensive theory to explain all the observations we have from neutron stars. That tells us we don't yet completely understand the origin, acceleration and other properties of the plasma environment around the pulsar," Brambilla said. "As PIC simulations grow in complexity, we can expect a clearer picture."
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