Modeled plasma discharges produce pulsar emission

Since their discovery in 1967, pulsars have been used to indirectly detect gravitational waves (see Physics Today, July 2017, page 26 ), learn more about neutron stars (see Physics Today, January 2011, page 12 ), and probe the interstellar medium. Despite the attention pulsars have attracted, how they produce their observed radio emission has puzzled astronomers. Now Alexander Philippov of the Flatiron Institute in New York City and his colleagues have learned more about the source of that emission. The researchers’ simulations show that electromagnetic waves can be emitted by discharges that generate nonuniform electron–positron pair plasma near the pulsar’s surface and across its magnetic field lines.

Earlier simulations had revealed that particles near the magnetic poles of neutron stars get accelerated to high energies, which leads to the emission of energetic photons and the creation of electron–positron pairs. The pair production occurs intermittently; that’s because electron–positron pairs in the polar region shield the accelerating electric field generated by the rotating neutron star. Philippov and his colleagues thought that those shielding events could produce electromagnetic waves, though they knew that their explanation would need to describe how the waves are excited and how they travel in the plasma.

To meet those criteria, the researchers analyzed pair discharges with a two-dimensional computational model that consists of a conducting neutron star surrounded by a magnetic field. The simulation then numerically solves Maxwell’s equations and the relativistic equations of motion for the particles near the neutron star’s surface. Philippov and his colleagues found that the nonuniform, intermittent production of electron–positron pairs results in oscillating transverse components of the electric and magnetic fields. That process launches powerful electromagnetic waves that can propagate through the dense plasma in the general direction of the background magnetic field. A set of high-resolution discharge simulations with parameters similar to those of pulsars shows that the emission frequencies are comparable to those of observed pulsar radio emission. (A. Philippov, A. Timokhin, A. Spitkovsky, Phys. Rev. Lett. 124, 245101, 2020 .)