One of the most remarkable things about astronomy is that we have the ability to understand things we can’t touch, measure, or otherwise experience up close. Here, from the surface of our pale blue dot, we can observe our universe and say, “Yes, I know how this formed”. We don’t know all the details, but our big picture understanding is capable of making remarkable predictions that prove true over and over again.

Sometimes though, our models for how objects work are remarkably vague. Consider the pulsar. These whirling blobs are a special kind of neutron star that appears to rotate tens to hundreds of times a second, and with each rotation, we see two pulses of radio light. First detected by Jocelyn Bell Burnell and her thesis advisor in 1967, the pulses have been described as bright spots or jets associated with the poles of the star’s magnetic field, which must not be aligned with the star’s rotational axis. The idea is, as the star rotates, the magnet poles come in and out of view, and it is only when they are pointed generally toward us that we see these pulses.

Why are there bright spots? Well, that part has been a bit of a mystery for many decades. Pulsars are weird objects. They aren’t really stars, but instead are the dense cores of former stars that blew off their atmospheres in supernova explosions, while their cores collapsed down so small that only neutrons can get close enough to exist. They are a kind of dense we can’t comprehend. They are “1.5 solar masses resting within the diameter of Manhattan Island” dense. They are “a tablespoon’s worth of mass weighs ten million tons” dense. They are “so dense that physics goes relativistic in a lot of cases” dense. Figuring out anything about these objects has been a challenge.

But now, one team has found a way to use physics to explain the bright magnetic poles of pulsars. In a paper appearing in Physical Review Letters, researchers describe how the interplay between magnetic and electric fields can drive what might be described as photon-producing lightning. Now, that is one heck of an oversimplification of work that is rich in detail. It is the detail in this work that makes it so interesting. 

Here is what they think is happening: the neutron star’s massive magnetic field is tearing electrons from the star’s surface and accelerating them like a naturally occurring rail gun. This acceleration drives the release of gamma-ray energy photons that interact with the magnetic field to transform energy to matter, creating pairs of electrons and positrons. Initially, some of these particles are accelerated, leading to further pair-production. These newly coalesced particles eventually fill the region and dampen the electric field momentarily, creating an oscillation in the field, and we observe this oscillation as radio waves. 

According to lead author Alexander Philippov: The process is a lot like lightning. Out of nowhere, you have a powerful discharge producing a cloud of electrons and positrons, and then, as an afterglow, there are electromagnetic waves.

This work takes advantage of modern computing technology. These are exceedingly complex systems, and past attempts to simulate what is going on could only follow a single line of points radiating up from the pole, and that one-dimensional kind of a model can’t simulate the electric fields that are so important in this result. In this paper, these researchers, who are part of New York’s Flatiron Institute, were able to create a two-dimensional model that cuts a plane through the magnetic pole and thus can follow some of the electric field phenomena. Now, we need to point out, the universe is three dimensional, and this two-dimensional model will be missing things. We won’t know how important those missing unknowns might be until we can do three-dimensional models. Luckily, this is a pretty symmetric kind of a system, so the model should be getting the big picture right, and this should be one more phenomenon that has gone from a vague explanation to a science-based explanation.

More Information

Simons Foundation news article 

“Origin of Pulsar Radio Emission,” Alexander Philippov, Andrey Timokhin, and Anatoly Spitkovsky, 2020 June 15, Physical Review Letters (Preprint on arxiv.org)