There are some research papers I read and realize I am really glad I’m not in grad school anymore because the paper I’m reading would be a homework problem. There are a remarkable number of problems in astronomy where lots and lots of folks have said “that could be X”, but no one really wanted to do the maths to figure it out. Then, once one person has done it, lots of folks assign what they did as homework, using their paper as the solution guide.

A remarkable amount of grad school was learning how to find the papers our homework problems were stolen from. I am still bitter about the homework problem that came from a paper that resulted in the original researcher getting a Nobel Prize.

Anyway…

A pair of new papers in The Astrophysical Journal Letters and Nature Astronomy, led respectively by E. de Oña Wilhelmi and R. López-Coto, looks at the regions around young neutron stars and asks the simple question: Are the magnetic fields sufficient to accelerate electrons to the kinds of speeds necessary for them to produce gamma-ray light when they collide with photons?

They did this exercise specifically to prove that this is not what was happening, but the math replied with “well, actually…“, and they found themselves proving that yes, the magnetic fields around young stars are that strong.

Here is what is happening. As the young neutron stars – less than 200,000 years old – spin, they generate powerful magnetic fields. Their surroundings aren’t exactly empty. Neutron stars are the leftovers of massive stars that had previously undergone all sorts of mass loss, and the young objects are extremely hot and produce ionizing radiation that can separate electrons from their atoms. Those electrons, in the powerful magnetic field, get accelerated like electrons in the large hadron collider, just more so.

While some of these electrons will eventually reach Earth as cosmic rays, their paths are hard to trace because they are affected by intervening magnetic fields from other stars and black holes. Some of these electrons, however, interact with photons in the vicinity of the pulsar and pump the energy of the photons to the gamma-ray part of the spectrum. These extremely high-energy bits of light then fly to Earth on straight lines that ignore magnetic fields. We knew that pulsars had gamma rays, but the why of it wasn’t detailed out prior to these papers.

Now, we have a nice, consistent answer to both where some cosmic rays come from and why pulsars produce gamma rays. It is a beautiful result, and I’m really glad I don’t have to replicate their math.

More Information

Pulsars may power cosmic rays with the highest-known energies in the universe (Science News)

“On the Potential of Bright, Young Pulsars to Power Ultrahigh Gamma-Ray Sources,” Emma de Oña Wilhelmi, Rubén López-Coto, Elena Amato, and Felix Aharonian, 2022 April 28, The Astrophysical Journal Letters

“Gamma-ray haloes around pulsars as the key to understanding cosmic-ray transport in the Galaxy,” Rubén López-Coto et al., 2022 February 14, Nature Astronomy