One of the problems with the electromagnetic spectrum is at a certain point we just kind of stopped naming things. Photons, the particles that make up light, come in a lot of different energies that our eyes perceive as color, our skin perceives as heat, and our radios convert into music. Those photons we see with our eyes get grouped by color in the visible spectrum: r-o-y-g-b-i-v. Sunburns do a good job highlighting the UVA and UVB parts of the spectrum. Beyond that, medicine highlights the use of X-rays, and catastrophic nuclear events teach us about gamma rays. And really, anything higher energy than X-rays, we call those gamma rays, and there are a whole lot of energies beyond X-rays that we are all lumping together.

Gamma-ray light comes from a lot of different sources, none of which are beakers in labs that can produce Spider-Man. One of the most common sources is particles getting accelerated to high velocities and then hitting something. That sudden stop converts kinetic energy into myriad things, including high-energy gamma rays.

In astronomy, rogue high-energy particles are called cosmic rays. This name can get a bit confusing, because high-energy, like gamma ray, doesn’t really have a clear definition. If your home is built on granite bedrock, you may have radon in your basement and cosmic rays creating bright spots on camera images. This is because radium-226 in granite and other rocks can decay, releasing zippy little protons and helium nuclei in the process that are essentially cosmic rays from our own planet.

These aren’t particularly high energy; they don’t generally cause gamma rays to form, but they are still called cosmic rays. So now you know that there are many reasons why astronomers shouldn’t name things — lack of clarity, or over clarity, generally being problems.

On the other extreme, some supernovae, jets from stellar mass black holes and neutron star accretion disks, and other situations with massive magnetic fields accelerate protons to noticeable fractions of the speed of light, and when those particles hit things, they can generate gamma-ray photons.

And we now know, thanks to a new paper in Physical Review Letters, that if you move from the powerful magnetic fields associated with stars to the more powerful magnetic fields associated with the supermassive black holes in the centers of galaxies, you can produce even higher energy protons, and when those protons hit random gas molecules in our galaxy, they produce gamma rays with such high energy that researchers made up a new word: PeVatrons. 

Essentially, cosmic rays from the Milky Way’s core produce gamma rays with energies measured in petaelectron volts. While some of these cosmic rays are inevitably hitting our world now and then, that’s not how these were detected. Instead, researchers from the Tibet ASγ Collaboration detected PeVatrons from the gas along the Milky Way. Cosmic rays originate in the core, fly across the galaxy to hit gas along the galaxy’s arms, and those collisions produce PeVatron energy gamma rays that then travel across the galaxy until they hit our atmosphere and trigger a detector on the Tibetan plateau. There really is no limit to just how much energy one particle can have; we just stop naming them after a while, at least until they are found.

PeVatrons – we welcome you to the Gamma Ray part of the EM Spectrum.

More Information

Chinese Academy of Sciences press release

APS press release (EurekAlert)

“First Detection of sub-PeV Diffuse Gamma Rays from the Galactic Disk: Evidence for Ubiquitous Galactic Cosmic Rays beyond PeV Energies,” M. Amenomori et al., 2021 April 5, Physical Review Letters