Even Microquasars are Powerful Particle Accelerators

The Earth is bathed in high-energy particles. Known as cosmic rays, most of them are protons striking us at nearly the speed of light. Fortunately, the atmosphere protects us from any significant harm, though the particles can strike with so much energy that they create a shower of lower energy particles that do reach Earth’s surface. That’s actually how we can detect most cosmic rays.

We aren’t entirely sure what accelerates these particles so tremendously. There are phenomena that can do it. Nearby supernovae can generate lower energy cosmic rays, but the origin of the highest-energy cosmic rays is less understood. One clear source is quasars. These distant beacons are powered by supermassive black holes, which can generate tremendous jets of relativistic particles. Even across billions of light years, these particles can strike Earth with incredible energy. But the problem is that the number of particles quasars send our way isn’t enough to account for the number of cosmic rays we observe. So there must be another source as well.

The most obvious possibility is what are known as microquasars. These are exactly what they sound like. Whereas quasars are powered by supermassive black holes in the hearts of distant galaxies, microquasars are powered by stellar mass black holes in our own galaxy. Although they are tiny in comparison to quasars, they have very similar structures, with an accretion disk of material surrounding them and high-energy jets that stream away from their poles. Microquasars are so similar to their large cousins that astronomers can study them to better understand the evolution of quasars.

Not all stellar-mass black holes are microquasars. Regular quasars create jets by capturing material from the galaxy surrounding them, but microquasars need a companion star to pull material from. The amount of energy a microquasar can produce depends on the available material, so microquasars are often categorized by the mass of their companion star. For high-mass microquasars the black hole’s companion is several times the mass of the Sun, providing plenty of material to accelerate. Low-mass microquasars have small companion stars and therefore tend to be less energetic. One of the most energetic microquasars is SS 433, where the black hole’s companion is ten times the Sun’s mass.

Since high mass stars are much less common than low mass ones, high-mass microquasars are rare compared to low-mass microquasars. So there aren’t enough high-mass microquasars to account for all the cosmic rays we detect on Earth. But a new study finds that might not be a problem, since low-mass microquasars can also produce cosmic rays.

In this study, the team looked at a microquasar known as GRS 1915+105. It is a stellar-mass black hole with a companion star less massive than the Sun, so it shouldn’t be large enough to produce cosmic rays. However, using data from the Fermi satellite, the team found a source of gamma rays from the same location. The gamma-ray source is faint, so the team used 16 years of back data to confirm it. They found that some of the gamma rays have energies greater than 10 GeV, which is quite a kick. The gamma rays are likely produced when protons accelerated by the microquasar strike interstellar gas, generating high-energy photons. For this to work, the protons of the microquasar’s jets must have energies higher than 10 GeV. This puts them in the energy range of high-energy cosmic rays.

While this study shows that low-mass microquasars can produce high-energy cosmic rays, it doesn’t settle the question of whether this would solve the mystery of their source. Some low-mass microquasars don’t produce cosmic rays, so more study will be needed to determine why some microquasars are so energetic while others aren’t.

Reference: Martí-Devesa, Guillem, and Laura Olivera-Nieto. “Persistent GeV counterpart to the microquasar GRS 1915+ 105.” The Astrophysical Journal Letters 979.2 (2025): L40.

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