Black holes snacking on small stars create particle accelerators that bombard Earth with cosmic rays

Using 16 years of data from NASA's gamma-ray detecting Fermi spacecraft, astronomers have discovered that "microquasars," systems in which a black hole is slowly devouring a star, may be small, but they pack one heck of a punch.

Despite their diminutive nature, this research suggests even microquasars snacking on small stars can have an impressive cosmic influence, becoming powerful natural particle accelerators.

This means black holes indulging in stellar meals of all sizes could be responsible for a higher-than-suspected amount of high-energy charged particles called "cosmic rays," which are constantly bombarding Earth. The mechanism for this particle acceleration is the near-light-speed jets that blast out from microquasars.

"Earth is constantly bombarded by particles accelerated elsewhere within our galaxy. These are mainly protons and electrons, which are commonly known as cosmic rays," team co-leader Guillem Martí-Devesa from the Università di Trieste told Space.com. "However, their origin has been debated for decades."

What Martí-Devesa and team co-leader Laura Olivera-Nieto from the Max Planck Institute for Nuclear Physics found is a new source of gamma-rays that is consistent with the position of GRS 1915+105, a well-known microquasar in which a black hole is slowly feasting on a low-mass star.

"Finding sources that can accelerate particles and understanding what makes them special is the first step towards uncovering why and how the universe sometimes provides a very small fraction of particles with huge amounts of energy," Olivera-Nieto told Space.com. "In order to accelerate particles, you typically need a few ingredients: strong magnetic fields, high amounts of power, and also the presence of particles to accelerate.

"Microquasar jets have them all!"

Quasars vs. miniquasars

Standard quasars are powered by supermassive black holes feasting on surrounding matter and gas in central regions of some galaxies called active galactic nuclei (AGNs).

Quasars are some of the brightest light sources in the cosmos, often outshining the combined light of billions of stars in the galaxy that surrounds them.

Whereas supermassive black holes have masses from millions to billions of times that of the sun, the star-snacking stellar black holes at the heart of microquasars have masses of no more than a few hundred solar masses.

"Most stars in the galaxy are not alone, but actually orbiting another star. Microquasars are a special type of stair pair in which one of the two stars already 'died,' that is, ran out of fuel and exploded," Olivera-Nieto said. "What is left behind is a black hole. If the normal star is close enough to the black hole, the black hole will start ripping matter out from it and swallowing it.

"We call these objects 'microquasars' because they resemble quasars, an analogous phenomenon but with supermassive black holes in the center of galaxies."

Black holes snacking on stars is a fairly common phenomenon. When a star ventures too close to a supermassive black hole, the tremendous gravitational influence of that cosmic titan generates tidal forces that squash the star horizontally while stretching it vertically.

This "spaghettification" process turns the unfortunate star into a noodle of stellar material that wraps around the supermassive black hole and is gradually fed to it. These powerful and violent occurrences are referred to as "tidal disruption events" or "TDEs."

Microquasars differ from TDEs because the stars involved aren't rapidly destroyed, with these more diminutive black holes preferring to nibble on their victim star. It isn't the mass of the black hole that is responsible for this savoring of this stellar meal, though.

"A microquasar is in a stable orbit, with the black hole only taking mass from the star at a very slow pace," Olivera-Nieto said. "That means that it is not destroying the star with its tides and will not do so during its evolution."

Microquasars as cosmic particle accelerators

This new research shows that despite the diminutive size of their black hole engines (compared to supermassive black holes) and their slow approach to eating stars, microquasars can have an impressive cosmic influence, becoming powerful natural particle accelerators.

"In the case of a microquasar, we have a star slowly being swallowed by a black hole," Martí-Devesa said. "As a result, the black hole can generate powerful relativistic jets, which is truly the distinctive feature of a microquasar."

These outflows become the most potent astrophysical jets found within our galaxy and, thus, excellent cosmic particle accelerators. The question is, just how much of the Milky Way's cosmic ray content is contributed by microquasars, especially those with very low-mass black hole engines?

To answer this query, Olivera-Nieto and Martí-Devesa turned to 16 years' worth of data collected by NASA's Fermi spacecraft using its Large Area Telescope (LAT) detector.

They found a gamma-ray signal associated with the microquasar GRS 1915+105, also known as "V1487 Aquilae," something that came as a major surprise.

The reason for the surprise is GRS 1915+105 is a low-mass binary consisting of a black hole with 14 solar masses slowly eating a star with around half the mass of the sun.

This is in stark contrast with previously known particle-accelerating microquasars, which only hosted massive stars. For instance, the microquasar SS 433 hosts a black hole snacking on a star that has ten times the mass of the sun.

"Something that makes this system special is actually that it might be rather common," Olivera-Nieto said. "The number of stars in the galaxy drops steeply as their mass increases: low-mass stars are much more common than high-mass stars. As a consequence, the same must be true for the microquasar systems."

That means finding that even a system with a black hole slowly devouring such a small star can accelerate particles enough to create gamma-ray photons implies that the contribution of microquasars as a whole to the cosmic ray content of our galaxy may be higher than scientists expected.

"There are many more microquasars known where there is no evidence of gamma-ray emission, which means no evidence of particle acceleration to high energies," Olivera-Nieto said. "Some of these are because we didn't look with sensitive enough telescopes, but others seem to simply not be efficient accelerators.

"We would like to understand the difference between these systems, which holds the clue to understand just how many cosmic rays are produced in the jets of microquasars."

Thus, with this new evidence that microquasars with low-mass stars can also be particle accelerators and contribute to the cosmic rays that arrive at Earth, Martí-Devesa explained that it may be time to re-visit previously discovered microquasar systems.

"We hope that our study will be an important step forward to understanding the real contribution of microquasars to the cosmic ray abundance in our galaxy," Martí-Devesa concluded. "In this way, we will be able to re-evaluate the whole population of microquasars and their true relevance as cosmic ray producers in our galaxy."

The team's research was published in January in The Astrophysical Journal Letters.