Our home planet, Earth, is perpetually bombarded by particles from outer space. While many people are familiar with the dramatic sights of meteors—rocky fragments from within our solar system that streak across the night sky—it’s the much smaller particles that intrigue scientists, offering vital clues about the universe’s workings. Among these particles, cosmic rays, a type of subatomic particle traveling at incredibly high speeds, are central to our understanding of cosmic phenomena.
Cosmic rays are high-energy particles, including electrons, protons, and atomic nuclei, originating from distant parts of the universe. These particles often possess energies so high that they approach the limit set by our current understanding of particle physics. Their origins, as well as the mechanisms through which they are accelerated to such extreme speeds, remain one of the most fascinating mysteries in the realm of astrophysics. Answering these questions could deepen our understanding of the processes occurring in some of the universe’s most enigmatic regions, such as black holes and supernova remnants.
Understanding Cosmic Rays
Cosmic rays, which include subatomic particles like electrons and protons, travel through space at speeds close to the speed of light. They constantly rain down on Earth, interacting with the Earth’s atmosphere and creating a cascade of secondary particles that scientists can study. These high-energy particles have far more energy than the particles found in the highest energy accelerators on Earth. The discovery of cosmic rays dates back to 1912 when the physicist Victor Hess first observed them through experiments in a hot-air balloon.
While cosmic rays can originate from our Sun (solar cosmic rays), most of the highest-energy cosmic rays come from interstellar space or beyond. The primary focus of modern astrophysics is on understanding the nature of these fast-moving particles, particularly the most energetic ones, and identifying their sources. Recent research has shown that one potential source of these cosmic rays is microquasars—a system involving a stellar-mass black hole and a normal star.
Microquasars: Particle Accelerators in the Cosmos
Microquasars are compact, high-energy systems comprising a stellar-mass black hole in orbit with a normal star. These systems are particularly exciting because of the extreme environments around black holes, where enormous amounts of energy are concentrated. When a black hole interacts with a companion star, material from the star may be sucked in by the black hole’s powerful gravitational pull. As matter is pulled in, it forms a accretion disk that accelerates particles to high speeds. Alongside this, black holes launch energetic jets—high-speed streams of matter that travel far from the black hole.
These jets can serve as powerful cosmic particle accelerators. Particle acceleration refers to the process by which charged particles, like protons or electrons, gain tremendous amounts of energy. In a similar fashion to how particles are accelerated in particle colliders, these cosmic jets are believed to accelerate particles to extraordinary speeds, converting them into cosmic rays. While earlier studies hinted at the particle-acceleration potential of these jets, the detailed mechanisms that govern this process have largely remained an enigma.
Most of the microquasars studied in this context were high-mass systems, where the star is much larger than the Sun, often several times its size. For example, the famous microquasar SS 433 has a star that’s approximately ten times as massive as our Sun. These systems were long thought to be the primary accelerators of cosmic rays within our galaxy, especially because their high-energy outputs aligned with the intensity of cosmic rays detected on Earth.
However, in a new turn of events, recent observations have begun challenging the earlier assumptions about the relationship between particle acceleration and the size of the stellar companion in microquasar systems. Specifically, low-mass microquasars—those with a star smaller than the mass of our Sun—have now been identified as potential particle accelerators capable of producing cosmic rays, a finding that opens up new avenues in the study of cosmic radiation.
The Discovery: Low-Mass Microquasar GRS 1915+105
In a groundbreaking paper published in The Astrophysical Journal Letters, a team of scientists from the Max-Planck-Institut für Kernphysik (MPIK) and the Università di Trieste, including Dr. Laura Olivera-Nieto and Dr. Guillem Martí-Devesa, have challenged the conventional view by presenting evidence of particle acceleration in a low-mass microquasar system known as GRS 1915+105.
Located in the Milky Way galaxy, GRS 1915+105 contains a stellar-mass black hole in orbit with a star that has a mass smaller than our Sun. Previous models of cosmic ray production suggested that such low-mass microquasars were unlikely to be powerful enough to produce the high-energy gamma-rays associated with cosmic ray acceleration. However, the scientists used data from NASA’s Fermi Gamma-ray Space Telescope to unveil a faint yet significant gamma-ray signal from this system, which represents the accelerated particles.
Using over 16 years of data collected by Fermi’s Large Area Telescope (LAT), the team detected a gamma-ray signal originating from the location of GRS 1915+105. The signal indicated particle energies higher than 10 GeV (giga-electron volts), suggesting that protons within the system are accelerated to extremely high speeds. This discovery provided compelling evidence that even low-mass microquasars can act as effective cosmic particle accelerators.
This was a game-changing finding, as it was previously believed that only microquasars with massive stellar companions could generate the required energy to accelerate particles to such high energies. The gamma-ray signal detected from GRS 1915+105 was evidence that even smaller systems can unleash immense energy in the form of accelerated protons, possibly contributing to the population of cosmic rays that reach Earth.
The Process: Proton Acceleration and Gamma-Ray Production
The detailed mechanisms of particle acceleration in GRS 1915+105 were further examined by the researchers. Their findings suggest that protons—charged particles—are accelerated within the powerful jets ejected by the black hole. As these high-speed protons escape the black hole’s immediate vicinity, they travel outward into the surrounding gas. In their journey, these protons interact with the surrounding matter, causing energy conversion into gamma photons.
A key piece of supporting evidence comes from Nobeyama Radio Telescope data, which revealed that there was an adequate amount of gas surrounding the microquasar for these interactions to occur. The study proposed that the protons from the jets interact with this surrounding gas, producing gamma rays in the process. These gamma rays are then detectable by instruments such as Fermi’s telescope.
By studying such gamma rays, scientists can trace the origins of high-energy particles and better understand the processes through which they are accelerated. This discovery not only provides insights into GRS 1915+105 but also opens up new questions regarding the potential for other low-mass microquasars to contribute to the larger cosmic ray population.
Implications: The Role of Microquasars in Cosmic Ray Generation
The newly observed particle acceleration capabilities of low-mass microquasars have significant implications for our understanding of cosmic rays. Previously, it was assumed that only certain systems, particularly the high-mass microquasars with the largest stellar companions, were efficient enough to contribute significantly to the cosmic ray budget of the galaxy. However, with the discovery of cosmic-ray acceleration in a system with a low-mass star, this assumption no longer holds.
Since low-mass microquasars are far more numerous than their high-mass counterparts, this discovery suggests that they could contribute a much larger fraction of cosmic rays than previously thought. This discovery could alter current estimates about the sources of high-energy cosmic rays, offering a revised understanding of their origins in our galaxy.
However, many questions still remain. Not all microquasars appear to be capable of accelerating particles to high energies, so the next steps for researchers are to identify why some microquasar systems—whether high-mass or low-mass—are particularly effective at particle acceleration while others are not. Future research, especially in the form of more multi-wavelength observations across radio, X-ray, and gamma-ray frequencies, will be essential to refining our understanding of these enigmatic objects.
Conclusion
The detection of particle acceleration in low-mass microquasars such as GRS 1915+105 represents a remarkable step forward in our understanding of cosmic ray origins. By challenging established views about which systems can produce the high-energy particles that bombard our planet, these findings open new research avenues, potentially reshaping our understanding of the sources and mechanisms that drive cosmic ray production. The insights provided by this discovery will have far-reaching implications not only in the study of microquasars but in the quest to answer some of the deepest questions in astrophysics, including the acceleration of matter in extreme cosmic environments.
Reference: Guillem Martí-Devesa et al, Persistent GeV Counterpart to the Microquasar GRS 1915+105, The Astrophysical Journal Letters (2025). DOI: 10.3847/2041-8213/ada14f