Extreme particle acceleration: New source of cosmic rays discovered

Cosmic rays contain some of the most energetic particles in the universe, moving at nearly the speed of light. These subatomic particles, such as electrons and protons, travel through space, carrying essential clues about astrophysical phenomena.

While some originate from within our solar system, the most powerful ones come from distant cosmic sources.

The origin and acceleration of these high-energy particles remain one of astrophysics’ greatest puzzles. Scientists have long theorized that extreme environments, such as the jets of black holes, could provide the necessary conditions for this extreme particle acceleration.

However, the details of how and under what circumstances these processes occur are still not well understood.

Microquasars and cosmic rays

Microquasars, a type of binary system composed of a stellar-mass black hole and a companion star, have been identified as promising sites for high-energy particle acceleration. In these systems, the black hole pulls in material from its companion, forming a swirling accretion disk.

This process generates powerful jets that shoot matter away at exceptionally high speeds.

Such jets are thought to play a significant role in accelerating cosmic rays but, until now, observational evidence has been scarce and limited to specific systems.

Microquasars in high-energy physics

Microquasars exist in two main forms that are differentiated by the mass of their companion stars. High-mass microquasars contain stars that are significantly larger than the Sun, while low-mass microquasars have much smaller companion stars.

High-energy particle acceleration had previously been observed only in the high-mass category, leading many researchers to believe that low-mass microquasars lacked the necessary conditions to produce cosmic rays.

One of the most well-known examples of a high-mass microquasar is SS 433, which was recently revealed to be one of the most powerful particle accelerators in the galaxy.

This system contains a companion star with a mass approximately ten times that of the Sun. Its powerful jets emit high-energy gamma rays, reinforcing the idea that such systems could significantly contribute to the total cosmic ray production in the Milky Way.

The question remained whether all microquasars could accelerate cosmic particles, or if only the most massive ones had this capability.

Since low-mass microquasars are much more abundant than their high-mass counterparts, answering this question was essential for understanding their role in cosmic ray production.

Low-mass microquasar emitting gamma rays

A recent discovery by Dr. Laura Olivera-Nieto from the Max-Planck-Institut für Kernphysik and Dr. Guillem Martí-Devesa from Università di Trieste has challenged previous assumptions about particle acceleration.

Using 16 years of observational data from NASA’s Fermi Large Area Telescope, the experts detected a faint gamma-ray signal linked to GRS 1915+105, a microquasar with a low-mass companion star.

The presence of gamma rays at energies exceeding 10 GeV suggests that this system is accelerating particles to even higher levels. This marks the first concrete evidence of particle acceleration occurring in a low-mass microquasar.

The discovery fundamentally shifts the understanding of which systems contribute to cosmic ray production in the galaxy.

Mechanism behind the acceleration

The study proposes that the detected gamma rays result from proton acceleration within the microquasar’s jets.

These high-energy protons escape the system and collide with nearby gas, producing gamma-ray photons. This mechanism aligns with previous theoretical predictions about how cosmic rays might originate from black hole environments.

Further supporting this hypothesis, additional data from Japan’s Nobeyama 45-meter Radio Telescope indicate that there is enough surrounding gas to sustain this process.

The presence of this gas is crucial, as it provides the necessary medium for the accelerated protons to interact with and generate the observed gamma-ray emission.

Implications for cosmic ray research

This discovery reshapes the understanding of how cosmic rays are produced and distributed throughout the galaxy.

Given that low-mass microquasars are significantly more common than their high-mass counterparts, their total contribution to the cosmic ray population could be far greater than previously assumed. This challenges earlier models that relied primarily on the influence of high-mass microquasars and supernova remnants.

Despite this breakthrough, many questions remain. Are all low-mass microquasars capable of accelerating cosmic rays, or is GRS 1915+105 an exceptional case? What specific conditions enable efficient particle acceleration in these systems?

Further observations and multi-wavelength studies will be required to refine these findings and better understand the underlying mechanisms.

Future of high-energy astrophysics

The discovery that low-mass microquasars can act as cosmic ray accelerators opens new research avenues. If similar gamma-ray signals are detected in other low-mass systems, it could lead to a major revision of current cosmic ray models.

Future missions involving more sensitive space- and ground-based telescopes will play a critical role in identifying additional sources and confirming the extent of their contributions.

This research also highlights the importance of long-term observational studies. By analyzing extensive datasets spanning multiple years, researchers can uncover faint signals that might otherwise go undetected.

The use of multi-wavelength approaches – combining gamma-ray, radio, and X-ray observations – will continue to refine the understanding of how these extreme environments function.

Microquasars, once thought to be relatively minor players in cosmic ray production, are now emerging as significant contributors.

Whether high-mass or low-mass, these systems demonstrate the universe’s ability to accelerate particles to incredible energies, reshaping how scientists view the origin of some of the most energetic particles in space.

The study is published in The Astrophysical Journal Letters.

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