A collection of telescopes in Namibia has shed light on the origin of some of the most powerful particles in the galaxy. The observations suggest that these particles are produced in a region known as the Manatee Nebula, where a black hole accelerates matter to incredibly high speeds, approaching the speed of light.
On January 25th, scientists from the HESS (High Energy Stereoscopic System) published their discoveries in Science, bringing us closer to unraveling the mystery of cosmic rays. These rays consist of rapidly moving atomic nuclei and other particles, which consistently collide with the Earth’s upper atmosphere.
According to Sera Markoff, a theoretical astrophysicist at the University of Amsterdam, the data provided by HESS is remarkable for individuals such as herself who are interested in studying astrophysical jets, including their internal structure, movement, and development.
Space rain
Cosmic rays possess varying levels of energy. The most prevalent and least energetic cosmic rays are composed of solar wind particles that descend upon Earth’s atmosphere after moving in a spiral pattern within the planet’s magnetic field. On the other hand, cosmic rays with significantly higher energies are believed to be generated by the explosive demise of massive stars known as supernovae.
Moreover, cosmic rays with even greater levels of energy originate from outside of our Galaxy, particularly from quasars – supermassive black holes that emit jets of plasma traveling at nearly the speed of light. These jets can possess energies up to eight magnitudes higher than those produced in particle accelerators.
Scientists specializing in astrophysics have suggested that plasma jets originating from black holes that are smaller than quasars, yet significantly larger than the Sun, may play a role in the cosmic-ray population. These jets, known as “microquasars,” emit X-rays and radio waves, in addition to their high energies, which fall between the levels produced by supernovas and quasars.
In a recent investigation, scientist Laura Olivera-Nieto, stationed at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, and her colleagues, examined a microquasar known as SS 433.
This black hole is situated in the Aquila Constellation, approximately 18,000 light years (5.5 kiloparsecs) away from our solar system, and is part of a binary system with a massive star. The black hole is surrounded by matter that is expelled from the star and then sucked into it, producing immensely powerful jets.
The binary system is enveloped by a nebula known as the Manatee, named for its elongated shape, which is composed of leftover dust and gas from a supernova that occurred between 10,000 and 100,000 years ago.
The supernova’s core collapsed, forming a black hole, and the subsequent outflow of matter produced cosmic rays for thousands of years. However, this activity subsided over time. Interestingly, the system reactivated between 10,000 and 30,000 years ago, when the black hole formed its jets, which the researchers believe is when it began producing cosmic rays once again.
Celestial clues
Cosmic-ray particles that come from a microquasar will travel in spiral patterns throughout the Galaxy before reaching Earth due to the influence of magnetic fields. This bending of their trajectories makes it challenging to track down their exact source.
Instead, scientists studying cosmic rays focus on γ-ray photons as indicators, since they are expected to be generated in the same processes that accelerate cosmic-ray particles. Unlike the cosmic rays, these γ-ray photons travel to Earth in straight paths.
In 2018, scientists initially detected γ-rays originating from SS 433 at the HAWC observatory located in Mexico’s Pico de Orizaba National Park. However, unlike the HESS team, they faced difficulty in accurately determining the specific origin of these γ-rays.
HAWC and HESS both detect γ-ray photons, albeit through distinct methods. When a γ-ray interacts with an atmospheric nucleus, it generates a cascade of secondary particles, including electrons and muons.
HAWC employs water-filled tanks to capture these particles as they reach the ground, while HESS uses its five dishes to image the light pulses produced by the particles as they travel through the atmosphere. By pointing the dishes in a specific direction, HESS can track the source of the γ-rays in the sky.
By utilizing this technique, HESS was able to accurately determine the precise location within the Manatee Nebula where the γ-rays originated, and concentrate on differentiating those with specific energies.
Through more than 200 hours of observations conducted over a span of 3 years, it was revealed that the emission of γ-rays initiates approximately halfway between the black hole and the supernova remnant, gradually diminishing afterwards.
Olivera-Nieto emphasizes the significance of the discovery, stating that the highest-energy photons are exclusively emitted from closer proximity to the black hole.
According to Olivera-Nieto, the origin of the γ-rays, and consequently the cosmic rays, can be traced back to the internal mechanisms of the jets, rather than external collisions with other matter. The surrounding space is devoid of any obstructions, having been cleared by the supernova’s powerful shockwave.
The discovery reinforces the notion that X-ray binaries share similarities with supermassive black holes and are equally efficient in propelling cosmic rays, according to Markoff. He commends Olivera-Nieto’s methodology, which enabled the utilization of a larger dataset and increased the sensitivity, thereby enabling this groundbreaking research and paving the way for future studies.