Telescopes in Namibia have made a groundbreaking discovery, revealing the origin of some of the most energetic particles in the galaxy. The High Energy Stereoscopic System (HESS), a group of telescopes, has identified a region in the Manatee Nebula where particles of matter expelled by a black hole are accelerated to near-light-speed. This finding, published in Science by researchers at HESS, is a significant step forward in understanding the origins of cosmic rays.
Cosmic rays are fast-moving atomic nuclei and other particles that constantly collide with Earth’s upper atmosphere. They can have a wide range of energies, with the lowest-energy cosmic rays consisting of particles from the solar wind. On the other hand, the highest-energy cosmic rays are believed to be produced by supernovae, the explosive deaths of massive stars. Even more energetic cosmic rays originate from quasars, super-massive black holes that generate jets of plasma traveling at near-light-speed.
Astrophysicists have suggested that plasma jets from smaller black holes, known as microquasars, could also contribute to the cosmic-ray population. These microquasars, which are brighter sources of X-rays and radio waves, could produce energies intermediate between those from supernovae and quasars.
In a recent study, astrophysicist Laura Olivera-Nieto and her colleagues at the Max Planck Institute for Nuclear Physics focused on a microquasar called SS 433. Located in the Aquila Constellation, approximately 18,000 light years away from the Solar System, SS 433 forms a binary system with a large star. Matter ejected from the star spirals around the black hole and generates highly energetic jets. Surrounding this binary system is a nebula nicknamed the Manatee due to its elongated shape. The nebula is composed of dust and gas left over from a supernova that occurred between 10,000 and 100,000 years ago.
The researchers believe that the black hole in SS 433 started producing cosmic rays again between 10,000 and 30,000 years ago when it formed its jets. To trace the origins of cosmic rays, astrophysicists search for γ-ray photons, which are produced in the same processes that accelerate cosmic-ray particles. However, the trajectories of cosmic-ray particles are bent by magnetic fields as they move across the galaxy, making it impossible to trace their paths back to a specific source. Therefore, astronomers rely on detecting γ-ray photons that travel to Earth in straight lines.
In 2018, γ-rays from SS 433 were first observed by the High Altitude Water Cherenkov (HAWC) observatory in Mexico’s Pico de Orizaba National Park. However, the HAWC team was unable to precisely locate the exact source of these γ-rays. On the other hand, HESS, which also detects γ-ray photons indirectly, uses a different approach. HESS consists of five dishes that can be pointed in a specific direction in the sky. By imaging flashes of light produced by secondary particles as they move down the atmosphere, HESS was able to precisely locate where in the Manatee Nebula the γ-rays were produced.
Over 200 hours of observations made over three years revealed that the γ-ray emission starts halfway between the black hole and the supernova remnant and gradually diminishes. The crucial discovery was that the highest-energy photons come from closer to the black hole. This suggests that the γ-rays and cosmic rays are produced by mechanisms internal to the jets rather than by collisions with other matter. The space surrounding the black hole is otherwise empty, cleared by the supernova’s expanding shockwave.
This finding strengthens the case that X-ray binaries, such as microquasars, are smaller analogues to supermassive black holes and are equally capable of accelerating cosmic rays. The data analysis conducted by Olivera-Nieto allowed for the use of more data and amplified the sensitivity enough to conduct this remarkable study. It sets the stage for further research in this field.
The discovery made by the telescopes in Namibia provides valuable insights into the origins of cosmic rays and contributes to our understanding of the universe. By pinpointing the location where particles are accelerated to near-light-speed, astrophysicists can continue to unravel the mysteries of cosmic rays and their impact on our planet.
