Physicists have known that supermassive black holes at the center of galaxies, also known as active galactic nuclei (AGNs), produce the ultrahigh-energy cosmic rays (UHECRs) detected on Earth. However, the mechanism behind this was unknown. A team of scientists from the SLAC National Accelerator Laboratory in Menlo Park, California, has now come up with a theory that depicts black holes as giant particle accelerators in space.
The rotation of AGNs emits astrophysical jets, or beams of ionized matter, in both directions along the axis of rotation. Most of the jets’ energy is stored in powerful magnetic fields that assume a helical shape. The previous simulations done by SLAC scientists had modeled the jet plasma as a fluid. But for this specific study, the team went ahead and modeled the plasma as individual particles.
To be exact, the team’s supercomputer ran simulations of 550 billion particles and observed as they moved through the jet, focusing their activity near the “kink instabilities,” a region where the jet twists and bends. They discovered that continuous wiggling of the magnetic field produced an electric field. The kink instabilities tangled the magnetic field line to an extent that particles crossed them and were accelerated by the electric fields into energies that could turn into UHECRs.
“What is distinctive and exciting about our recent work… is that it shows that a mechanism that operates naturally in the magnetic field structure known to exist in AGN jets can simultaneously explain the generation of high-energy radiation and high-energy cosmic rays,” Frederico Fiuza, who led the study, said to APS Physics.
In another breakthrough, scientists from the IceCube Collaboration have recently traced the origins of a high-energy cosmic particle called a neutrino that traveled 3.7 billion light years to arrive on Earth. It was identified by sensors of the IceCube detector deep inside the Antarctic Ice.
IceCube is an international observatory that consists of more than 5,000 photomultiplier tubes inside a grid that covers one cubic kilometer of ice. When a neutrino hits an atomic nucleus on the ice, it generates a blue light called Cerenkov radiation. With this specific neutrino, the photomultipliers of IceCube picked up blue light and alerted the scientists.
“This identification launches the new field of high-energy neutrino astronomy, which we expect will yield exciting breakthroughs in our understanding of the universe and fundamental physics, including how and where these ultra-high-energy particles are produced… For 20 years, one of our dreams as collaboration was to identify the sources of high-energy cosmic neutrinos, and it looks like we’ve finally done it!” Doug Cowen, a founding member of the IceCube collaboration, said in a statement (CNN).
The neutrino was identified as having come from a galaxy that has a supermassive, spinning black hole (blazar) in its center and is about 4 billion light-years away from Earth. Though IceCube scientists were able to zero in on the source of the neutrinos, they are yet to finalize where exactly they were produced. “It is clear that the supermassive black hole provides the accelerator power,” Francis Halzen from the University of Wisconsin, Madison, said to The New York Times.
The reason why neutrinos attract the interest of physicists is that they have no electrical charge. As such, they are not affected by intergalactic magnetic fields or other forces that might scramble their paths. So if the scientists identify a specific neutrino, it is possible to trace it back to its source with very high accuracy.