New theory for why black holes and neutron stars shine bright
Research points to interaction between chaotic motion and reconnection of magnetic fields.
Scientists have long speculated about the origin of the electromagnetic radiation emitted from celestial regions that host black holes and neutron stars.
Astrophysicists believe this high-energy radiation is generated by electrons that move at nearly the speed of light, but the physics underlying the process has remained unclear.
Now, researchers from Columbia University in the US have presented a new theory in a paper in The Astrophysical Journal.
Luca Comisso and Lorenzo Sironi used super-computer simulations to calculate the mechanisms that accelerate these particles and concluded that it is a result of the interaction between chaotic motion and reconnection of super-strong magnetic fields.
"Turbulence and magnetic reconnection – a process in which magnetic field lines tear and rapidly reconnect – conspire together to accelerate particles, boosting them to velocities that approach the speed of light," says Comisso.
The region that hosts black holes and neutron stars is permeated by an extremely hot gas of charged particles, he adds, and the magnetic field lines dragged by the chaotic motions of the gas, drive vigorous magnetic reconnection.
"It is thanks to the electric field induced by reconnection and turbulence that particles are accelerated to the most extreme energies, much higher than in the most powerful accelerators on Earth, like the Large Hadron Collider at CERN."
When studying turbulent gas, scientists cannot predict chaotic motion precisely. To address this challenge, Comisso and Sironi used super-computer simulations they say are among the largest ever developed in this research area.
"We used the most precise technique – the particle-in-cell method – for calculating the trajectories of hundreds of billions of charged particles that self-consistently dictate the electromagnetic fields,” says Sironi. “And it is this electromagnetic field that tells them how to move."
The crucial point of the study was to identify the role magnetic reconnection plays within the turbulent environment. Sironi says the simulations show that reconnection is the key mechanism that selects the particles that will be subsequently accelerated by the turbulent magnetic fields up to the highest energies.
The simulations also revealed that particles gained most of their energy by bouncing randomly at an extremely high speed off the turbulence fluctuations. When the magnetic field is strong, this acceleration mechanism is very rapid. But the strong fields also force the particles to travel in a curved path, and by doing so, they emit electromagnetic radiation.
"This is indeed the radiation emitted around black holes and neutron stars that make them shine, a phenomenon we can observe on Earth," Sironi says.