Unraveling the Mystery of Jets in Black Holes

Friday - April 26 2024, 02:13 UTC - 6 months ago

A team of researchers used three-dimensional simulations to compare the jet formation models for the supermassive black hole in M87 with actual observations from the Event Horizon Telescope, finding that the BZ-jet model accurately predicted the observed morphology of the jet while the disk-jet model struggled to explain it. The study provides new insights into the mechanisms behind the formation of jets in black holes.

Black holes are fascinating cosmic entities characterized by gravitational pulls so intense that not even light can escape once it crosses into their event horizons. Yet, intriguingly, more than a hundred years ago, it was discovered that right outside the event horizon, black holes can produce potent streams of matter and energy, referred to as jets, which can travel nearly as fast as light. Telescopic observations have shown these jets extending directly outward in focused streams, resembling laser beams, with some jets reaching lengths that exceed entire galaxies.

Since the discovery of jets, many scholars, including Nobel Laureate Sir Roger Penrose, have studied the formation of these enigmatic phenomena. Currently, two main models attempt to explain jet formation: The “BZ-jet model,” named for the researchers Blandford and Znajek and now the most influential model, posits that a jet is formed by extracting spin energy from a black hole via magnetic field lines connected to the black hole’s event horizon. In contrast, the second model posits that a jet is formed by extracting rotational energy from a black hole’s accretion disk. The latter is a collection of ionized gas rotating around the black hole due to its strong gravitational force. The second model may be described as the “disk-jet model.” .

Although the BZ-jet model had already been used by other researchers to simulate general relativistic collimated outflows—effectively, jets—it was unclear whether the BZ-jet model could explain the observed morphology of an actual jet, including its elongated structure, width, and limb-brightening, i.e., its increased brightness near the edge of the jet.

To investigate the validity of these two models, an international team led by Dr. Yuan Feng from the Shanghai Astronomical Observatory of the Chinese Academy of Sciences calculated the jets respectively predicted by these two models for the supermassive black hole at the center of Messier 87 (M87), a giant galaxy in the constellation Virgo. The team then compared its calculations with actual observations of the M87 jet, which had been recorded in the first-ever image of a black hole captured by the Event Horizon Telescope (EHT). The team’s research showed that the BZ-jet model accurately predicted the morphology of the observed M87 jet, while the disk-jet model struggled to explain the observations. The study was published in Science Advances.

In terms of methods, the team first employed three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations to reproduce the structure of the M87 jet. To calculate the radiation from the simulated jets and compare the radiation with observations, the energy spectrum and spatial distribution of radiating electrons were crucial. The team hypothesized that electron acceleration occurred through “magnetic reconnection,” i.e., a process whereby magnetic energy is converted into kinetic energy, thermal energy, and particle acceleration. Based on this hypothesis, the team combined the results of particle acceleration studies using kinetic theory and GRMHD simulations to obtain the energy spectrum and spatial distribution of radiating electrons.

Under the BZ-jet model, which predicts that ejecta leave the black hole in a vacuum, the team found that the magnetic field and high-speed plasma motions at the base of the jet were so strong that the electrons could be accelerated to highly relativistic speeds. The team determined that, in most cases, the energy spectrum of the radiating electrons was power-law-shaped, with energies at TeV energies dominating the non-thermal synchrotron emission (synchrotron emission refers to radiation produced by charged particles moving at relativistic speeds in the presence of a magnetic field). Moreover, the team found that this emission may play a key role in the radio emission of the jet. Meanwhile, the simulated M87 jet was elongated and had a width similar to that observed.