Black hole jets create ‘negative energy’ particles, simulation reveals
29 Jan 2019
New insights into how luminous jets form around rapidly-rotating black holes have been provided by advanced computer simulations done by astrophysicists in the US and France. Their model could play an important role in interpreting future electromagnetic and gravitational-wave observations of black holes.
Black hole jets are some of the brightest sources of X-ray and radio emissions known to astronomers. These jets are formed when black hole surfaces spinning at relativistic speeds are threaded with magnetic field lines. Interactions with infalling gas cause these fields to become tightly wound into helixes around the black hole’s axis of rotation. Huge amounts of energy within the coiled field lines is dissipated gradually through the creation of electron-positron pairs. A cascade process creates a huge jet of energetic plasma that emits vast amounts of electromagnetic radiation.
Much of the jet-formation process remains a mystery, however, and astrophysicists are trying to improve their knowledge using general relativistic magnetohydrodynamics (GRMHD) computer simulations. Despite successes in recreating energy transfer between black holes and magnetic fields, these simulations face a major shortcoming. Rather than regarding the plasma formed by pair-creation as a collection of individual particles, GRMHD treats it as a continuous fluid. Such a crude approximation means that small-scale variations in plasma density; important for accurately modelling its overall dynamics, are disregarded.
Realistic density variations
To create a more physically-accurate description, Kyle Parfrey at Lawrence Berkeley National Laboratory, Alexander Philippov at the Flatiron Institute and Benoît Cerutti of the University of Grenoble Alpes modelled the plasma as being “collisionless” – whereby collisions between particles can be ignored. By accounting for pair-creation within the electric fields induced by dynamic magnetic fields, the team’s simulation could model plasmas with far more realistic density variations than achieved previously. Parfrey and colleagues ran two simulations to test their model. The first run had a high plasma density due to a low threshold for pair-creation, and the second with a low density due to a high threshold.
Both simulations ran for about 12 black hole rotations, allowing them to settle to quasisteady states. The results showed some intriguing differences to previous GRMHD approximations. From the vantage of a distant observer, some relativistic particles in the models appeared to have “negative energies”. As these particles fall into the black hole, they reduce its rotational energy in a process that was first predicted in 1971 by Roger Penrose. Surprisingly, the amount of energy these particles extract from the black hole is on par with the energy extracted by the magnetic field as it winds into a helix.
In future studies, Palfrey’s team hope to use even more realistic treatments of pair-creation to study the flow of these negative-energy particles in more detail. The physicists now believe their simulation will become an important tool for interpreting an ever-expanding number of gravitational wave observations, as well as future observations of the jets surrounding supermassive black holes.
The simulations are described in Physical Review Letters.
Sam Jarman is a science writer based in the UK
physicsworld.com 19/2/2019
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