Simulations of accretion flows around black holes, involving General Relativity and relativistic plasma physics, have led to a new model of how extreme particle acceleration is achieved in the hearts of galaxies, gamma-ray bursts, and elsewhere.
Visualization of a simulation for a black hole accretion flow. Shown, in a vertical slice through the accretion disk, are magnetic field lines and mass density (blue low to red high). Matter pushes into the hole through the bulging magnetic field.
The cosmos contains particle accelerators that dwarf anything we can build on Earth, in the form of systems of black holes and their accretion disks of orbiting matter that spew enormous relativistic jets of high energy particles and radiation. These range in size from microquasars in our own Galaxy to gamma-ray bursts in the far Universe to active nuclei at the centers of distant galaxies where the supermassive black holes have a billion or more times the mass of our Sun. In all cases, the interaction of the black hole and infalling matter sends out two opposing jets which we see glowing across the electromagnetic spectrum, and from where the mysterious ultra-high energy cosmic rays we receive on Earth may originate. The commonly accepted paradigm for how the jets come about is the Blandford-Znajek mechanism, developed more than 30 years ago by KIPAC director Roger Blandford and Roman Znajek, in which the rotational energy of the black hole is extracted to accelerate the particles that make their way in. However, there are still many crucial questions and mysteries about nature's giant accelerators, such as how the matter reaches the black hole, how the jets are collimated, what gives rise to particular fluctuations in jet output, and many others.
Because these cosmic accelerators are far away, and because an accretion disk of matter surrounds the black hole, the small scale details in the immediate neighborhood of the black hole and thus the launching of the jets can't be seen directly. Enter simulations, which allow us to probe the extreme physics within accretion disk and black hole systems with computers, by putting in the initial conditions and allowing systems to evolve with interactions according to the relevant physics which governs them. In such extreme systems as the vicinity of black holes, the important physical laws are General Relativity, which is needed to describe how gravity works at high densities, and Magnetohydrodynamics, which describes the physics of high energy charged particles in an environment of strong magnetic fields. Simulations combining General Relativity and Magnetohydrodynamics, or "GRMHD" for short, involve state-of-the-art computing, and require a deep understanding of the extreme physics to properly undertake and interpret.
In a recent paper, KIPAC postdoc Jonathan McKinney, along with Blandford and Alexander Tchekhovskoy of Princeton University, have reported on the results from a GRMHD simulation program of thick disk accretion flows around black holes that has very encouragingly reproduced observed properties of jet systems and has overcome some previous conflicts of simulations with observation and theory.
The authors show that magnetic flux should accumulate near the black hole to such a degree that the magnetic field becomes strong enough to suppress the previously assumed standard mechanism for getting accreting matter in near the black hole. Instead, they find that accretion is driven by large-scale magnetic torques due to the accumulation of ordered magnetic flux in a so-called "magnetically choked accretion flow" (MCAF) state. Next, they crucially show that in the MCAF state the Blandford-Znajek mechanism of extracting energy from a rotating black hole is optimized. Greater than 100% of the energy of the matter that goes into the black hole actually comes back out in the form of relativistic jets, having extracted some of the black hole's rotational energy.
Additionally, the team's results point to the spontaneous production of a large scale dipolar magnetic field near the black hole, the first time this has been seen to happen in simulations. A dipolar magnetic field has the simplest large-scale shape and may be necessary to mold the outflowing jets into the spectacular columns that we observe. Another feature that they see in their systems are so-called high frequency quasi-periodic oscillations, in which the system varies on semi-regular timescales and which has been observed in a variety of astrophysical contexts. These oscillations, which are sensitive to the details of the gravitational behavior in the neighborhood of the black hole, can be used to probe General Relativity in the strong field regime, which other, more traditional probes, cannot access.
The MCAF state is very different from most prior simulations, yet observations already suggest it is the preferred state to explain properties of the supermassive black hole system at the center of our own Galaxy and the much larger one at the center of the nearby active galaxy M87. For example, the radio source at the center of our own Galaxy shows a constant sign for circular polarization over 40 years and a similar constancy of the Faraday rotation measure, which probes the magnetic field environment around it. This is only possible if the field is ordered and has constant polarity in the disk or jet, which is a result of the MCAF state. Both these systems are intense targets for probing Einstein's general relativity and have been special science targets of the Fermi Gamma-ray Space Telescope.
These GRMHD simulations are yet another example of the profound interdependence of astrophysics and high energy physics, as they combine the study of relativistic plasmas, cutting-edge computing, gravity at extreme energies, nature's largest particle accelerators, and some of astronomy's most spectacular and enigmatic phenomena which pervade the Universe. The researchers will next focus their simulation efforts on extracting a more complete picture of the nature of the emission from accretion in the MCAF state in various systems, such as its spectra, variability, and spatial structure, and compare that with observations. A more thorough understanding of the jet-launching mechanism near black holes will profoundly affect many areas of investigation in astrophysics and the techniques developed will inform, and be informed by, plasma physics.
This work is based in part on a paper to appear in Monthly Notices of the Royal Astronomical Society, and is available from astro-ph at arXiv:1201.4163.