The Event Horizon Telescope (EHT) established a reputation worldwide in 2019 when it released the first-ever image of a black hole. This was made possible by the science of Very Long Baseline Interferometry (VLBI), a technique in which multiple instruments collect light to create a complete picture of what an object looks like. In this case, the image was of the supermassive black hole (SMBH) at the center of Messier 87, a massive galaxy 55 million light-years from Earth. This was followed by images of the relativistic jets emanating from two bright galaxies, and of Sagitarius A*, the SMBH at the center of the Milky Way.
Meanwhile, scientists with the EHT Collaboration are employing supercomputer simulations to sharpen their understanding of the environment beyond the outer boundary of black holes (aka. the event horizon). Among them is the team led by Andrew Chael, an associate research scholar at Princeton University and a fellow of the Princeton Gravity Initiative. He and his team conducted simulations of M87's SMBH using the Stampede2 and Stampede3 supercomputers at the Texas Advanced Computing Center (TACC). The resulting image (at top) shows how light from hot electrons spirals just beyond the black hole's "shadow."
Chael's research group is one of many using advanced simulations to model the dynamics of black hole shadows, including high-energy plasma, magnetic fields, and powerful gravity. All of these interact in a complex system that allows black holes to accrete infalling matter around them, release immense amounts of radiation, and produce relativistic jets that can extend for millions of light-years. The simulations consisted of 11 general relativistic magnetohydrodynamic simulations (GRMHDS), which take a fluid dynamics approach to simulating plasma interacting with gravity and magnetic field lines.
The image of the SMBH at the center of M87, captured by the Event Horizon Telescope in 2019. Credit: EHT Collaboration
Said Chael in a TACC press release:
Ever since we made that first black hole image, there's been a lot of work trying to understand the environment just around the black hole. We want to understand the nature of the particles of this plasma that the black hole is eating, and the details of the magnetic fields commingled with the plasma that in M87 launches huge, luminous jets of subatomic particles.
Since graduate school, Chael has been conducting simulations using the Extreme Science and Engineering Discovery Environment (XSEDE) and resources provided by TACC's Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS) program. Thanks to recent advancements he and his team made using his own code, their simulation reaches beyond traditional models that treat electrically-charged protons and electrons as a single entity.
"This paper is a first attempt [at] using a more advanced," added Chael, "more computationally expensive technique to directly model these separate particle species of electrons and protons to try to understand how they interact, and in particular, what the relative temperature of the two is."
Their simulations revealed that the temperature of the electrons around M87 is much higher than previously thought, about 100 times cooler than the protons. This is significant since temperature differences between these and the protons determine the brightness and other properties in the image. Therefore, the results highlight a fundamental tension between current models in plasma physics and the observations provided by the EHT. Looking ahead, Chael and his team plan to apply their simulation code to more EHT data of M87 to produce a movie that tracks its evolution over time.
Images from the 11 radiative simulations, showing the multiply-lensed photon ring (cyan) and the event horizon (magenta). Credit: DOI: 10.1093/mnras/staf200
A study Chael and his team conducted back in January compared the EHT's M87 black hole image to a wide range of simulations using the Stampede2 and Jetstream supercomputers. These revealed that while the size and structure of the SMBH's "shadow" remains consistent, it is subject to change. They further revealed that the brightest spot on the photon ring shifts over time because of the chaotic processes at work with dynamic plasma flows near the event horizon. As various plasma regions heat up and cool down, the appearance of the black hole undergoes subtle changes with time. Said Chael:
Black holes are extremely complicated environments. The best available tools we have are supercomputing simulations. It's amazing that we've been able to build these computers and codes that allow us to create accurate models of what's going on in such a strange and complicated relationship. Simulations give us confidence that we are accounting for all these effects, which are all interacting in complicated and sometimes unpredictable ways.
Further Reading: TACC
