The Mystery of Black Hole Flares

In 2019, a team of researchers used an international network of radio telescopes—called the Event Horizon Telescope—to take the first photo of a supermassive black hole in the center of the elliptical galaxy Messier 87 (M87). On that team of researchers was JILA Fellow Jason Dexter. Since then, Dexter has been studying M87's black hole further using simulations, with code written by researchers at the University of Illinois. As described in a new paper published in the Monthly Notices of the Royal Astronomical Society (MNRAS), Dexter, and his team of graduate students and postdoctoral researchers, collaborated with researchers at the Los Alamos National Laboratory and the University of Illinois to create a new simulation studying the edge of a black hole.

The researchers specifically focused on the phenomenon of flaring that occurs around the black hole. Flares, also called jets, are outputs of hot gas. The source of these flares and the explanations for their behavior are still unknown. These unanswered questions intrigued first author Philippe Z. Yao, who led this study as a CU Boulder undergraduate student. “I started talking to Jason Dexter about what we can do to help us better understand the processes around black holes. And since we also see these very high-energy flares that come from M87, we do not know what processes really trigger them,”said Yao.  Dexter echoed this uncertainty about where the flares around the black hole occur: “The brightness changes can occur in about the same amount of time it would take for light to cross roughly the size of the event horizon of the black hole. So, it's possible that the flares happen close to the black hole.” The event horizon is the term for the edge of the black hole. The telescope images that showed the flares were confusing to the researchers, as the flares could have been either close to the event horizon, or farther away, just flaring at a faster rate.

In order to better understand where these flares were occurring, the researchers created General Relativistic Magneto-Hydro-Dynamic  (GRMHD) simulations, which naturally follow how gas flows along magnetic fields that thread through a black hole. The GRMHD simulations looked at accretion flow of the gas, where the gas spinning around the black hole slowly moves toward the black hole, and eventually falls in. Dexter explained this accretion flow in the simulation by describing the gas as: “an ionized fluid made of protons and electrons. The reason the gas falls into the black hole is that the charged particles conduct magnetic fields and the magnetic fields actually cause the gas to become unstable and turbulent. The turbulence causes collisions which knock the gas off of stable orbits around the black hole and cause it to fall in through the event horizon. And the magnetic fields also end up launching the jet that we see.” In studying how the gas moves toward the event horizon of the black hole, the researchers hoped to find an explanation for the cause of the flares as well as their positions relative to the black hole.

Gas and Light

The simulation not only modeled the flow of this gas, but also how it interacted with light particles, called photons. According to Dexter, the gas cools by radiating light and gamma rays. “The gas heats up, and then we can track its cooling,” Dexter said. “That puts us in a unique position, because all the mechanisms for how you might produce flares close to the black hole rely on how photons interact with matter. We're able to realize what we think is a physically realistic description of the gas and the photons near the black hole.” The study of the photons in the gas shed new light (no pun intended) on more processes occurring at the edge of a black hole.

This simulation is also important because it builds on previous black hole research. “Previous work on the flares have made simplifying assumptions about the low-energy photons near the black hole,” said postdoctoral researcher Alexander Chen, who had worked on flare models before this paper. “One of the results of this work is that we have a much better understanding of how these low-energy photons are distributed.” The team is planning to continue building on these simulations to find ways to answer questions about the black hole flares. “Our calculations provide new background conditions for calculations of the flaring process itself,” Dexter stated. “What we find is that how the light is moving inside the jet is not at all uniform, with photons moving mostly inwards close to the black hole and away from it as well.” Chen, for his part, is excited to continue the research: “We can now take this result and compute more accurately the gamma-ray flares, and compare with data. This is what we're going to do next.”

Written by Kenna Castleberry, JILA Science Communicator