Reading between the lines to model our galaxy’s central black hole

Reading between the lines to model our galaxy’s central black hole

A Flicker of Darkness: Reading Between the Lines to Model Our Galaxy's Central Black Hole

Simulation of incandescent gas around a black hole. Credit: Chris White, Princeton University

Appearances can be deceiving. The light from an incandescent bulb seems steady, but it flashes 120 times per second. Because the brain only perceives an average of the information it receives, this flicker is blurred and the perception of constant lighting is only an illusion.

While light cannot escape from a black hole, the bright glow of fast orbiting gas has its own unique twinkle. In a recent article, published in Astrophysical Journal Letters, Lena Murchikova, William D. Loughlin Member of the Institute for Advanced Study; Chris White of Princeton University; and Sean Ressler of the University of California, Santa Barbara were able to use this subtle flicker to build the most accurate model yet of our own galaxy’s central black hole – Sagittarius A* (Sgr A*) – giving insight into properties such as its structure and motion.

For the first time, researchers have shown in a single model the full story of how gas moves through the center of the Milky Way, from its expulsion by the stars to its fall into the black hole. Reading between the proverbial lines (or flickering light), the team concluded that the most likely picture of a black hole feeding into the galactic center involves a direct fall of gas from great distance, rather than a slow siphoning of material into orbit over a long period of time.

Simulation of incandescent gas around a black hole. Credit: Chris White, Princeton University

“Black holes are keepers of their own secrets,” Murchikova said. “In order to better understand these mysterious objects, we depend on direct observation and high-resolution modelling.”

Although the existence of black holes was predicted about 100 years ago by Karl Schwarzschild, based on Albert Einstein’s new theory of gravity, researchers are only now beginning to probe them with observations.

In October 2021, Murchikova published an article in Astrophysical Journal Letters, introducing a method to study black hole flicker on a timescale of seconds, instead of minutes. This advance allowed a more precise quantification of the properties of Sgr A* as a function of its scintillation.

White worked on the details of what happens to gas near black holes (where the strong effects of general relativity are important) and how it affects the light reaching us. A Astrophysical Journal publication earlier this year summarizes some of his findings.

Ressler has spent years trying to build the most realistic simulations yet of the gas around Sgr A*. He did this by incorporating observations of nearby stars directly into the simulations and meticulously tracking the material they release as it falls into the black hole. His recent work has resulted in a Astrophysical Journal Letters paper in 2020.

Murchikova, White, and Ressler then teamed up to compare the observed flicker pattern of Sgr A* with those predicted by their respective numerical models.

“The result turned out to be very interesting,” explained Murchikova. “For a long time, we thought we could largely ignore where the gas around the black hole was coming from. Typical models imagine a man-made, roughly doughnut-shaped ring of gas at a great distance from the black hole. We found that such patterns produce flicker patterns inconsistent with observations.”

Ressler’s stellar wind model takes a more realistic approach, in which gas consumed by black holes is initially expelled by stars near the galactic center. When this gas falls into the black hole, it replicates the correct pattern of flickering. “The model was not built with the intention of explaining this particular phenomenon. Success was by no means a guarantee,” Ressler commented. “So it was very encouraging to see the model succeed so dramatically after years of work.”

“When we study flicker, we can see changes in the amount of light emitted by the black hole second by second, taking thousands of measurements over a single night,” White explained. “However, this does not tell us how the gas is arranged in space as a large-scale image would. By combining these two types of observations, it is possible to soften the limits of each, thus obtaining the most authentic image.”

NASA visualization brings together the best-known black hole systems

More information:
Lena Murchikova et al, Remarkable match of Sagittarius A* submillimeter variability with a stellar wind-powered accretion flow model, Letters from the Astrophysical Journal (2022). DOI: 10.3847/2041-8213/ac75c3

Lena Murchikova et al, Sagittarius A* second-scale submillimeter variability during 2019 flaring activity: on the origin of near-infrared flares, Letters from the Astrophysical Journal (2021). DOI: 10.3847/2041-8213/ac2308

Christopher J. White et al, The effects of tilt on the temporal variability of millimeter and infrared emissions from Sagittarius A*, The Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac423c

Sean M. Ressler et al, Ab Initio Horizon-scale Simulations of Magneticly Arrested Accretion in Sagittarius A* Fed by Stellar Winds, The Astrophysical Journal (2020). DOI: 10.3847/2041-8213/ab9532

Provided by the Institute for Advanced Study

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