Flicker from the dark: reading between the lines to model our galaxy’s central black hole

Appearances are deceiving. The light from an incandescent lamp appears stable, but flickers 120 times per second. Because the brain perceives only an average of the information it receives, this flicker is fuzzy and the perception of constant lighting is just an illusion.

While light cannot escape a black hole, the bright glow of rapidly spinning gas (remember the images of the black hole from M87 and Sgr A*) has its own unique flicker. In a recent article, published in Astrophysical Journal Letters, Lena Murchikova, William D. Loughlin Member at 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 construct the most accurate model yet of our own galaxy’s central black hole — Sagittarius A* (Sgr A*) — that provided insight into properties such as its structure and movement.

For the first time, researchers have shown in a single model the full story of how gas travels through the center of the Milky Way — from being blown away by stars to falling into the black hole. By reading between the proverbial lines (or flickering light), the team concluded that the most likely picture of a black hole’s feeding at the galactic center is directly incident gas from great distances, rather than a slow transfer of orbital material across a long period of time.

“Black holes are the gatekeepers of their own secrets,” Murchikova said. “To better understand these mysterious objects, we depend on direct observation and high-resolution modeling.”

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 investigate them through observations.

In October 2021, Murchikova published a paper in Astrophysical Journal Letters, which introduced a method to study black hole flicker on a timescale of a few seconds instead of a few minutes. This advancement allowed a more accurate quantification of the properties of Sgr A* based on its flicker.

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

Ressler has spent years trying to create the most realistic simulations to date of the gas around Sgr A*. He did this by including observations of nearby stars directly into the simulations and closely monitoring what material they release when it falls into the black hole. His recent work culminated in a Astrophysical Journal paper in 2020.

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

“The result turned out to be very interesting,” explains Murchikova. “For a long time, we thought we could largely ignore where the gas around the black hole came from. Typical models imagine an artificial ring of gas, roughly shaped like a donut, some great distance from the black hole. We discovered that such models produce patterns of flicker that are inconsistent with observations.”

Ressler’s stellar wind model takes a more realistic approach, where the gas consumed by black holes is originally repelled by stars near the galactic center. When this gas falls into the black hole, it reproduces the correct flicker pattern. “The model was not built with the intention of explaining this particular phenomenon. Success was by no means a guarantee,” Ressler said. “So it was very encouraging to see the model become such a huge success after years of work.”

“If we study flicker, we can see changes in the amount of light emitted by the black hole, second by second, and take thousands of measurements over the course of a single night,” explains White. “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 reduce the constraints of each, yielding the most authentic image.”

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