Appearances can be deceiving. An incandescent bulb's light appears constant, but it flickers 120 times per second. Because the brain only perceives an average of the information it receives, the flickering is distorted, and the perception of constant illumination is merely an illusion.
While light cannot escape a black hole, the bright glow of rapidly orbiting gas has its own distinct flicker (recall the images of M87's black hole and Sgr A*). In a recent paper published in Astrophysical Journal Letters, Lena Murchikova, a 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 flickering to construct the most accurate model to date of our galaxy's central black hole, Sagittarius A* (Sgr A*), providing insight into properties such as structure and motion.
For the first time, researchers have depicted the entire path of gas in the center of the Milky Way in a single model, from being blown off by stars to falling into a black hole. The team concluded that the most likely picture of black hole feeding in the galactic center involves directly infalling gas from great distances, rather than a slow siphoning off of orbiting material over a long period of time, by reading between the lines (or flickering light).
"Black holes are the gatekeepers of their own secrets," Murchikova explained. "We rely on direct observation and high-resolution modeling to better understand these mysterious objects."
Although Karl Schwarzschild predicted the existence of black holes around 100 years ago, based on Albert Einstein's new theory of gravity, researchers are only now beginning to investigate them through observations.
Murchikova published a paper in Astrophysical Journal Letters in October 2021, introducing a method for studying black hole flickering on a timescale of a few seconds rather than a few minutes. This advancement allowed for a more precise quantification of Sgr Aproperties *'s based on flickering.
White has been researching the specifics of what happens to gas near black holes (where the strong effects of general relativity are important) and how this affects the light that reaches us. Some of his findings were summarized in an earlier issue of the Astrophysical Journal.
Ressler has spent years attempting to create the most realistic simulations of the gas surrounding Sgr A* to date. He accomplished this by directly incorporating observations of nearby stars into the simulations and meticulously tracking the material shed as it falls into the black hole. His most recent work will be published in the Astrophysical Journal in 2020.
Murchikova, White, and Ressler then collaborated to compare the observed Sgr A* flickering pattern to those predicted by their respective numerical models.
"The outcome was very interesting," Murchikova explained. "For a long time, we assumed that we could ignore the source of the gas surrounding the black hole. Typical models envision an artificial ring of gas, roughly donut-shaped, at a great distance from the black hole. We discovered that such models produce flickering patterns that contradict observations."
Ressler's stellar wind model is more realistic, as the gas consumed by black holes is first shed by stars near the galactic center. When this gas falls into the black hole, it replicates the correct flickering pattern. "The model was not designed to explain this specific phenomenon. Success was far from certain "Ressler commented. "Seeing the model succeed so dramatically after years of work was very encouraging."
"When we study flickering, we can see changes in the amount of light emitted by the black hole second by second, taking thousands of measurements over the course of a single night," White explained. "However, this does not tell us how the gas is arranged in space, as a large-scale image would. Combining these two types of observations allows one to overcome the limitations of each, resulting in the most authentic picture."
The Astrophysical Journal Letters, 2022; 932 (2): L21 DOI: 10.3847/2041-8213/ac75c3
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