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

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 (remember the black hole images of M87 and Sgr A*) has its own unique flicker. 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 a 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 moves through the center of the Milky Way – from being blown up by stars to falling into the black hole. Reading between the proverbial lines (or flickering light), the team concluded that the most likely image of a black hole feeding at the galactic center involves gas falling directly from great distances, rather than siphoning it off. slow orbiting material over a long period of time.

“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 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.”

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Materials provided by Institute for Advanced Studies. Note: Content may be edited for style and length.

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