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

June 22, 2022

(News from Nanowerk) Appearances can be deceiving. Light from an incandescent bulb seems steady, but it actually flickers 120 times per second. Because the brain only perceives an average of the information it receives, this flickering is blurry. The perception of constant lighting is only an illusion.

While light can’t escape from a black hole, the bright glow of fast-orbiting gas (remember the 2019 images of the M87 black hole) has its own unique shimmer. In a recent article submitted to Astrophysical Journal Letters (“Remarkable match of Sagittarius A* submillimeter variability with a stellar wind-powered accretion flow pattern”), Sean Ressler of UC Santa Barbara, Lena Murchikova of the Institute for Advanced Study, and Chris White of Princeton University were able to use this subtle flicker to build the most accurate model yet of the central black hole of our own galaxy – Sagittarius A* (Sgr A*) – giving insight into properties such as its structure and motion.

Simulation of incandescent gas around a black hole. (Video: Chris White, Princeton University)

There has been a lot of excitement recently about the new image of the black hole at the center of our galaxy, and with good reason. “But a single image only tells part of the story,” said Ressler, a postdoctoral researcher at UCSB’s Kavli Institute for Theoretical Physics (KITP). Ressler is supported by a grant to KITP from the Gordon and Betty Moore Foundation.

A video would be ideal, he noted, but for now we can only build blurry, flickering images. Fortunately, the flicker pattern encodes a lot of information. “Here we have shown that our model of gas falling inward from nearby stars reproduces this same pattern much better than previous models,” Ressler added.

It’s 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 picture of a black hole feeding into the galactic center involves direct gas falling from great distances, rather than a slow siphoning of material into orbit over a long period of time. 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.”

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

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 they orbit the black hole. His recent work has resulted in a Astrophysics Journal Letter paper in 2020 (“Horizon-scale ab initio simulations of magnetically arrested accretion in Sagittarius A* powered by stellar winds”).

In October 2021, Murchikova published an article in Astrophysical Journal Letters (“Sub-millimeter variability at the second Sagittarius A* scale during 2019 flaring activity: behind near-infrared flares”), introducing a method to study black hole flicker on a time scale 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, a former KITP postdoc, has been working on the details of what happens to gas near black holes — where strong general relativity effects matter — and how it affects light reaching us. A Astrophysical Journal publication earlier this year summarizes some of his findings (“The effects of tilt on the temporal variability of millimeter and infrared emissions from Sagittarius A*”).

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 came from around the black hole. Typical models imagine a roughly donut-shaped ring of man-made 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. In this simulation, the incoming gas replicates the correct flicker pattern. “The model was not built with the aim of explaining this particular phenomenon. Success was by no means a guarantee,” Ressler said. “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, making 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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