A flicker from the dark

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

While light can’t escape a black hole, the bright glow of rapidly spinning gas (think of the 2019 images of the M87 black hole) has its own unique flicker. In a recent article filed with Astrophysical Journal Letters, UC Santa Barbara’s Sean ResslerLena Murchikova of the Institute for Advanced Study and Chris White of Princeton University 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* ) – which provided insight into properties such as structure and movement.

There has been a lot of excitement recently, and for good reason, about the new image of the black hole at the center of our galaxy. “But a single photo only tells part of the story,” said Ressler, a postdoctoral researcher at the 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 from now on we can only create blurry, flickering images. Fortunately, the flickering pattern encodes a lot of information. “Here we have shown that our model of gas ingress from nearby stars reproduces that same pattern much better than previous models,” Ressler added.

This is 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 material into a job over a long period of time. 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.”

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

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 tracking the material they repel as they orbit the black hole. His recent work culminated in an Astrophysical Journal Letter paper in 2020.

In October 2021, Murchikova . published a newspaper in Astrophysical Journal Letters, introducing 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, a former KITP postdoc, has been working 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. An astrophysical journal publication earlier this year he summarized some of his findings.

Murchikova, White and Ressler then collaborated 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 envision an artificial ring of gas, roughly shaped like a donut, some great distance from the black hole. We found 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. In this simulation, the incident gas 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, providing the most authentic image.”

This release is co-written by Lee Sandberg at the Institute for Advanced Study

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