Mark A. Garlick/Simons Foundation

Black Holes Flicker When They Eat, Revealing Their Size

The feeding patterns of black holes offer insight into their size, researchers report in a new paper. The study, published in the August 13 issue of Science, reveals that the flickering in the brightness observed in actively feeding supermassive black holes is related to their mass.

“Black hole mass is the most important quantity for characterizing a black hole system, and typically it’s very hard to measure,” says study co-author Yan-Fei Jiang, an associate research scientist at the Flatiron Institute’s Center for Computational Astrophysics in New York City. “The relationship between flickering and black hole mass may provide a much simpler and more accurate way of determining this fundamental quantity.”

Supermassive black holes (SMBHs) are millions to billions of times more massive than the sun and usually reside at the center of massive galaxies. When dormant and not feeding on the gas and stars surrounding them, SMBHs emit very little light, and the only way astronomers can detect them is through their gravitational influences on the stars and gas in their vicinity. In the early universe, however, when SMBHs were rapidly growing, they were actively feeding on, or accreting, materials at intensive rates and emitting an enormous amount of radiation — sometimes outshining the entire galaxy in which they reside, the researchers say.

The new study, led by the University of Illinois Urbana-Champaign astronomy graduate student Colin Burke and professor Yue Shen, uncovers a definitive relationship between the mass of actively feeding SMBHs and the characteristic timescale in the light flickering pattern.

The observed light from an accreting SMBH is not constant. It displays a ubiquitous flickering over timescales ranging from hours to decades due to physical processes not yet understood. “There have been many studies that explored possible relations of the observed flickering and the mass of the SMBH, but the results have been inconclusive and sometimes controversial,” Burke says.

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This animation shows an artist’s impression of an accretion disk rotating around a central unseen supermassive black hole (SMBH). The accretion disk is feeding the SMBH and produces an enormous amount of radiation, allowing these SMBHs to be “seen” from faraway. The accretion process produces random fluctuations in the luminosity from the disk over time, mimicked by the “lightening” features across the disk. The pattern of these luminosity fluctuations is found to be related to the mass of the central SMBH in this new study. Mark A. Garlick/Simons Foundation

The team compiled a large dataset of actively feeding SMBHs to study the variability pattern of flickering. They identified a characteristic timescale, over which the pattern changes, that tightly correlates with the mass of the SMBH.

That timescale is known as the thermal time. Black holes don’t neatly follow our standard timekeeping of seconds, hours and days. Instead, one of their natural clocks is the time it would take for all the heat energy in the material flowing around the black hole to radiate away as photons. “It’s the most important timescale, since the photons are what we observe,” says Jiang, who worked on the theoretical component of the study. Thermal time allowed the scientists to connect what was going on in the black hole’s environment — such as the black hole’s mass — with the observed light fluctuations.

The researchers then compared the results with accreting white dwarfs — the remnants of stars like our sun — and found that the same timescale-mass relation holds, even though white dwarfs are millions to billions times less massive than SMBHs.

The light flickers are random fluctuations in a black hole’s feeding process, the researchers say. Astronomers can quantify this flickering pattern by measuring the power of the variability as a function of timescales. For accreting SMBHs, the variability pattern changes from short to long timescales. This transition of variability pattern happens at a characteristic timescale that is longer for more massive black holes.

The team compares black hole feeding to the way we eat or drink by equating this transition to a human belch. Babies frequently burp while drinking milk, while adults can hold in the burp for a more extended amount of time. Black holes kind of do the same thing while feeding, they say.

“These results suggest that the processes driving the flickering during accretion are universal, whether or not the central object is a supermassive black hole or a much more lightweight white dwarf,” Shen says.

“The firm establishment of a connection between the observed light flicker and fundamental properties of the accretor will certainly help us better understand accretion processes,” Jiang says.

Lucy Reading-Ikkanda/Simons Foundation

But the team says that there is an even more exciting application of their results. Astrophysical black holes come in a broad spectrum of mass and size. In between the population of stellar-mass black holes — which weigh less than several tens’ times the mass of the sun — and SMBHs, there is a population of black holes called intermediate-mass black holes (IMBHs) that weigh anywhere from 100 to 100,000 times the mass of the sun. IMBHs are expected to form in large numbers through the history of the universe, and they may provide the seeds necessary to grow into SMBHs later. Observationally, this population of IMBHs are surprisingly elusive, however. There is only one indisputably confirmed IMBH that weighs about 150 times the mass of the sun. And that IMBH was serendipitously discovered by the gravitational wave radiation from the coalescence of two less massive black holes.

“Now that there is a correlation between the flickering pattern and the mass of the central accreting object, we can use it to predict what the flickering signal from an IMBH might look like,” explains Burke.

The timing may just be perfect, the researchers say. Astronomers worldwide are waiting for the official kickoff of an era of massive surveys that monitor the dynamic and variable sky. Starting in late 2023, the Vera C. Rubin Observatory’s Legacy Survey of Space and Time will survey the sky over a decade and collect light flickering data for billions of objects.

“Mining the LSST dataset to search for flickering patterns that are consistent with accreting IMBHs has the potential to discover and fully understand this long sought mysterious population of black holes,” says co-author Xin Liu, an astronomy professor at the University of Illinois.

The study is a collaboration with astronomy and physics professor Charles Gammie and astronomy postdoctoral researcher Qian Yang, at the Illinois Center for Advanced Study of the Universe, and researchers at the University of California, Santa Barbara; the University of St Andrews, Scotland; the Flatiron Institute; the University of Southampton, U.K.; the United States Naval Academy; and the University of Durham, U.K.

Burke, Shen and Liu also are affiliated with the Center for Astrophysical Surveys at the National Center for Supercomputing Applications at the University of Illinois.

The National Science Foundation, the Science and Technology Facilities Council and the Illinois Graduate Survey Science Fellowship supported this research.

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