The feeding patterns of black holes offer insight into their size,
researchers report. A new study revealed that the flickering in the
brightness observed in actively feeding supermassive black holes is related
to their mass.
Supermassive black holes 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; the only way astronomers can detect them is through their
gravitational influences on stars and gas in their vicinity. However, in the
early universe, when SMBHs were rapidly growing, they were actively feeding
– or accreting – materials at intensive rates and emitting an enormous
amount of radiation – sometimes outshining the entire galaxy in which they
reside, the researchers said.
The new study, led by the University of Illinois Urbana-Champaign astronomy
graduate student Colin Burke and professor Yue Shen, uncovered a definitive
relationship between the mass of actively feeding SMBHs and the
characteristic timescale in the light-flickering pattern. The findings are
published in the journal Science.
The observed light from an accreting SMBH is not constant. Due to physical
processes that are not yet understood, it displays a ubiquitous flickering
over timescales ranging from hours to decades. “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 said.
The team compiled a large data set 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. 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 said. 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
timescales to long timescales. This transition of variability pattern
happens at a characteristic timescale that is longer for more massive black
holes.
The team compared black hole feeding to our eating or drinking activity 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 said.
“These results suggest that the processes driving the flickering during
accretion are universal, whether the central object is a supermassive black
hole or a much more lightweight white dwarf,” Shen said.
“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,” said Yan-Fei Jiang, a researcher at the
Flatiron Institute and study co-author.
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 of times the mass of the sun, and SMBHs, there is a population
of black holes called intermediate-mass black holes that weigh between about
100 and 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.
However, observationally this population of IMBHs is surprisingly elusive.
There is only one indisputably confirmed IMBH that weighs about 150 times
the mass of the sun. But 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,” Burke said.
Astronomers worldwide are waiting for the official kickoff of an era of
massive surveys that monitor the dynamic and variable sky. The Vera C. Rubin
Observatory in Chile’s Legacy Survey of Space and Time will survey the sky
over a decade and collect light flickering data for billions of objects,
starting in late 2023.
“Mining the LSST data set 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,” said
co-author Xin Liu, an astronomy professor at the U. of I.
Reference:
A characteristic optical variability timescale in astrophysical accretion
disks by Colin J. Burke, Yue Shen, Omer Blaes, Charles F. Gammie, Keith
Horne, Yan-Fei Jiang, Xin Liu, Ian M. McHardy, Christopher W. Morgan, Simone
Scaringi and Qian Yang, 12 August 2021, Science.
DOI: 10.1126/science.abg9933
Tags:
Space & Astrophysics