‘Astronomical burps’ can determine black hole size

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Supermassive Black Holes (SMBHs) are the largest type of black hole, defined to be millions to billions of times larger than the Sun. The variability in the size of black holes in this classification is huge, but now a new study has linked the size of SMBHs to their feeding patterns.

The research was led by graduate student Colin Burke and Professor Yue Shen from the University of Illinois, and involved Professor Simone Scaringi from Durham University’s Department of Physics.

Observational evidence has shown that SMBHs are found at the centres of most large galaxies, including the Milky Way. Black holes are astronomical objects that have undergone gravitational collapse, leaving behind a region of space where nothing can escape. They are surrounded by an accretion disk, which is made up of gas, particles and stars spiralling inwards towards the centre of the SMBH. Black holes themselves cannot be seen, however, the accretion disks around them can be observed.

The material making up the accretion disk is accelerated to extremely high speeds by the gravitational force of the black hole, causing huge releases of electromagnetic radiation at ultraviolet and optical wavelengths. This is observed as a flickering light emitted from the accretion disk.

When black holes are dormant and not feeding on the material around them, very little radiation is emitted from the accretion disk, making it difficult for astronomers to detect them.

The study found a relationship between the mass of active SMBHs and the characteristic timescale in the flickering light pattern. The researchers tracked the radiation emitted from 67 active galactic nuclei over time as they fed. They studied the link between when an SMBH feeds and when light is emitted from its accretion disk, as an ‘astronomical burp’.

The team discovered that smaller SMBHs with smaller accretion disks go through the feeding process quicker than larger SMBHs with bigger accretion disks. They observed this by measuring the frequency of these ‘burps’.

Plotting the accretion disk brightness over time showed that the difference in brightness between two random timestamps increases as the timescale between the timestamps increases. By measuring the timescale at which variability in the light emitted from active galactic nuclei flattens, the team discovered that this pattern changes above a certain timescale. This timescale closely correlates with the characteristic mass of an SMBH.

How SMBHs form is still unknown. One idea is that Stellar Black Holes, which are the remnants of some collapsed stars, consume enormous amounts of material over millions of years to grow to the size of an SMBH. The connection between the light emissions and feeding of SMBHs will allow researchers to understand their change in mass in relation to their feeding processes, and possibly discover how SMBHs form.

The team also compared these findings to the feeding processes of accreting white dwarfs, which are remnants of stars similar in size to the Sun. White dwarfs are therefore millions to billions of times smaller than an SMBH, however, they found similarities between their feeding processes.

Durham’s Professor Scaringi, co-author of the study, said: “Both have an accretion disk, but due to the large differences in size, we did not expect the accretion process and properties to be similar to one another”.

“However, when comparing the data from the SMBHs to the white dwarfs, it is remarkable to see that the feeding process for SMBHs, which can take days to weeks, closely resemble those observed in accreting white dwarfs, which can take only minutes.”

“It is as if the physics driving the accretion process, and thus the light variations, can be attuned to the size of the accretor. The exact process that links these seemingly different objects remains a mystery, however.”

With these new findings, the researchers are hoping to discover that these same principles around feeding habits will apply to all classifications of black holes found in our universe.

Image: Mark A. Garlick / Simons Foundation

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