An artist's impresion of a bright disk of light around a black hole.

How big is a black hole? Watch how it eats

A new study in Science has revealed that the feeding patterns of a black hole can tell us about its size.

Supermassive black holes (SMBH) are voracious, using their powerful gravitational pull to gobble up immense amounts of gas, dust and stars. When actively consuming matter, they release dazzling bursts of radiation-like burps, in flickering patterns that vary over timescales from hours to decades.

Researchers have long wondered whether the timing of these patterns could tell us more about the black hole itself.

“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,” says Colin Burke from the University of Illinois Urbana-Champaign, US.

Burke and team aimed to clarify whether such a relationship existed.

They compiled a large data set of actively feeding SMBHs to study how the flickering varied over time. They then compared these variations with data sets from accreting white dwarf systems (where a stellar corpse pulls in matter from a companion star).

While white dwarfs are millions to billions of times less massive than a black hole, the team found a relationship between the changing brightness of an accreting system and the mass of the system.

Essentially, the more massive the object, the longer the timescale over which the light pattern will vary.

“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,” says Yue Shen, also from the University of Illinois Urbana-Champaign.

This relationship could therefore help astronomers study other populations of lighter black holes, including stellar-mass ones that are just a few dozen times the mass our Sun – and elusive intermediate-mass black holes (IMBH). Thought to span between 100 and 100,000 solar masses, little is known about these medium-sized cosmic monsters. In fact, we’ve only spotted a couple so far.

“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 says.

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