Black holes do not emit light, which makes them difficult to study. Fortunately, many black holes are noisy eaters. As they consume nearby matter, the surrounding material becomes overheated. This allows the material to glow intensely or to be thrown away from the black hole as a relativistic jet. By examining the light from this material, we can examine black holes. And as a recent study shows, we can even determine their size.
Active supermassive black holes, also known as active galactic nuclei (AGN). Don’t just shine with constant brightness. Their luminosity can change slightly over time. The time scale of this flickering can be between hours and years. Early studies have argued that this could be related to the size of the black hole, but the relationship is not always clear.
Artist’s impression of a dust torus surrounding a black hole. Image: ESA, V. Beckmann (GSFC)
The basic idea is that the flicker is caused by an accretion disk around a black hole. The disc can be light hours or light days long. Since the speed of light is the maximum cosmic speed limit, it means that the total changes in the disk will take at least hours or days. This works pretty well to tell you the maximum size of a black hole. For example, the rapid flickering of quasars tells us that they need to be powered by black holes, not some cross-galaxy effect. However, this does not mean that AGNs that flicker on a ten-year timescale are ten light-years in diameter. The speed of light is just an upper limit, and most effects spread much more slowly.
In this new study, the team examined not a simple flicker rate, but rather the distribution of flicker rates known as Power Spectrum Density (PSD). They found that the scale on which the PSD flattens out correlates with the size of the black hole. A given black hole may have faster or slower rates of flicker, but the overall distribution of flicker depends on the mass of the black hole. This is a much more reliable measure of size.
Interestingly, the team also applied their method to white dwarfs. These planet-sized solar mass stars can also have accretion disks, and the team found that their model applies just as well to these accretion disks. This suggests that the model is describing something basic about accretion disks, not just black holes. For this reason, the method could be used to study elusive medium-mass black holes (IMBHs).
Medium mass black holes are the least understood types of black holes. They have masses of around 1,000 to 100,000 suns and are typically found in dense globular clusters. If the Vera Rubin Observatory goes online, its sky survey could allow us to study the flickering of medium-mass black holes, and this method could reveal the mass distribution of IMBHs.
There are still aspects of the relationship that the team does not fully understand. Further studies will investigate how the rotation of a black hole or the magnetic field of the accretion disk could affect the relationship. But the tool itself can give a good estimate of the size of the black hole. At least if they’re big eaters.
Relation: Burke, Colin J., et al. “A characteristic timescale of optical variability in astrophysical accretion disks.” Science 373,6556 (2021): 789-792.
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