The most mysterious Space objects are the Black Holes as they don’t emit light, which makes them difficult to study. Fortunately, many black holes are loud eaters. As they consume nearby matter, their surrounding materials are superheated. As a result, these materials glow intensely or be thrown away from the black hole as relativistic jets. By studying the light coming from these materials we can study black holes and even determine their size too.
Supermassive active black holes are also known as Active Galactic Nuclei (AGN). They don’t simply shine with constant brightness as their luminosity can change slightly over time. The timescale of this flickering can be anywhere from hours to years. Early studies have argued that this could be related to the size of the black hole, but the relationship was not clear.
The basic idea is that the flickering is caused by an accretion disk around a black hole. The disk can be light-hours or light-days across as the speed of light is the maximum cosmic speed limit, which means overall changes in the disk will take at least hours or days. For example, the rapid flickering of quasars (a massive and extremely remote celestial object, emitting exceptionally large amounts of energy, and typically having a star-like image in a telescope. It has been suggested that quasars contain massive black holes and may represent a stage in the evolution of some galaxies) This tells us that they must be powered by black holes rather than some galaxy-spanning effect. Light speed is only an upper limit, and most effects propagate much more slowly.
Modern Scientists looked not only at a simple flicker rate but also the distribution of flicker rates known as the Power Spectrum Density (PSD). They found that the scale at which the PSD levels off correlates to the size of the black hole. A particular black hole may have faster or slower flicker rates, but the overall distribution of flickers depends on the black hole mass. This is a much more reliable measure of size.
Interestingly, scientists also applied their method to white dwarfs which are planet-sized solar-mass stars that also have accretion disks and found that their model applied to these accretion disks just as well. This suggests that the model describes something fundamental about accretion disks, not simply black holes.
Still, there are various space events that Scientists doesn’t fully understand and doing further studies on the relation of rotation of a black hole or the magnetic field of the accretion disk. But current studies also provides a good estimate of black hole size, at least when they happen to be big eaters….Burp!!