The flickering light emitted by astrophysical accretion disks can reveal the mass of the supermassive black hole (SMBH) at their center, according to a new study. The findings provide a novel method for characterizing the masses of SMBHs using optical observations and help to constrain the poorly understood processes that occur within accretion disks. Accretion disks – made of gas, dust and plasma – surround the SMBHs located at the centers of active galaxies. As material from the accretion disk falls toward the black hole, it heats up, emitting an enormous amount of radiation, including ultraviolet and optical light. Although these disks are much smaller than their host galaxy – roughly the size of the Solar System – they can often out-shine the entire rest of the galaxy. However, accretion disks flicker for unknown reasons, causing their luminosity to fluctuate over a wide range of time scales. Colin Burke and colleagues report that a characteristic time scale measured from the optical variability of accretion disks is correlated with the masses of the SMBHs they surround. The authors measured the optical variability of 67 well-observed active galaxies to determine the time scale on which the fluctuations became noticeably smaller, known as the “damping” time scale (usually several hundred days). They find that this damping time scale is related to SMBH mass over the entire range of SMBH masses observed in active galaxies and may even extend to smaller accretion discs around other objects. “One of the most interesting aspects of the study of Burke et al. is that it extends its findings to much less massive objects, such as white dwarf stars, which emit radiation through a similar accretion disk mechanism and can be regarded as miniature accreting SMBHs,” write Paulina Lira and Patricia Arevalo in a related Perspective.
A characteristic optical variability timescale in astrophysical accretion disks
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