Accretion discs of gas, dust, and plasma surround SMBHs, which dominate the centres of many galaxies – including the Milky Way. As material from the disc falls into a SMBH, it is rapidly compressed and heated. This causes it to emit vast amounts of radiation at optical and ultraviolet wavelengths. These emitting discs are called active galactic nuclei (AGN) and can sometimes outshine all the stars in their galaxy. Optical emissions from these discs undergo seemingly random fluctuations in intensity, which can vary over a wide range of timescales. So far, the mechanisms underlying this flickering behaviour have remained a mystery.

To investigate the process in more detail, the team studied previous observations of 67 AGN, spanning the entire mass range of known SMBHs: from 10,000 to 10 billion solar masses. After measuring fluctuations in the optical emissions of each object, the researchers used a “damped random walk” model to determine the timescales over which these variations became noticeably smaller – or dampened. Across the entire range they studied, the astronomers spotted a correlation between the timescales of emission variability, and SMBH masses.

Mysterious mechanisms

Although the physical mechanisms driving these fluctuations remain a mystery, Burke’s team speculate that the damping timescales could closely trace fluctuation timescales in the heat emitted by the inner part of the accretion disc. Regardless of the cause, the researchers propose that the correlation they uncovered could be used to estimate the masses of SMBHs, simply by observing the variability of AGN associated with them.