Correlation Between Optical and X-ray Break Frequencies in Power Spectra of Active Galactic Nuclei

We analyze the optical power spectral density (PSD) for 22 active galactic nuclei (AGN) with measured X-ray PSDs using light curves from the All-Sky Automated Survey for SuperNovae (ASAS-SN) and the Transiting Exoplanet Survey Satellite (TESS). The joint optical PSD is measured over up to six orders of magnitude in frequency space from timescales of minutes to a decade. We fit either a damped random walk (DRW) or a broken power law model to constrain the PSD model and break frequency. For the broken power-law fits to the joint PSDs, we find a high-frequency timescale which is proportional to both the X-ray timescales and the black hole masses, but the optical timescale is 2.7 dex longer. Assuming the optical and X-ray breaks are related by a physical process, such as reprocessing of X-ray emission, the break frequency difference interpreted as a light crossing time is consistent with the expected size difference between the optical and X-ray emission regions. On timescales of months to a decade, we also measured a correlation between the low-frequency optical break timescales and the X-ray break timescales, but with a much shallower slope. The DRW model provides acceptable fits and we generally confirm previously reported correlations between the DRW timescales and the black hole mass.