High-frequency breaks in the optical active galactic nucleus power spectral density

Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks St., Norman, OK 73019, USA

^⋆ Corresponding author.

Received: 2 October 2024
Accepted: 24 March 2025

Abstract

Context. Variability is a ubiquitous feature of active galactic nuclei (AGNs), and the characterisation of this variability is crucial to constraining its physical mechanism and proper applications in AGN studies. The advent of all-sky and high-cadence optical surveys allows more accurate measurements of AGN variability down to short timescales as well as direct comparisons with X-ray variability from the same sample of sources.

Aims. We aim to analyse the optical power spectral density (PSD) of AGNs with measured X-ray PSDs.

Methods. We used light curves from the All-Sky Automated Survey for SuperNovae (ASAS-SN) and the Transiting Exoplanet Survey Satellite (TESS) and used the Lomb-Scargle periodogram to obtain PSDs. The joint optical PSD is measured over up to six orders of magnitude in frequency space on timescales of minutes to a decade. We fitted either a damped random walk (DRW) or a broken power law (BPL) model to constrain the PSD model and break frequency.

Results. We find a set of break frequencies (≲10⁻² day⁻¹) from DRW and BPL fits that generally confirm previously reported correlations between break frequencies and the black hole mass. In addition, we find a second set of break frequencies at higher frequencies (> 10⁻² day⁻¹). We observe a potential weak correlation between the high-frequency breaks with the X-ray break frequencies and the black hole mass. We further explored the dependence of the correlations on other AGN parameters, finding that adding X-ray, optical, or bolometric luminosity as the third correlation parameter can substantially improve the correlation significances. The newly identified high-frequency optical breaks can constrain different aspects of the physics of AGNs.