High frequency breaks in optical active galactic nuclei power spectral density

TL;DR

This study analyzes optical variability in AGNs across a wide frequency range, identifying new high-frequency breaks in PSDs that relate to black hole mass and X-ray variability, enhancing understanding of AGN physics.

Contribution

It introduces the detection of high-frequency breaks in optical PSDs of AGNs and explores their correlations with black hole mass and X-ray properties, expanding previous variability studies.

Findings

Confirmed correlations between low-frequency break and black hole mass.

Discovered new high-frequency breaks in optical PSDs.

Found potential weak correlation between high-frequency breaks and X-ray break frequencies.

Abstract

Context. Variability is a ubiquitous feature of active galactic nuclei (AGNs), and the characterisation of the variability is crucial to constrain its physical mechanism and proper applications in AGN studies. The advent of all-sky and high-cadence optical surveys allows more accurate measurements of AGNs variability down to short timescales and direct comparisons with X-ray variability from the same sample of sources. Aims. We aim to analyse the optical power spectral density (PSDs) of AGNs with measured X-ray PSDs. Methods. We use light curves from the All-Sky Automated Survey for SuperNovae (ASAS-SN) and the Transiting Exoplanet Survey Satellite (TESS) and use the Lomb-Scargle periodogram to obtain PSDs. 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 (BPL) model to constrain the PSD model and break frequency. Results. We find a set of break frequencies ($\lesssim10^{-2}$ 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 black hole mass. We further explore the dependence of the correlations on other AGN parameters, finding that adding either 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 active galactic nuclei.