Probing the rest-frame wavelength dependence of quasar variability : Insights from the Zwicky Transient Facility Survey

Abstract

Context: Quasar variability can potentially unlock crucial insights into the accretion process. Understanding how this variability is influenced by wavelength is crucial for validating and refining quasar variability models. Aims: This paper aims to enhance the understanding of the dependence of variability on the rest-frame wavelength ($\lambda_{RF}$) by isolating the variance in different timescales in well-defined wavelength bins and examining the corona-heated accretion-disk (CHAR) model. Methods: We investigated the relation between variance and rest-frame wavelength ($\lambda_{RF}$) using optical g- and r-band light curves from the Zwicky Transient Facility (ZTF) Data Release 15 for $\sim 5000$ quasars within narrow ranges of black hole mass ($M_{BH}$) and Eddington ratio ($R_{Edd}$). A spectral model taking into account disk continuum emission, Balmer transitions, Fe II pseudo-continuum emission, and other emission lines is necessary to best interpret the variance spectrum. Results: Our analysis indicates a strong anticorrelation between median variance and $\lambda_{RF}$ for quasars with $M_{BH} = 10^{8}$ and $R_{Edd} = 10^{-1}$ at different timescales. This anti-correlation is more pronounced at shorter timescales. The results align well with a bending power-law power spectrum density (PSD) model with both the damping timescale and the high-frequency slope of the PSD depending on the wavelength. The predictions provided by the CHAR model on the variance spectrum across most timescales studied showcase its potential in constraining temperature variations within the accretion disk. Key words. accretion, accretion discs; galaxies:active; quasars: supermassive black holes