Probing the rest-frame wavelength dependence of quasar variability

Insights from the Zwicky Transient Facility Survey

¹ Departamento de Astronomía, Universidad de Chile, Camino el Observatorio 1515, Santiago, Chile
² Millennium Nucleus on Transversal Research and Technology to Explore Supermassive Black Holes (TITANS), Chile
³ Instituto de Física y Astronomía, Universidad de Valparaíso, Gran Bretaña 1111, Valparaíso, Chile
⁴ Department of Astronomy, Xiamen University, Xiamen, Fujian 361005, People’s Republic of China
⁵ Astronomy Department, Universidad de Concepción, Barrio Universitario S/N, Concepción 4030000, Chile
⁶ Millennium Institute of Astrophysics MAS, Nuncio Monsenor Sotero Sanz 100, Of. 104, Providencia, Santiago, Chile

^⋆ Corresponding authors; priya@das.uchile.cl; patricia.arevalo@uv.cl

Received: 6 February 2024
Accepted: 17 September 2024

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 (λ_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 (λ_RF) using optical g- and r-band light curves from the Zwicky Transient Facility (ZTF) Data Release 15 (DR15) for ∼5000 quasars within narrow ranges of the 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 λ_RF for quasars with M_BH = 10⁸ M_⊙ and R_Edd = 10⁻¹ at different timescales. This anticorrelation is more pronounced at shorter timescales. The results align well with a bending power-law power spectral 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.