Ensemble power spectral density of SDSS quasars in UV/optical bands

¹ Dipartimento di Fisica “Ettore Pancini”, Università di Napoli Federico II, via Cinthia 9, 80126 Napoli, Italy
e-mail: vincenzo.petrecca@unina.it
² INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
³ Department of Physics and Institute of Theoretical and Computational Physics, University of Crete, 71003 Heraklion, Greece
⁴ Institute of Astrophysics, FORTH, 71110 Heraklion, Greece
⁵ INFN – Sezione di Napoli, via Cinthia 9, 80126 Napoli, Italy
⁶ Instituto de Astrofísica and Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile, Campus San Joaquín, Av. Vicuña Mackenna 4860, Macul Santiago 7820436, Chile
⁷ Millennium Institute of Astrophysics, Nuncio Monseñor Sótero Sanz 100, Of 104, Providencia, Santiago, Chile
⁸ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA

Received: 4 January 2024
Accepted: 8 April 2024

Abstract

Context. Quasar variability has proven to be a powerful tool to constrain the properties of their inner engine and the accretion process onto supermassive black holes. Correlations between UV/optical variability and physical properties have been long studied with a plethora of different approaches and time-domain surveys, although the detailed picture is not yet clear.

Aims. We analysed archival data from the SDSS Stripe-82 region to study how the quasar power spectral density (PSD) depends on the black hole mass, bolometric luminosity, accretion rate, redshift, and rest-frame wavelength. We developed a model-independent analysis framework that could be easily applied to upcoming large surveys such as the Legacy Survey of Space and Time (LSST).

Methods. We used light curves of 8042 spectroscopically confirmed quasars, observed in at least six yearly seasons in five filters ugriz. We split the sample into bins of similar physical properties containing at least 50 sources, and we measured the ensemble PSD in each of them.

Results. We find that a simple power law is a good fit to the power spectra in the frequency range explored. Variability does not depend on the redshift at a fixed wavelength. Instead, both PSD amplitude and slope depend on the black hole mass, accretion rate, and rest-frame wavelength. We provide scaling relations to model the observed variability as a function of the physical properties, and discuss the possibility of a universal PSD shape for all quasars, where frequencies scale with the black hole mass, while normalization and slope(s) are fixed (at any given wavelength and accretion rate).