The universal shape of the X-ray variability power spectrum of AGN up to z ∼ 3

¹ Dipartimento di Fisica “Ettore Pancini”, Università di Napoli Federico II, Via Cintia 80126, Italy
² INAF – Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Naples, Italy
³ INFN – Unità di Napoli, Via Cintia 9, 80126 Napoli, Italy
e-mail: paolillo@na.infn.it
⁴ Department of Physics and Institute of Theoretical and Computational Physics, University of Crete, Voutes University Campus, 71003 Heraklion, Greece
⁵ Institute of Astrophysics–FORTH, N. Plastira 100, 70013 Vassilika Vouton, Greece
⁶ Department of Astronomy and Astrophysics, 525 Davey Lab., The Pennsylvania State University, University Park, PA 16802, USA
⁷ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
⁸ Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA
⁹ Instituto de Astrofísica and Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
¹⁰ Millennium Institute of Astrophysics, Nuncio Monseñor Sótero Sanz 100, Of 104, Providencia Santiago, Chile
¹¹ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA
¹² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
¹³ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
¹⁴ Department of Physics, University of North Texas, Denton, TX 76203, USA
¹⁵ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
¹⁶ School of Astronomy and Space Science, Nanjing University, Nanjing, Jiangsu 210093, PR China
¹⁷ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, PR China
¹⁸ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, Firenze, Italy
¹⁹ CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China
²⁰ School of Astronomy and Space Sciences, University of Science and Technology of China, Hefei 230026, PR China

Received: 25 October 2022
Accepted: 15 February 2023

Abstract

Aims. We study the ensemble X-ray variability properties of active galactic nuclei (AGN) over large ranges of timescale (20 ks ≤ T ≤ 14 yr), redshift (0 ≤ z ≲ 3), luminosity (10⁴⁰ erg s⁻¹ ≤ L_X ≤ 10⁴⁶ erg s⁻¹), and black hole (BH) mass (10⁶ ≤ M_⊙ ≤ 10⁹).

Methods. We propose the use of the variance-frequency diagram as a viable alternative to the study of the power spectral density (PSD), which is not yet accessible for distant, faint, and/or sparsely sampled AGN.

Results. We show that the data collected from archival observations and previous literature studies are fully consistent with a universal PSD form, which does not show any evidence for systematic evolution of shape or amplitude with redshift or luminosity, even if there may be differences between individual AGN at a given redshift or luminosity. We find new evidence that the PSD bend frequency depends on BH mass and possibly on accretion rate. We finally discuss the implications for current and future AGN population and cosmological studies.