A structure function analysis of VST-COSMOS AGN^⋆,⋆⋆

¹ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
² Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sotero Sanz 100, Of. 104, Providencia, Santiago, Chile
³ Dipartimento di Fisica, Università degli Studi di Napoli “Federico II”, Via Cinthia 9, 80126 Napoli, Italy
e-mail: demetra.decicco@unina.it, demetradecicco@gmail.com
⁴ Space Science Institute, 4750 Walnut Street, Suite 2015, Boulder, CO, 80301 USA
⁵ INFN – Sezione di Napoli, Via Cinthia 9, 80126 Napoli, Italy
⁶ INAF – Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
⁷ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁸ Department of Astronomy and Astrophysics, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA, 16802 USA
⁹ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA, 16802 USA
¹⁰ Department of Physics, 104 Davey Laboratory, The Pennsylvania State University, University Park, PA, 16802 USA
¹¹ Dipartimento di Fisica, Università di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹² INAF – Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Caveliere 100, 00133 Roma, Italy
¹³ Departamento de Ciencias Fisicas, Universidad Andres Bello, Avda. Republica 252, Santiago, Chile
¹⁴ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁵ Inter-university Institute for Data Intensive Astronomy, Department of Physics and Astronomy, University of the Western Cape, 7535 Bellville Cape Town, South Africa
¹⁶ Inter-university Institute for Data Intensive Astronomy, Department of Astronomy, University of Cape Town, 7701 Rondebosch, Cape Town, South Africa
¹⁷ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy

Received: 25 November 2021
Accepted: 23 May 2022

Abstract

Context. We present our sixth work in a series dedicated to variability studies of active galactic nuclei (AGN), based on the survey of the COSMOS field by the VLT Survey Telescope (VST). Its 54 r-band visits over 3.3 yr and single-visit depth of 24.6 r-band mag make this dataset a valuable scaled-down version that can help forecast the performance of the Rubin Observatory Legacy Survey of Space and Time (LSST).

Aims. This work is centered on the analysis of the structure function (SF) of VST-COSMOS AGN, investigating possible differences in its shape and slope related to how the AGN were selected, and explores possible connections between the AGN ensemble variability and the black-hole mass, accretion rate, bolometric luminosity, redshift, and obscuration of the source. Given its features, our dataset opens up the exploration of samples ∼2 mag fainter than most literature to date.

Methods. We identified several samples of AGN – 677 in total – obtained through a variety of selection techniques partly overlapping. Our analysis compares the results for the various samples. We split each sample in two based on the median of the physical property of interest, and analyzed the differences in the SF shape and slope, and their possible causes.

Results. While the SF shape does not change with depth, it is highly affected by the type of AGN (unobscured or obscured) included in the sample. Where a linear region can be identified, we find that the variability amplitude is anticorrelated to the accretion rate and bolometric luminosity, consistent with previous literature on the topic, while no dependence on black-hole mass emerges from this study. With its longer baseline and denser and more regular sampling, the LSST will allow for an improved characterization of the SF and its dependencies on the mentioned physical properties over much larger AGN samples.