Multiband optical variability on diverse timescales of the blazar Ton 599 from 2011 to 2023

¹ Astronomical Observatory, Volgina 7 11060 Belgrade, Serbia
² Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China
³ INAF, Osservatorio Astrofisico di Torino, Via Osservatorio 20 I-10025, Pino Torinese, Italy
⁴ Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital 263001, India
⁵ Xinjiang Astronomical Observatory, CAS, 150 Science-1 Street, Urumqi 830011, China
⁶ Department of Astronomy, University of Belgrade – Faculty of Mathematics, Studentski Trg 16 11000 Belgrade, Serbia
⁷ Department of Physical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Knowledge City, Sector 81, SAS Nagar, Manauli 140306, Punjab, India
⁸ Ulugh Beg Astronomical Institute, Astronomy Street 33, Tashkent 100052, Uzbekistan
⁹ National University of Uzbekistan, Tashkent 100174, Uzbekistan
¹⁰ EPT Observatories, Tijarafe, La Palma, Spain
¹¹ INAF, TNG Fundación Galileo Galilei, La Palma, Spain
¹² Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
¹³ Saint Petersburg State University, 7/9 Universitetskaya Nab., St. Petersburg 199034, Russia
¹⁴ Pulkovo Observatory, St. Petersburg 196140, Russia
¹⁵ Department of Physics Florida International University and the SARA Observatory, Miami, FL 33199, USA
¹⁶ Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA
¹⁷ Institute of Astronomy, National Central University, 300 Zhongda Rd., Taoyuan 320317, Taiwan
¹⁸ Taiwan Astronomical Research Alliance, 300 Zhongda Rd., Taoyuan 320317, Taiwan
¹⁹ Crimean Astrophysical Observatory RAS, P/O Nauchny 298409, Russia
²⁰ Abastumani Observatory, Mt. Kanobili 0301, Abastumani, Georgia
²¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69 53121 Bonn, Germany
²² Engelhardt Astronomical Observatory, Kazan Federal University, Tatarstan, Russia
²³ Center for Astrophysics, Guangzhou University, Guangzhou 510006, China
²⁴ Landessternwarte, Zentrum für Astronomie der Universitat Heidelberg, Knigstuhl 12 69117 Heidelberg, Germany
²⁵ Instituto de Astrofisica de Canarias (IAC), E-38200, La Laguna, Tenerife, Spain
²⁶ Universidad de La Laguna (ULL), Departamento de Astrofisica, E-38206 La Laguna, Tenerife, Spain
²⁷ Special Astrophysical Observatory of Russian Academy of Sciences, Nyzhnij Arkhyz, Karachai-Circassia 369167, Russia
²⁸ Nordic Optical Telescope, Apartado 474 E-38700, Santa Cruz de La Palma, Santa Cruz de Tenerife, Spain
²⁹ Hans-Haffner-Sternwarte (Hettstadt), Naturwissenschaftliches Labor für Schüler am FKG, Friedrich-Koenig-Gymnasium, D-97082 Würzburg, Germany
³⁰ Department of Physics, TU Dortmund University, Otto-Hahn-Str. 4A D-44227, Dortmund, Germany
³¹ Instituto de Astrofísica de Andalucía, IAA-CSIC, Glorieta de la Astronomía s/n E-18008, Granada, Spain
³² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
³³ INAF Osservatorio Astronomico di Brera, Via E. Bianchi 46 23807 Merate, (LC), Italy
³⁴ Department of Astronomy, Faculty of Physics, Sofia University ‘St. Kliment Ohridski’, 5 James Bourchier Blvd. BG-1164, Sofia, Bulgaria
³⁵ Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarigradsko Shosse Blvd. 1784 Sofia, Bulgaria
³⁶ Department of Physics, University of Colorado Denver, Denver, Colorado 80204, USA
³⁷ National Research Institute of Astronomy and Geophysics (NRIAG), 11421 Helwan, Cairo, Egypt
³⁸ Astronomical Observatory, University of Siena, Via Roma 56 53100 Siena, Italy

^⋆ Corresponding author: ovince@aob.rs

Received: 17 June 2025
Accepted: 1 September 2025

Abstract

Context. We analyze the optical variability of the flat-spectrum radio quasar (FSRQ) Ton 599 using BVRI photometry from the Whole Earth Blazar Telescope (WEBT) collaboration (2011–2023), complemented by photometric and spectroscopic data from the Steward Observatory monitoring program.

Aims. We aim to characterize short- and long-term optical variability – including flux distributions, intranight changes, color evolution, and spectra – to constrain physical parameters and processes in the central engine of this active galactic nucleus (AGN).

Methods. We tested flux distributions in each filter against normal and log-normal models and explored the root mean square (RMS)–flux relation. We derived power spectral densities (PSDs) to assess red-noise behavior. We quantified intranight variability using a χ² test and fractional variability. From variability timescales, we estimated the emitting region size and magnetic field. Long-term variability was studied by segmenting the light curve into 12 intervals and analyzing flux statistics. For multi-filter flares, we computed spectral slopes, redshift-corrected fluxes, and monochromatic luminosities. Color-magnitude and color-time diagrams traced color evolution over different flux regimes and timescales. From low-flux spectra, we measured Mg II line properties (correcting for Fe II) to estimate the black hole mass via single-epoch scaling.

Results. During the monitoring period, Ton 599 showed strong optical variability. Log-normal distributions fit the fluxes better than normal ones, and all bands display a positive RMS–flux relation. The PSDs follow red-noise trends. Intranight variability is detected, with derived timescales constraining the emission region and magnetic field. The R band reaches a peak flux of 23.5 mJy, corresponding to a monochromatic luminosity of log(νL_ν) = 48.48 [erg s⁻¹]. Color-magnitude diagrams reveal a redder-when-brighter trend at low fluxes (thermal dominance), achromatic behavior at intermediate levels (possibly due to jet orientation changes), and a bluer-when-brighter trend at high fluxes (synchrotron dominance). While long-term color changes are modest, short-term variations are significant, with a negative correlation between the amplitude of color changes and the average flux. The estimated supermassive black hole mass is on the order of 10⁸ M_⊙, which is in agreement with previous estimates.

Conclusions. Our results underscore the complexity of blazar variability, pointing to multiple emission processes at work. The joint photometric and spectroscopic approach constrains key physical parameters and deepens our understanding of the blazar central engine.