Optical variability of the blazar 3C 371: From minute to year timescales

¹ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
e-mail: joteros@iaa.es
² Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
³ INAF-Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁴ Universidad de La Laguna (ULL), Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁵ Saint Petersburg State University, 7/9 Universitetskaya Nab., St. Petersburg 199034, Russia
⁶ Special Astrophysical Observatory, Russian Academy of Sciences, 369167 Nizhnii Arkhyz, Russia
⁷ Pulkovo Observatory, St. Petersburg 196140, Russia
⁸ EPT Observatories, Tijarafe, La Palma, Spain
⁹ INAF, TNG Fundación Galileo Galilei, La Palma, Spain
¹⁰ Institute of Astronomy, National Central University, Taoyuan 32001, Taiwan
¹¹ Abastumani Observatory, Mt. Kanobili, 0301 Abastumani, Georgia
¹² Department of Physics and Astronomy, N283 ESC, Brigham Young University, Provo, UT 84602, USA
¹³ Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarigradsko Shosse Blvd., 1784 Sofia, Bulgaria
¹⁴ Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Santa Cruz de Tenerife, Spain
¹⁵ Department of Physics and Astronomy, Aarhus University, Munkegade 120, 8000 Aarhus C, Denmark
¹⁶ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 70-264, CDMX 04510, Mexico
¹⁷ Astronomical Observatory, Volgina 7, 11060 Belgrade, Serbia
¹⁸ Crimean Astrophysical Observatory RAS, P/O Nauchny 298409, Russia
¹⁹ Department of Astronomy, Faculty of Physics, Sofia University, “St. Kliment Ohridski”, 5 James Bourchier Blvd., 1164 Sofia, Bulgaria
²⁰ Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Plaza San Juan 1, 44001 Teruel, Spain
²¹ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 106, Ensenada, 22800 Baja California, Mexico
²² Department of Physics and Astronomy, Faculty of Natural Sciences, University of Shumen, 115, Universitetska Str., 9712 Shumen, Bulgaria
²³ Engelhardt Astronomical Observatory, Kazan Federal University, Tatarstan, Russia
²⁴ National Sun Yat-sen University, No. 70 Lien-hai Road, Kaohsiung 804201, Taiwan

Received: 17 February 2024
Accepted: 22 March 2024

Abstract

Context. The BL Lac object 3C 371 was observed by the Transiting Exoplanet Survey Satellite (TESS) for approximately a year, between July 2019 and July 2020, with an unmatched two-minute imaging cadence. In parallel, the Whole Earth Blazar Telescope (WEBT) Collaboration organized an extensive observing campaign, providing three years of continuous optical monitoring between 2018 and 2020. These datasets allow for a thorough investigation of the variability of the source.

Aims. The goal of this study is to evaluate the optical variability of 3C 371. Taking advantage of the remarkable cadence of TESS data, we aim to characterize the intra-day variability (IDV) displayed by the source and identify its shortest variability timescale. With this estimate, constraints on the size of the emitting region and black hole mass can be calculated. Moreover, WEBT data are used to investigate long-term variability (LTV), including in terms of the spectral behavior of the source and the polarization variability. Based on the derived characteristics, we aim to extract information on the origin of the variability on different timescales.

Methods. We evaluated the variability of 3C 371 by applying the variability amplitude tool, which quantifies variability of the emission. Moreover, we employed common tools, such as ANOVA (ANalysis Of VAariance) tests, wavelet and power spectral density (PSD) analyses to characterize the shortest variability timescales present in the emission and the underlying noise affecting the data. We evaluated the short- and long-term color behavior to understand its spectral behavior. The polarized emission was analyzed, studying its variability and possible rotation patterns of the electric vector position angle (EVPA). Flux distributions of the IDV and LTV were also studied with the aim being to link the flux variations to turbulent and/or accretion-disk-related processes.

Results. Our ANOVA and wavelet analyses reveal several entangled variability timescales. We observe a clear increase in the variability amplitude with increasing width of the time intervals evaluated. We are also able to resolve significant variations on timescales of as little as ∼0.5 h. The PSD analysis reveals a red-noise spectrum with a break at IDV timescales. The spectral analysis shows a mild bluer-when-brighter (BWB) trend on long timescales. On short timescales, mixed BWB, achromatic and redder-when-brighter signatures can be observed. The polarized emission shows an interesting slow EVPA rotation during the flaring period, where a simple stochastic model can be excluded as the origin with a 3σ significance. The flux distributions show a preference for a Gaussian model for the IDV, and suggest it may be linked to turbulent processes, while the LTV is better represented by a log-normal distribution and may have a disk-related origin.