Multi-wavelength picture of the misaligned BL Lac object 3C 371

¹ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
² Istituto Nazionale di Fisica Nucleare, Sezione di Padova, 35131 Padova, Italy
³ INAF-Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁴ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁵ Instituto de Astrofísica de Canarias (IAC), E-38200 La Laguna, Tenerife, Spain
⁶ Universidad de La Laguna (ULL), Departamento de Astrofísica, E-38206 La Laguna, Tenerife, Spain
⁷ Institute of Applied Astronomy, Russian Academy of Sciences, St. Petersburg, Russia
⁸ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
⁹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁰ Saint Petersburg State University, 7/9 Universitetskaya Nab., St. Petersburg 199034, Russia
¹¹ Special Astrophysical Observatory, Russian Academy of Sciences, 369167 Nizhnii Arkhyz, Russia
¹² Pulkovo Observatory, St. Petersburg 196140, Russia
¹³ EPT Observatories, Tijarafe, La Palma, Spain
¹⁴ INAF, TNG Fundación Galileo Galilei, La Palma, Spain
¹⁵ Institute of Astronomy, National Central University, Taoyuan 32001, Taiwan
¹⁶ Abastumani Observatory, Mt. Kanobili, 0301 Abastumani, Georgia
¹⁷ Department of Physics and Astronomy, N283 ESC, Brigham Young University, Provo, UT 84602, USA
¹⁸ Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarigradsko Shosse Blvd., 1784 Sofia, Bulgaria
¹⁹ Nordic Optical Telescope, Apartado 474, E-38700 Santa Cruz de La Palma, Santa Cruz de Tenerife, Spain
²⁰ Department of Physics and Astronomy, Aarhus University, Munkegade 120, DK-8000 Aarhus C, Denmark
²¹ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 70-264, CDMX 04510, Mexico
²² Astronomical Observatory, Volgina 7, 11060 Belgrade, Serbia
²³ Crimean Astrophysical Observatory RAS, P/O Nauchny 298409, Crimea
²⁴ Department of Astronomy, Faculty of Physics, Sofia University “St. Kliment Ohridski”, 5 James Bourchier Blvd., BG-1164 Sofia, Bulgaria
²⁵ Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Plaza San Juan 1, 44001 Teruel, Spain
²⁶ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 106, Ensenada, 22800 Baja California, Mexico
²⁷ Department of Physics and Astronomy, Faculty of Natural Sciences, University of Shumen, 115, Universitetska Str., 9712 Shumen, Bulgaria
²⁸ Engelhardt Astronomical Observatory, Kazan Federal University, Tatarstan, Russia
²⁹ National Sun Yat-sen University, No. 70 Lien-hai Road, Kaohsiung 804201, Taiwan
³⁰ Taiwan Astronomical Research Alliance, Taoyuan 32001, Taiwan

^⋆ Corresponding author; joteros@iaa.es

Received: 8 July 2024
Accepted: 4 December 2024

Abstract

Context. The BL Lac object 3C 371 is one of the targets regularly monitored by the Whole-Earth Blazar Telescope (WEBT), a collaboration of observers studying blazar variability on both short and long timescales.

Aims. We aim to evaluate the long-term multi-wavelength (MWL) behaviour of 3C 371, comparing it with results derived from its optical emission in our previous study. For this, we make use of the multi-band campaigns organised by the WEBT collaboration in optical and radio between January 2018 and December 2020, and of public data from Swift and Fermi satellites and the MOJAVE Very Large Interferometry programme.

Methods. We evaluated the variability shown by the source in each band by quantifying the amplitude variability parameter, and also looked for a possible inter-band correlation using the z-discrete correlation function. We also present a deep analysis of the optical-UV, X-ray, and γ-ray spectral variability. With the MOJAVE data, we performed a kinematics analysis, looking for components propagating along the jet and calculating its kinematics parameters. We then used this set of parameters to interpret the source MWL behaviour, modelling its broadband spectral energy distribution (SED) with theoretical blazar emission scenarios.

Results. The MWL variability of the source in the UV, X-ray, and γ-ray bands is comparable to that in optical, especially considering the lower coverage of the first two wavebands. On the other hand, the radio bands show variability of much lower magnitude. Moreover, this MWL emission shows a high degree of correlation, which is compatible with zero lag, again with the exception of the radio emission. The radio VLBI images reveal super-luminal motion of one of the identified components, which we used to set constraints on the jet kinematics and parameters, and to estimate a viewing angle of θ = (9.6 ± 1.6)°, a Doppler factor of δ = 6.0 ± 1.1, and a Lorentz factor of Γ = 6.0 ± 1.8. The polarised radio emission was found to be anti-correlated with the total flux, and to follow the same behaviour as the polarised optical radiation. The optical-UV spectral behaviour shows a mild harder-when-brighter trend on long timescales, and other trends such as redder-when-brighter on shorter timescales. We successfully modelled the broadband emission with a leptonic scenario, where we compared the low and high emission states during the period of complete MWL coverage. The difference between these two states can be ascribed mainly to a hardening of the distribution of particles. The derived features of the source confirm that 3C 371 is a BL Lac whose jet is not well aligned with the line of sight.