A wiggling filamentary jet at the origin of the blazar multi-wavelength behaviour

¹ INAF, Osservatorio Astrofisico di Torino, Via Osservatorio 20, I-10025 Pino Torinese, Italy
² Abastumani Observatory, Mt. Kanobili, 0301 Abastumani, Georgia
³ Ulugh Beg Astronomical Institute, Astronomy Street 33, Tashkent 100052, Uzbekistan
⁴ Instituto de Astronomía, Universidad Nacional Autónoma de México, AP 70-264 CDMX 04510, Mexico
⁵ INAF, Osservatorio Astronomico di Brera, Via Emilio Bianchi 46, 23807 Merate, (LC), Italy
⁶ EPT Observatories, Tijarafe, La Palma, Spain
⁷ INAF, TNG Fundación Galileo Galilei, La Palma, Spain
⁸ Instituto de Astrofísica de Canarias, Via Lactea, E-38200 La Laguna, Tenerife, Spain
⁹ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía, E-18008 Granada, Spain
¹⁰ Institute of Applied Astronomy of RAS, Kutuzova Embankment 10, St. Petersburg 191187, Russia
¹¹ Radioastronomical Observatory “Svetloe”, 188833 Leningradskaya Oblast, Priozerskiy district, village Svetloe, Russia
¹² Brigham Young University Department of Physics and Astronomy, N284 ESC, Provo, UT 84602, USA
¹³ Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarigradsko shosse Blvd., 1784 Sofia, Bulgaria
¹⁴ Crimean Astrophysical Observatory RAS, P/O Nauchny 298409, Crimea
¹⁵ Department of Astronomy, Faculty of Physics, Sofia University “St. Kliment Ohridski”, 5 James Bourchier Blvd, BG-1164 Sofia, Bulgaria
¹⁶ Connecticut College, 270 Mohegan Ave., New London, CT, USA
¹⁷ Institute of Astrophysics, Foundation for Research and Technology – Hellas, 7110 Heraklion, Greece
¹⁸ National Central University, 300 Zhongda Road, Zhongli, 32001 Taoyuan, Taiwan
¹⁹ Astronomical Observatory, Volgina 7, 11060 Belgrade, Serbia
²⁰ National University of Uzbekistan, Tashkent 100174, Uzbekistan
²¹ Hans-Haffner-Sternwarte (Hettstadt), Naturwissenschaftliches Labor für Schüler, Friedrich-Koenig-Gymnasium, D-97082 Würzburg, Germany
²² Astroteilchenphysik, TU Dortmund, Otto-Hahn-Str. 4A, D-44227 Dortmund, Germany
²³ Hypatia Observatory, 19 Via Sacco e Vanzetti, Viserba, Rimini, Italy
²⁴ Section of Astrophysics, Astronomy and Mechanics, Dept. of Physics, National and Kapodistrian Univ. of Athens, 15784 Zografos, Athens, Greece
²⁵ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
²⁶ Saint Petersburg State University, 7/9 Universitetskaya Nab., St. Petersburg 199034, Russia
²⁷ Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital 263001, India
²⁸ Xinjiang Astronomical Observatory, CAS, 150 Science-1 Street, Urumqi 830011, China
²⁹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
³⁰ National Research Institute of Astronomy and Geophysics (NRIAG), 11421 Helwan, Cairo, Egypt
³¹ Instituto de Astronomía, Universidad Nacional Autónoma de México, AP 106, Ensenada, 22800 B.C., Mexico
³² Owens Valley Radio Observatory, California Institute of Technology, Pasadena, CA 91125, USA
³³ IAR, Boston University, 725 Commonwealth Ave., Boston, MA 02215, USA
³⁴ Department of Physics, University of Crete, GR-70013 Heraklion, Greece
³⁵ Astro Space Center of Lebedev Physical Institute, Profsoyuznaya 84/32, 117997 Moscow, Russia
³⁶ Institute for Nuclear Research, Russian Academy of Sciences, 60th October Anniversary Prospect 7a, Moscow 117312, Russia
³⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³⁸ Engelhardt Astronomical Observatory, Kazan Federal University, Tatarstan, Russia
³⁹ Facultad de Ciencias, Universidad Autónoma de Baja California, El Sauzal, Ensenada, 22800 B.C., Mexico
⁴⁰ Astronomical Observatory, University of Siena, Via Roma 56, 53100 Siena, Italy
⁴¹ Astronomical Institute, Osaka Kyoiku University, Osaka 582-8582, Japan
⁴² Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁴³ Special Astrophysical Observatory of RAS, Nizhny Arkhyz 369167, Russia
⁴⁴ Kazan Federal University, 18 Kremlyovskaya St, Kazan 420008, Russia
⁴⁵ Institut de Radioastronomie Milimétrique, Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain
⁴⁶ Moscow Institute of Physics and Technology, Institutsky Per. 9, Dolgoprudny 141700, Russia
⁴⁷ CePIA, Departamento de Astronomía, Universidad de Concepción, Concepción, Chile
⁴⁸ American Association of Variable Star Observers (AAVSO), 185 Alewife Brook Parkway, Suite 410, Cambridge, MA 02138, USA
⁴⁹ Burke-Gaffney Observatory. Saint Mary’s University, 923 Robie Street, Halifax, NS B3H 3C3, Canada
⁵⁰ Abbey Ridge Observatory, 45 Abbey Road, Stillwater Lake, Nova Scotia, Canada
⁵¹ Pulkovo Observatory, St. Petersburg 196140, Russia
⁵² National Sun Yat-sun University, 70 Lienhai Rd., Kaohsiung 80424, Taiwan
⁵³ Florida International University, The SARA Observatories, FIU, Miami, Florida, USA

^⋆ Corresponding author; claudia.raiteri@inaf.it

Received: 19 September 2024
Accepted: 18 October 2024

Abstract

Context. Blazars are beamed active galactic nuclei (AGNs) known for their strong multi-wavelength variability on timescales ranging from years down to minutes. Many different models have been proposed to explain this variability.

Aims. We aim to investigate the suitability of the twisting jet model presented in previous works to explain the multi-wavelength behaviour of BL Lacertae, the prototype of one of the blazar classes. According to this model, the jet is inhomogeneous, curved, and twisting, and the long-term variability is due to changes in the Doppler factor due to variations in the orientation of the jet-emitting regions.

Methods. We analysed optical data of the source obtained during monitoring campaigns organised by the Whole Earth Blazar Telescope (WEBT) in 2019–2022, together with radio data from the WEBT and other teams, and γ-ray data from the Fermi satellite. In this period, BL Lacertae underwent an extraordinary activity phase, reaching its historical optical and γ-ray brightness maxima.

Results. The application of the twisting jet model to the source light curves allows us to infer the wiggling motion of the optical, radio, and γ-ray jet-emitting regions. The optical-radio correlation shows that the changes in the radio viewing angle follow those in the optical viewing angle by about 120 days, and it suggests that the jet is composed of plasma filaments, which is in agreement with some radio high-resolution observations of other sources. The γ-ray emitting region is found to be co-spatial with the optical one, and the analysis of the γ-optical correlation is consistent with both the geometric interpretation and a synchrotron self-Compton (SSC) origin of the high-energy photons.

Conclusions. We propose a geometric scenario where the jet is made up of a pair of emitting plasma filaments in a sort of double-helix curved rotating structure, whose wiggling motion produces changes in the Doppler beaming and can thus explain the observed multi-wavelength long-term variability.