Where within the 3C 84 jet are γ-rays produced?

¹ Finnish Centre for Astronomy with ESO, University of Turku, 20014 Turku, Finland
² Aalto University Metsähovi Radio Observatory, Metsähovintie 114, FI-02540 Kylmälä, Finland
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
⁴ Institute of Astrophysics, Foundation for Research and Technology – Hellas, Voutes 7110, Heraklion, Greece
⁵ Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA
⁶ Saint Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg, 199034, Russia
⁷ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁸ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁹ Interdisziplinäres Zentrum für wissenschaftliches Rechnen (IWR), Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 205, 69120 Heidelberg, Germany
¹⁰ Instituto de Astrofísica de Andalucía, IAA-CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain
¹¹ Department of Physics and Astronomy and Space Science Center, University of New Hampshire, Durham, NH 03824, USA
¹² ASI – Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
¹³ INAF, Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁴ Physical Research Laboratory, Thaltej, Ahmedabad, Gujarat 380009, India
¹⁵ Science & Technology Institute, Universities Space Research Association, 320 Sparkman Drive, Huntsville, AL 35805, USA
¹⁶ Dipartimento di Fisica, Universitá degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹⁷ Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133, Roma, Italy
¹⁸ Crimean Astrophysical Observatory, 298409 Nauchny, Crimea
¹⁹ Institute for Nuclear Research of the Russian Academy of Sciences, 60th October Anniversary Prospect 7a, Moscow 117312, Russia
²⁰ Lebedev Physical Institute, Pushchino Radio Astronomy Observatory, Radiotelescopnaya 1a, Pushchino 142290, Russia
²¹ Aalto University Department of Electronics and Nanoengineering, PL 15500, FI-00076 Aalto, Finland
²² Institut de Radioastronomie Millimétrique, Avenida Divina Pastora, 7, Local 20, E–18012 Granada, Spain
²³ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
²⁴ Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Yuseong-gu, Daejeon, 34055, Korea
²⁵ University of Science and Technology, Korea, 217 Gajeong-ro, Yuseong-gu, Daejeon, 34113, Korea
²⁶ Department of Physics, University of Crete, 70013 Heraklion, Greece
²⁷ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, D-85741 Garching, Germany
²⁸ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, Via Alfonso Corti 12, I – 20133 Milano, Italy
²⁹ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr.1, 81679 München, Germany
³⁰ INFN, Sezione di Pavia, Via A. Bassi 6, 27100 Pavia, Italy
³¹ INAF Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, (LC), Italy
³² Department of Astronomy and Astrophysics, The University of Chicago, Chicago, IL 60637, USA

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Received: 31 January 2026
Accepted: 19 March 2026

Abstract

The location in which γ-ray are created and emitted within extra-galactic jets is a matter of active debate. One particularly well-suited source for determining the location is the nearby bright radio galaxy 3C 84, harbouring a powerful jet. We investigated the origin of γ-rays that were measured during a recent γ-ray flare by analysing the linear polarisation signal of close-in-time very long baseline interferometry (VLBI) observations at centimetre and millimetre wavelengths. While 3C 84 is almost unpolarised overall, we find that close in time to the γ-ray flare peak regions at parsec-scale distances from the central engine, the linear polarisation increases fractionally. Under the physically well-motivated assumption of a causal relation between this polarisation enhancement and the γ-ray flare, and combined with insights from concurrent X-ray polarisation measurements, a physically motivated scenario is that the γ-rays are created in this region, in a process consistent with the synchrotron self-Compton mechanism.