Letter to the Editor

Optical circular polarization of blazar S4 0954+65 during high linear polarized states

¹ Finnish Centre for Astronomy with ESO, University of Turku, 20014 Turku, Finland
e-mail: yannis.liodakis@gmail.com
² NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
³ Special astrophysical observatory of Russian Academy of Sciences, Nizhnij Arkhyz, Karachai-Cherkessian Republic 369167, Russia
⁴ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Santiago, Región Metropolitana 8370191, Chile
⁵ Institute of Astrophysics, Foundation for Research and Technology-Hellas, 70013 Heraklion, Greece
⁶ Department of Physics, University of Crete, 70013 Heraklion, Greece
⁷ Saint Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg 199034, Russia
⁸ Pulkovo Observatory, St. Petersburg 196140, Russia
⁹ Owens Valley Radio Observatory, California Institute of Technology, Pasadena, CA 91125, USA
¹⁰ South African Astronomical Observatory, PO Box 9 Observatory, 7935 Cape Town, South Africa
¹¹ Department of Physics, University of Johannesburg, PO Box 524 Auckland Park 2006, South Africa
¹² Institute for Theoretical Physics (ITP), University of Heidelberg, Philosophenweg 12, 69120 Heidelberg, Germany
¹³ Max Planck Institute for Plasma Physics, Boltzmannstraße 2, 85748 Garching, Germany
¹⁴ Crimean Astrophysical Observatory RAS, P/O Nauchny 298409, Crimea

Received: 9 October 2023
Accepted: 14 November 2023

Abstract

Optical circular polarization observations can directly test the particle composition in black hole jets. We report the first observations of the BL Lac-type object S4 0954+65 in high linear polarized states. While no circular polarization was detected, we were able to place upper limits of < 0.5% at 99.7% confidence. Using a simple model and our novel optical circular polarization observations, we can constrain the allowed parameter space for the magnetic field strength and composition of the emitting particles. Our results favor models that require magnetic field strengths of only a few Gauss and models in which the jet composition is dominated by electron-positron pairs. We discuss our findings in the context of typical magnetic field strength requirements for blazar emission models.