Do multifrequency polarimetric observations of BL Lac rule out a hadronic origin for its X-ray emission?

¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate, Italy
² DiSAT, Università dell’Insubria, Via Valleggio 11, I-22100 Como, Italy
³ Gran Sasso Science Institute, Viale F. Crispi 7, I-67100 L’Aquila, Italy

^⋆ Corresponding author: fabrizio.tavecchio@inaf.it

Received: 26 May 2025
Accepted: 6 July 2025

Abstract

Recent multifrequency polarimetric observations of the eponymous blazar BL Lac reveal an extremely large degree of polarization in the optical band (average of 25%, reaching 45%), together with a small degree of polarization in the X-ray band (≲7%). This has been interpreted as evidence that the X-rays are produced through inverse Compton emission by relativistic electrons, thus ruling out alternative models based on hadronic processes. Here we revisit the observational evidence, interpreting it in a framework where the observed radiation is entirely produced through synchrotron emission. Electrons produce the radio-to-optical component, and protons produce the X-rays and the gamma rays. We determined the jet magnetic fields from a magnetohydrodynamic model of magnetically dominated stationary axisymmetric outflows, and show that the X-ray emission from the protons is naturally less polarized than the optical emission from the electrons. The model parameters required to reproduce the multifrequency polarimetric observations are fully compatible with blazar jets.