High Optical-to-X-Ray Polarization Ratio Reveals Compton Scattering in BL Lacertae’s Jet
I. Agudo, Ioannis Liodakis, Jorge Otero-Santos, R. Middei, Alan P. Marscher, S. G. Jorstad, H. C. Zhang, Hui Li, Laura Di Gesu, Roger W. Romani, Dawoon E. Kim, Francesco Fenu, Herman L. Marshall, L. Pacciani, Juan Escudero, F. J. Aceituno, Beatriz Agı́s-González, G. Bonnoli, V. Casanova, Daniel Morcuende, V. Piirola, A. Sota, Pouya M. Kouch, E. Lindfors, Callum McCall, Helen Jermak, I. A. Steele, Г. А. Борман, T. S. Grishina, V. A. Hagen-Thorn, E. N. Kopatskaya, Е. Г. Ларионова, D. A. Morozova, С. С. Савченко, Ekaterina V. Shishkina, I. S. Troitskiy, Yu. V. Troitskaya, A. A. Vasilyev, А. В. Жовтан, I. Myserlis, Mark Gurwell, Garrett K. Keating, Ramprasad Rao, Sincheol Kang, Sang-Sung Lee, Sang-Hyun Kim, Whee Yeon Cheong, Hyeon‐Woo Jeong, E. Angelakis, A. Kraus, D. Blinov, Siddharth Maharana, R. Bachev, Jenni Jormanainen, K. Nilsson, V. Fallah Ramazanı, C. Casadio, Antonio Fuentes, Efthalia Traianou, Clemens Thum, José L. Gómez, L. A. Antonelli, Matteo Bachetti, L. Baldini, W. H. Baumgartner, R. Bellazzini, S. Bianchi, Stephen D. Bongiorno, R. Bonino, A. Brez, N. Bucciantini, Fiamma Capitanio, Simone Castellano, E. Cavazzuti, Chien‐Ting Chen, S. Ciprini, E. Costa, Alessandra De Rosa, E. Del Monte, Niccolò Di Lalla, Alessandro Di Marco, I. Donnarumma, Victor Doroshenko, Michal Dovčiak, Steven R. Ehlert, Teruaki Enoto, Y. Evangelista, Sergio Fabiani, Riccardo Ferrazzoli, Javier A. García, Shuichi Gunji, Kiyoshi Hayashida, Jeremy Heyl, W. Iwakiri, P. Kaaret, V. Karas, Fabian Kislat, Takao Kitaguchi, Jeffery J. Kolodziejczak, H. Krawczynski
Abstract
Abstract Blazars, supermassive black hole systems with highly relativistic jets aligned with the line of sight, are the most powerful long-lived emitters of electromagnetic emission in the Universe. We report here on a radio-to-gamma-ray multiwavelength campaign on the blazar BL Lacertae with unprecedented polarimetric coverage from radio to X-ray wavelengths. The observations caught an extraordinary event on 2023 November 10–18, when the degree of linear polarization of optical synchrotron radiation reached a record value of 47.5%. In stark contrast, the Imaging X-ray Polarimetry Explorer found that the X-ray (Compton scattering or hadron-induced) emission was polarized at less than 7.4% (3 σ confidence level). We argue here that this observational result rules out a hadronic origin of the high-energy emission and strongly favors a leptonic (Compton scattering) origin, thereby breaking the degeneracy between hadronic and leptonic emission models for BL Lacertae and demonstrating the power of multiwavelength polarimetry to address this question. Furthermore, the multiwavelength flux and polarization variability, featuring an extremely prominent rise and decay of the optical polarization degree, is interpreted for the first time by the relaxation of a magnetic “spring” embedded in the newly injected plasma. This suggests that the plasma jet can maintain a predominant toroidal magnetic field component parsecs away from the central engine.