High optical to X-ray polarization ratio reveals Compton scattering in BL Lacertae's jet
Abstract
Blazars, supermassive black hole systems (SMBHs) 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 (IXPE) found that the X-ray (Compton scattering or hadron-induced) emission was polarized at less than 7.4% (3sigma 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.
Keywords
Cite
@article{arxiv.2505.01832,
title = {High optical to X-ray polarization ratio reveals Compton scattering in BL Lacertae's jet},
author = {Ivan Agudo and Ioannis Liodakis and Jorge Otero-Santos and Riccardo Middei and Alan Marscher and Svetlana Jorstad and Haocheng Zhang and Hui Li and Laura Di Gesu and Roger W. Romani and Dawoon E. Kim and Francesco Fenu and Herman L. Marshall and Luigi Pacciani and Juan Escudero Pedrosa and Francisco Jose Aceituno and Beatriz Agis-Gonzalez and Giacomo Bonnoli and Victor Casanova and Daniel Morcuende and Vilppu Piirola and Alfredo Sota and Pouya M. Kouch and Elina Lindfors and Callum McCall and Helen E. Jermak and Iain A. Steele and George A. Borman and Tatiana S. Grishina and Vladimir A. Hagen-Thorn and Evgenia N. Kopatskaya and Elena G. Larionova and Daria A. Morozova and Sergey S. Savchenko and Ekaterina V. Shishkina and Ivan S. Troitskiy and Yulia V. Troitskaya and Andrey A. Vasilyev and Alexey V. Zhovtan and Ioannis Myserlis and Mark Gurwell and Garrett Keating and Ramprasad Rao and Sincheol Kang and Sang-Sung Lee and Sanghyun Kim and Whee Yeon Cheong and Hyeon-Woo Jeong and Emmanouil Angelakis and Alexander Kraus and Dmitry Blinov and Siddharth Maharana and Rumen Bachev and Jenni Jormanainen and Kari Nilsson and Vandad Fallah Ramazani and Carolina Casadio and Antonio Fuentes and Efthalia Traianou and Clemens Thum and Jose L. Gomez and Lucio Angelo Antonelli and Matteo Bachetti and Luca Baldini and Wayne H. Baumgartner and Ronaldo Bellazzini and Stefano Bianchi and Stephen D. Bongiorno and Raffaella Bonino and Alessandro Brez and Niccolo Bucciantini and Fiamma Capitanio and Simone Castellano and Elisabetta Cavazzuti and Chien-Ting Chen and Stefano Ciprini and Enrico Costa and Alessandra De Rosa and Ettore Del Monte and Niccolo Di Lalla and Alessandro Di Marco and Immacolata Donnarumma and Victor Doroshenko and Michal Dovciak and Steven R. Ehlert and Teruaki Enoto and Yuri Evangelista and Sergio Fabiani and Riccardo Ferrazzoli and Javier A. Garcia and Shuichi Gunji and Kiyoshi Hayashida and Jeremy Heyl and Wataru Iwakiri and Philip Kaaret and Vladimir Karas and Fabian Kislat and Takao Kitaguchi and Jeffery J. Kolodziejczak and Henric Krawczynski and Fabio La Monaca and Luca Latronico and Simone Maldera and Alberto Manfreda and Frederic Marin and Andrea Marinucci and Francesco Massaro and Giorgio Matt and Ikuyuki Mitsuishi and Tsunefumi Mizuno and Fabio Muleri and Michela Negro and Chi-Yung Ng and Stephen L. O'Dell and Nicola Omodei and Chiara Oppedisano and Alessandro Papitto and George G. Pavlov and Abel L. Peirson and Matteo Perri and Melissa Pesce-Rollins and Pierre-Olivier Petrucci and Maura Pilia and Andrea Possenti and Juri Poutanen and Simonetta Puccetti and Brian D. Ramsey and John Rankin and Ajay Ratheesh and Oliver J. Roberts and Carmelo Sgro and Patrick Slane and Paolo Soffitta and Gloria Spandre and Douglas A. Swartz and Toru Tamagawa and Fabrizio Tavecchio and Roberto Taverna and Yuzuru Tawara and Allyn F. Tennant and Nicholas E. Thomas and Francesco Tombesi and Alessio Trois and Sergey S. Tsygankov and Roberto Turolla and Jacco Vink and Martin C. Weisskopf and Kinwah Wu and Fei Xie and Silvia Zane},
journal= {arXiv preprint arXiv:2505.01832},
year = {2025}
}
Comments
Accepted for publication in ApJ Letters