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

description: 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.
description: Accepted for publication in ApJ Letters