Polarized blazar X-rays imply particle acceleration in shocks
Authors: Liodakis Ioannis , Marscher Alan P. , Agudo Iván , Berdyugin Andrei V. , Bernardos Maria I. , Bonnoli Giacomo , Borman George A. , Casadio Carolina , Casanova Vı́ctor , Cavazzuti Elisabetta , Rodriguez Cavero Nicole , Di Gesu Laura , Di Lalla Niccoló , Donnarumma Immacolata , Ehlert Steven R. , Errando Manel , Escudero Juan , Garcı́a-Comas Maya , Agı́s-González Beatriz , Husillos César , Jormanainen Jenni , Jorstad Svetlana G. , Kagitani Masato , Kopatskaya Evgenia N. , Kravtsov Vadim , Krawczynski Henric , Lindfors Elina , Larionova Elena G. , Madejski Grzegorz M. , Marin Frédéric , Marchini Alessandro , Marshall Herman L. , Morozova Daria A. , Massaro Francesco , Masiero Joseph R. , Mawet Dimitri , Middei Riccardo , Millar-Blanchaer Maxwell A. , Myserlis Ioannis , Negro Michela , Nilsson Kari , O’Dell Stephen L. , Omodei Nicola , Pacciani Luigi , Paggi Alessandro , Panopoulou Georgia V. , Peirson Abel L. , Perri Matteo , Petrucci Pierre-Olivier , Poutanen Juri , Puccetti Simonetta , Romani Roger W. , Sakanoi Takeshi , Savchenko Sergey S. , Sota Alfredo , Tavecchio Fabrizio , Tinyanont Samaporn , Vasilyev Andrey A. , Weaver Zachary R. , Zhovtan Alexey V. , Antonelli Lucio A. , Bachetti Matteo , Baldini Luca , Baumgartner Wayne H. , Bellazzini Ronaldo , Bianchi Stefano , Bongiorno Stephen D. , Bonino Raffaella , Brez Alessandro , Bucciantini Niccoló , Capitanio Fiamma , Castellano Simone , Ciprini Stefano , Costa Enrico , De Rosa Alessandra , Del Monte Ettore , Di Marco Alessandro , Doroshenko Victor , Dovčiak Michal , Enoto Teruaki , Evangelista Yuri , Fabiani Sergio , Ferrazzoli Riccardo , Garcia Javier A. , Gunji Shuichi , Hayashida Kiyoshi , Heyl Jeremy , Iwakiri Wataru , Karas Vladimir , Kitaguchi Takao , Kolodziejczak Jeffery J. , La Monaca Fabio , Latronico Luca , Maldera Simone , Manfreda Alberto , Marinucci Andrea , Matt Giorgio , Mitsuishi Ikuyuki , Mizuno Tsunefumi , Muleri Fabio , Ng Stephen C.-Y. , Oppedisano Chiara , Papitto Alessandro , Pavlov George G. , Pesce-Rollins Melissa , Pilia Maura , Possenti Andrea , Ramsey Brian D. , Rankin John , Ratheesh Ajay , Sgró Carmelo , Slane Patrick , Soffitta Paolo , Spandre Gloria , Tamagawa Toru , Taverna Roberto , Tawara Yuzuru , Tennant Allyn F. , Thomas Nicolas E. , Tombesi Francesco , Trois Alessio , Tsygankov Sergey , Turolla Roberto , Vink Jacco , Weisskopf Martin C. , Wu Kinwah , Xie Fei , Zane Silvia 
Published: November 2022

Most of the light from blazars, active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight, is produced by high-energy particles, up to around 1 TeV. Although the jets are known to be ultimately powered by a supermassive black hole, how the particles are accelerated to such high energies has been an unanswered question. The process must be related to the magnetic field, which can be probed by observations of the polarization of light from the jets. Measurements of the radio to optical polarization—the only range available until now—probe extended regions of the jet containing particles that left the acceleration site days to years earlier1–3, and hence do not directly explore the acceleration mechanism, as could X-ray measurements. Here we report the detection of X-ray polarization from the blazar Markarian 501 (Mrk 501). We measure an X-ray linear polarization degree ΠX of around 10%, which is a factor of around 2 higher than the value at optical wavelengths, with a polarization angle parallel to the radio jet. This points to a shock front as the source of particle acceleration and also implies that the plasma becomes increasingly turbulent with distance from the shock.