The X-Ray Polarization View of Mrk 421 in an Average Flux State as Observed by the Imaging X-Ray Polarimetry Explorer
Authors: Di Gesu Laura , Donnarumma Immacolata , Tavecchio Fabrizio , Agudo Iván , Barnounin Thibault , Cibrario Nicolò , Di Lalla Niccolò , Di Marco Alessandro , Escudero Juan , Errando Manel , Jorstad Svetlana G. , Kim Dawoon E. , Kouch Pouya M. , Liodakis Ioannis , Lindfors Elina , Madejski Grzegorz , Marshall Herman L. , Marscher Alan P. , Middei Riccardo , Muleri Fabio , Myserlis Ioannis , Negro Michela , Omodei Nicola , Pacciani Luigi , Paggi Alessandro , Perri Matteo , Puccetti Simonetta , Antonelli Lucio A. , Bachetti Matteo , Baldini Luca , Baumgartner Wayne H. , Bellazzini Ronaldo , Bianchi Stefano , Bongiorno Stephen D. , Bonino Raffaella , Brez Alessandro , Bucciantini Niccolò , Capitanio Fiamma , Castellano Simone , Cavazzuti Elisabetta , Ciprini Stefano , Costa Enrico , De Rosa Alessandra , Del Monte Ettore , Doroshenko Victor , Dovčiak Michal , Ehlert Steven R. , 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. , Krawczynski Henric , La Monaca Fabio , Latronico Luca , Maldera Simone , Manfreda Alberto , Marin Frédéric , Marinucci Andrea , Massaro Francesco , Matt Giorgio , Mitsuishi Ikuyuki , Mizuno Tsunefumi , Ng C.-Y. , O’Dell Stephen L. , Oppedisano Chiara , Papitto Alessandro , Pavlov George G. , Peirson Abel L. , Pesce-Rollins Melissa , Petrucci Pierre-Olivier , Pilia Maura , Possenti Andrea , Poutanen Juri , Ramsey Brian D. , Rankin John , Ratheesh Ajay , Romani Roger W. , Sgrò 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: October 2022

Abstract
Particle acceleration mechanisms in supermassive black hole jets, such as shock acceleration, magnetic reconnection, and turbulence, are expected to have observable signatures in the multiwavelength polarization properties of blazars. The recent launch of the Imaging X-Ray Polarimetry Explorer (IXPE) enables us, for the first time, to use polarization in the X-ray band (2–8 keV) to probe the properties of the jet synchrotron emission in high-synchrotron-peaked BL Lac objects (HSPs). We report the discovery of X-ray linear polarization (degree Πx = 15% ± 2% and electric vector position angle ψ
x = 35° ± 4°) from the jet of the HSP Mrk 421 in an average X-ray flux state. At the same time, the degree of polarization at optical, infrared, and millimeter wavelengths was found to be lower by at least a factor of 3. During the IXPE pointing, the X-ray flux of the source increased by a factor of 2.2, while the polarization behavior was consistent with no variability. The higher level of Πx compared to longer wavelengths, and the absence of significant polarization variability, suggest a shock is the most likely X-ray emission site in the jet of Mrk 421 during the observation. The multiwavelength polarization properties are consistent with an energy-stratified electron population, where the particles emitting at longer wavelengths are located farther from the acceleration site, where they experience a more disordered magnetic field.