X-Ray Polarization of the BL Lacertae Type Blazar 1ES 0229+200
Authors: Ehlert Steven R. , Liodakis Ioannis , Middei Riccardo , Marscher Alan P. , Tavecchio Fabrizio , Agudo Iván , Kouch Pouya M. , Lindfors Elina , Nilsson Kari , Myserlis Ioannis , Gurwell Mark , Rao Ramprasad , Aceituno Francisco José , Bonnoli Giacomo , Casanova Víctor , Agís-González Beatriz , Escudero Juan , Husillos César , Otero Santos Jorge , Sota Alfredo , Angelakis Emmanouil , Kraus Alexander , Keating Garrett K. , 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 , Cavazzuti Elisabetta , Chen Chien-Ting , Ciprini Stefano , Costa Enrico , De Rosa Alessandra , Del Monte Ettore , Di Gesu Laura , Di Lalla Niccoló , Di Marco Alessandro , Donnarumma Immacolata , Doroshenko Victor , Dovčiak Michal , Enoto Teruaki , Evangelista Yuri , Fabiani Sergio , Ferrazzoli Riccardo , Garcia Javier A. , Gunji Shuichi , Hayashida Kiyoshi , Heyl Jeremy , Iwakiri Wataru , Jorstad Svetlana G. , Kaaret Philip , Karas Vladimir , Kislat Fabian , Kitaguchi Takao , Kolodziejczak Jeffery J. , Krawczynski Henric , La Monaca Fabio , Latronico Luca , Maldera Simone , Manfreda Alberto , Marin Frédéric , Marinucci Andrea , Marshall Herman L. , Massaro Francesco , Matt Giorgio , Mitsuishi Ikuyuki , Mizuno Tsunefumi , Muleri Fabio , Negro Michela , Ng C.-Y. , O’Dell Stephen L. , Omodei Nicola , Oppedisano Chiara , Papitto Alessandro , Pavlov George G. , Peirson Abel L. , Perri Matteo , Pesce-Rollins Melissa , Petrucci Pierre-Olivier , Pilia Maura , Possenti Andrea , Poutanen Juri , Puccetti Simonetta , Ramsey Brian D. , Rankin John , Ratheesh Ajay , Roberts Oliver J. , Romani Roger W. , Sgró Carmelo , Slane Patrick , Soffitta Paolo , Spandre Gloria , Swartz Douglas A. , Tamagawa Toru , Taverna Roberto , Tawara Yuzuru , Tennant Allyn F. , Thomas Nicholas E. , Tombesi Francesco , Trois Alessio , Tsygankov Sergey S. , Turolla Roberto , Vink Jacco , Weisskopf Martin C. , Wu Kinwah , Xie Fei , Zane Silvia 
Published: December 2023

Abstract
We present polarization measurements in the 2–8 keV band from blazar 1ES 0229+200, the first extreme high synchrotron peaked source to be observed by the Imaging X-ray Polarimetry Explorer (IXPE). Combining two exposures separated by about two weeks, we find the degree of polarization to be ΠX = 17.9% ± 2.8% at an electric-vector position angle ψ
X = 25.°0 ± 4.°6 using a spectro-polarimetric fit from joint IXPE and XMM-Newton observations. There is no evidence for the polarization degree or angle varying significantly with energy or time on both short timescales (hours) or longer timescales (days). The contemporaneous polarization degree at optical wavelengths was >7× lower, making 1ES 0229+200 the most strongly chromatic blazar yet observed. This high X-ray polarization compared to the optical provides further support that X-ray emission in high-peaked blazars originates in shock-accelerated, energy-stratified electron populations, but is in tension with many recent modeling efforts attempting to reproduce the spectral energy distribution of 1ES 0229+200, which attribute the extremely high energy synchrotron and Compton peaks to Fermi acceleration in the vicinity of strongly turbulent magnetic fields.