X-Ray Polarization Observations of BL Lacertae
Authors: Middei Riccardo , Liodakis Ioannis , Perri Matteo , Puccetti Simonetta , Cavazzuti Elisabetta , Di Gesu Laura , Ehlert Steven R. , Madejski Grzegorz , Marscher Alan P. , Marshall Herman L. , Muleri Fabio , Negro Michela , Jorstad Svetlana G. , Agís-González Beatriz , Agudo Iván , Bonnoli Giacomo , Bernardos Maria I. , Casanova Víctor , García-Comas Maya , Husillos César , Marchini Alessandro , Sota Alfredo , Kouch Pouya M. , Lindfors Elina , Borman George A. , Kopatskaya Evgenia N. , Larionova Elena G. , Morozova Daria A. , Savchenko Sergey S. , Vasilyev Andrey A. , Zhovtan Alexey V. , Casadio Carolina , Escudero Juan , Myserlis Ioannis , Hales Antonio , Kameno Seiji , Kneissl Ruediger , Messias Hugo , Nagai Hiroshi , Blinov Dmitry , Bourbah Ioakeim G. , Kiehlmann Sebastian , Kontopodis Evangelos , Mandarakas Nikos , Romanopoulos Stylianos , Skalidis Raphael , Vervelaki Anna , Masiero Joseph R. , Mawet Dimitri , Millar-Blanchaer Maxwell A. , Panopoulou Georgia V. , Tinyanont Samaporn , Berdyugin Andrei V. , Kagitani Masato , Kravtsov Vadim , Sakanoi Takeshi , Imazawa Ryo , Sasada Mahito , Fukazawa Yasushi , Kawabata Koji S. , Uemura Makoto , Mizuno Tsunefumi , Nakaoka Tatsuya , Akitaya Hiroshi , Gurwell Mark , Rao Ramprasad , Lalla Niccoló Di , Cibrario Nicoló , Donnarumma Immacolata , Kim Dawoon E. , Omodei Nicola , Pacciani Luigi , Poutanen Juri , Tavecchio Fabrizio , 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 , Marco Alessandro Di , 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. , Krawczynski Henric , Monaca Fabio La , Latronico Luca , Maldera Simone , Manfreda Alberto , Marin Frédéric , Marinucci Andrea , Massaro Francesco , Matt Giorgio , Mitsuishi Ikuyuki , 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 , Ramsey Brian D. , Rankin John , Ratheesh Ajay , Romani Roger W. , Sgró Carmelo , Slane Patrick , Soffitta Paolo , Spandre Gloria , Tamagawa Toru , Taverna Roberto , Tawara Yuzuru , Tennant Allyn F. , Thomas Nicholas E. , Tombesi Francesco , Trois Alessio , Tsygankov Sergey , Turolla Roberto , Vink Jacco , Weisskopf Martin C. , Wu Kinwah , Xie Fei , Zane Silvia 
Published: January 2023

Blazars are a class of jet-dominated active galactic nuclei with a typical double-humped spectral energy distribution. It is of common consensus that the synchrotron emission is responsible for the low frequency peak, while the origin of the high frequency hump is still debated. The analysis of X-rays and their polarization can provide a valuable tool to understand the physical mechanisms responsible for the origin of high-energy emission of blazars. We report the first observations of BL Lacertae (BL Lac) performed with the Imaging X-ray Polarimetry Explorer, from which an upper limit to the polarization degree Π
X
< 12.6% was found in the 2–8 keV band. We contemporaneously measured the polarization in radio, infrared, and optical wavelengths. Our multiwavelength polarization analysis disfavors a significant contribution of proton-synchrotron radiation to the X-ray emission at these epochs. Instead, it supports a leptonic origin for the X-ray emission in BL Lac.