X-Ray and Multiwavelength Polarization of Mrk 501 from 2022 to 2023
Authors: Chen Chien-Ting J. , Liodakis Ioannis , Middei Riccardo , Kim Dawoon E. , Di Gesu Laura , Di Marco Alessandro , Ehlert Steven R. , Errando Manel , Negro Michela , Jorstad Svetlana G. , Marscher Alan P. , Wu Kinwah , Agudo Iván , Poutanen Juri , Mizuno Tsunefumi , Kouch Pouya M. , Lindfors Elina , Borman George A. , Grishina Tatiana S. , Kopatskaya Evgenia N. , Larionova Elena G. , Morozova Daria A. , Savchenko Sergey S. , Troitsky Ivan S. , Troitskaya Yulia V. , Vasilyev Andrey A. , Zhovtan Alexey V. , Aceituno Francisco José , Bonnoli Giacomo , Casanova Víctor , Escudero Juan , Agís-González Beatriz , Husillos César , Otero Santos Jorge , Sota Alfredo , Piirola Vilppu , Myserlis Ioannis , Angelakis Emmanouil , Kraus Alexander , Gurwell Mark , Keating Garrett , Rao Ramprasad , Kang Sincheol , Lee Sang-Sung , Kim Sang-Hyun , Cheong Whee Yeon , Jeong Hyeon-Woo , Song Chanwoo , Berdyugin Andrei V. , Kagitani Masato , Kravtsov Vadim , Nitindala Anagha P. , Sakanoi Takeshi , Imazawa Ryo , Sasada Mahito , Fukazawa Yasushi , Kawabata Koji S. , Uemura Makoto , Nakaoka Tatsuya , Akitaya Hiroshi , Casadio Carolina , Sievers Albrecht , Antonelli Lucio Angelo , 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 , Ciprini Stefano , Costa Enrico , De Rosa Alessandra , Del Monte Ettore , Di Lalla Niccoló , 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 , 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 , Muleri Fabio , Ng C.-Y. , O’Dell Stephen L. , Omodei Nicola , Oppedisano Chiara , Papitto Alessandro , Pavlov George G. , Peirson Abel Lawrence , Perri Matteo , Pesce-Rollins Melissa , Petrucci Pierre-Olivier , Pilia Maura , Possenti Andrea , 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 , Tavecchio Fabrizio , Taverna Roberto , Tawara Yuzuru , Tennant Allyn F. , Thomas Nicholas E. , Tombesi Francesco , Trois Alessio , Tsygankov Sergey S. , Turolla Roberto , Vink Jacco , Weisskopf Martin C. , Xie Fei , Zane Silvia 
Published: October 2024

Abstract
We present multiwavelength polarization measurements of the luminous blazar Mrk 501 over a 14 month period. The 2–8 keV X-ray polarization was measured with the Imaging X-ray Polarimetry Explorer (IXPE) with six 100 ks observations spanning from 2022 March to 2023 April. Each IXPE observation was accompanied by simultaneous X-ray data from NuSTAR, Swift/XRT, and/or XMM-Newton. Complementary optical–infrared polarization measurements were also available in the B, V, R, I, and J bands, as were radio polarization measurements from 4.85 GHz to 225.5 GHz. Among the first five IXPE observations, we did not find significant variability in the X-ray polarization degree and angle with IXPE. However, the most recent sixth observation found an elevated polarization degree at >3σ above the average of the other five observations. The optical and radio measurements show no apparent correlations with the X-ray polarization properties. Throughout the six IXPE observations, the X-ray polarization degree remained higher than, or similar to, the R-band optical polarization degree, which remained higher than the radio value. This is consistent with the energy-stratified shock scenario proposed to explain the first two IXPE observations, in which the polarized X-ray, optical, and radio emission arises from different regions.