Magnetic field properties inside the jet of Mrk 421
Authors: Kim Dawoon E. , Di Gesu Laura , Liodakis Ioannis , Marscher Alan P. , Jorstad Svetlana G. , Middei Riccardo , Marshall Herman L. , Pacciani Luigi , Agudo Iván , Tavecchio Fabrizio , Cibrario Nicoló , Tugliani Stefano , Bonino Raffaella , Negro Michela , Puccetti Simonetta , Tombesi Francesco , Costa Enrico , Donnarumma Immacolata , Soffitta Paolo , Mizuno Tsunefumi , Fukazawa Yasushi , Kawabata Koji S. , Nakaoka Tatsuya , Uemura Makoto , Imazawa Ryo , Sasada Mahito , Akitaya Hiroshi , Josè Aceituno Francisco , Bonnoli Giacomo , Casanova Vìctor , Myserlis Ioannis , Sievers Albrecht , Angelakis Emmanouil , Kraus Alexander , Yeon Cheong Whee , Jeong Hyeon-Woo , Kang Sincheol , Kim Sang-Hyun , Lee Sang-Sung , Agìs-Gonzàlez Beatriz , Sota Alfredo , Escudero Juan , Gurwell Mark , Keating Garrett K. , Rao Ramprasad , Kouch Pouya M. , Lindfors Elina , Bourbah Ioakeim G. , Kiehlmann Sebastian , Kontopodis Evangelos , Mandarakas Nikos , Romanopoulos Stylianos , Skalidis Raphael , Vervelaki Anna , Savchenko Sergey S. , Antonelli Lucio A. , Bachetti Matteo , Baldini Luca , Baumgartner Wayne H. , Bellazzini Ronaldo , Bianchi Stefano , Bongiorno Stephen D. , Brez Alessandro , Bucciantini Niccoló , Capitanio Fiamma , Castellano Simone , Cavazzuti Elisabetta , Chen Chien-Ting , Ciprini Stefano , De Rosa Alessandra , Del Monte Ettore , Di Lalla Niccoló , Di Marco Alessandro , 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 , 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 , 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 L. , Perri Matteo , Pesce-Rollins Melissa , Petrucci Pierre-Olivier , Pilia Maura , Possenti Andrea , Poutanen Juri , Ramsey Brian D. , Rankin John , Ratheesh Ajay , Roberts Oliver , Romani Roger W. , Sgró Carmelo , Slane Patrick , Spandre Gloria , Swartz Doug , Tamagawa Toru , Taverna Roberto , Tawara Yuzuru , Tennant Allyn F. , Thomas Nicholas E. , Trois Alessio , Tsygankov Sergey S. , Turolla Roberto , Vink Jacco , Weisskopf Martin C. , Wu Kinwah , Xie Fei , Zane Silvia 
Published: October 2023

Aims.We aim to probe the magnetic field geometry and particle acceleration mechanism in the relativistic jets of supermassive black holes.Methods.We conducted a polarimetry campaign from radio to X-ray wavelengths of the high-synchrotron-peak (HSP) blazar Mrk 421, including Imaging X-ray Polarimetry Explorer (IXPE) measurements from 2022 December 6–8. During the IXPE observation, we also monitored Mrk 421 usingSwift-XRT and obtained a single observation withXMM-Newtonto improve the X-ray spectral analysis. The time-averaged X-ray polarization was determined consistently using the event-by-event Stokes parameter analysis, spectropolarimetric fit, and maximum likelihood methods. We examined the polarization variability over both time and energy, the former via analysis of IXPE data obtained over a time span of 7 months.Results.We detected X-ray polarization of Mrk 421 with a degree of ΠX = 14 ± 1% and an electric-vector position angleψX = 107 ± 3° in the 2–8 keV band. From the time variability analysis, we find a significant episodic variation inψX. During the 7 months from the first IXPE pointing of Mrk 421 in 2022 May,ψXvaried in the range 0° to 180°, while ΠXremained relatively constant within ∼10–15%. Furthermore, a swing inψXin 2022 June was accompanied by simultaneous spectral variations. The results of the multiwavelength polarimetry show that ΠXwas generally ∼2–3 times greater than Π at longer wavelengths, whileψfluctuated. Additionally, based on radio, infrared, and optical polarimetry, we find that the rotation ofψoccurred in the opposite direction with respect to the rotation ofψXand over longer timescales at similar epochs.Conclusions.The polarization behavior observed across multiple wavelengths is consistent with previous IXPE findings for HSP blazars. This result favors the energy-stratified shock model developed to explain variable emission in relativistic jets. We considered two versions of the model, one with linear and the other with radial stratification geometry, to explain the rotation ofψX. The accompanying spectral variation during theψXrotation can be explained by a fluctuation in the physical conditions, for example in the energy distribution of relativistic electrons. The opposite rotation direction ofψbetween the X-ray and longer wavelength polarization accentuates the conclusion that the X-ray emitting region is spatially separated from that at longer wavelengths. Moreover, we identify a highly polarized knot of radio emission moving down the parsec-scale jet during the episode ofψXrotation, although it is unclear whether there is any connection between the two events.