The Imaging X-ray Polarimetry Explorer (IXPE) is a joint US and Italian mission. The US contribution is supported by the National Aeronautics and Space Administration (NASA) and led and managed by its Marshall Space Flight Center (MSFC), with industry partner Ball Aerospace (contract NNM15AA18C). The Italian contribution is supported by the Italian Space Agency (Agenzia Spaziale Italiana, ASI) through contract ASI-OHBI-2017-12-I.0, agreements ASI-INAF-2017-12-H0 and ASI-INFN-2017.13-H0, and its Space Science Data Center (SSDC), and by the Istituto Nazionale di Astrofisica (INAF) and the Istituto Nazionale di Fisica Nucleare (INFN) in Italy. This research used data products provided by the IXPE Team (MSFC, SSDC, INAF and INFN) and distributed with additional software tools by the High-Energy Astrophysics Science Archive Research Center (HEASARC), at NASA Goddard Space Flight Center (GSFC). The IAA-CSIC group acknowledges financial support from the grant CEX2021-001131-S funded by MCIN/AEI/10.13039/501100011033 to the Instituto de Astrofisica de Andalucia-CSIC and through grant PID2019-107847RB-C44. The POLAMI observations were carried out at the IRAM 30?m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). The Submillimetre Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. Mauna Kea, the location of the SMA, is a culturally important site for the indigenous Hawaiian people; we are privileged to study the cosmos from its summit. Some of the data reported here are based on observations made with the Nordic Optical Telescope, owned in collaboration with the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. E.L. was supported by Academy of Finland projects 317636 and 320045. The data presented here were obtained (in part) with ALFOSC, which is provided by the Instituto de Astrofisica de Andalucia (IAA) under a joint agreement with the University of Copenhagen and NOT. We are grateful to V. Braga, M. Monelli and M. Saenchez Benavente for performing the observations at the Nordic Optical Telescope. Part of the French contributions is supported by the Scientific Research National Center (CNRS) and the French spatial agency (CNES). The research at Boston University was supported in part by National Science Foundation grant AST-2108622, NASA Fermi Guest Investigator grants 80NSSC21K1917 and 80NSSC22K1571, and NASA Swift Guest Investigator grant 80NSSC22K0537. This research was conducted in part using the Mimir instrument, jointly developed at Boston University and Lowell Observatory and supported by NASA, NSF and the W.M. Keck Foundation. We thank D. Clemens for guidance in the analysis of the Mimir data. This work was supported by JST, the establishment of university fellowships towards the creation of science and technology innovation, grant number JPMJFS2129. This work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI grant number JP21H01137. This work was also partially supported by the Optical and Near-Infrared Astronomy Inter-University Cooperation Program from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. We are grateful to the observation and operating members of the Kanata Telescope. Some of the data are based on observations collected at the Observatorio de Sierra Nevada, owned and operated by the Instituto de Astrofisica de Andalucia (IAA-CSIC). Further data are based on observations collected at the Centro Astronomico Hispano en Andalucia (CAHA), operated jointly by Junta de Andalucia and Consejo Superior de Investigaciones Cientificas (IAA-CSIC). This research has made use of data from the RoboPol programme, a collaboration between Caltech, the University of Crete, IA-FORTH, IUCAA, the MPIfR and the Nicolaus Copernicus University, which was conducted at Skinakas Observatory in Crete, Greece. D.B., S.K., R.S. and N.M., acknowledge support from the European Research Council (ERC) under the European Unions Horizon 2020 Research and Innovation programme under grant agreement no. 771282. C.C. acknowledges support from the European Research Council (ERC) under the HORIZON ERC Grants 2021 programme under grant agreement no. 101040021. The research at Boston University was supported in part by National Science Foundation grant AST-2108622, NASA Fermi Guest Investigator grant 80NSSC21K1917 and 80NSSC22K1571, and NASA Swift Guest Investigator grant 80NSSC22K0537. This work was supported by NSF grant AST-2109127. We acknowledge the use of public data from the Swift data archive. Data from the Steward Observatory spectropolarimetric monitoring project were used. This programme is supported by Fermi Guest Investigator grants NNX08AW56G, NNX09AU10G, NNX12AO93G and NNX15AU81G. We acknowledge funding to support our NOT observations from the Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Finland (Academy of Finland grant no 306531). This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. The Asteroid Terrestrial-impact Last Alert System (ATLAS) project is primarily funded to search for near-Earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284 and 80NSSC18K1575; by-products of the NEO search include images and catalogues from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889, and STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen's University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory and The Millennium Institute of Astrophysics (MAS), Chile. The Very Long Baseline Array is an instrument of the National Radio Astronomy Observatory. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under a cooperative agreement by Associated Universities, Inc.

Full list of the authors: Di Gesu, Laura; Marshall, Herman L.; Ehlert, Steven R.; Kim, Dawoon E.; Donnarumma, Immacolata; Tavecchio, Fabrizio; Liodakis, Ioannis; Kiehlmann, Sebastian; Agudo, Iván; Jorstad, Svetlana G.; Muleri, Fabio; Marscher, Alan P.; Puccetti, Simonetta; Middei, Riccardo; Perri, Matteo; Pacciani, Luigi; Negro, Michela; Romani, Roger W.; Di Marco, Alessandro; Blinov, Dmitry; Bourbah, Ioakeim G.; Kontopodis, Evangelos; Mandarakas, Nikos; Romanopoulos, Stylianos; Skalidis, Raphael; Vervelaki, Anna; Casadio, Carolina; Escudero, Juan; Myserlis, Ioannis; Gurwell, Mark A.; Rao, Ramprasad; Keating, Garrett K.; Kouch, Pouya M.; Lindfors, Elina; Aceituno, Francisco José; Bernardos, Maria I.; Bonnoli, Giacomo; Casanova, Víctor; García-Comas, Maya; Agís-González, Beatriz; Husillos, César; Marchini, Alessandro; Sota, Alfredo; Imazawa, Ryo; Sasada, Mahito; Fukazawa, Yasushi; Kawabata, Koji S.; Uemura, Makoto; Mizuno, Tsunefumi; Nakaoka, Tatsuya; Akitaya, Hiroshi; Savchenko, Sergey S.; Vasilyev, Andrey A.; Gómez, José L.; Antonelli, Lucio A.; Barnouin, Thibault; Bonino, Raffaella; Cavazzuti, Elisabetta; Costamante, Luigi; Chen, Chien-Ting; Cibrario, Nicolò; De Rosa, Alessandra; Di Pierro, Federico; Errando, Manel; Kaaret, Philip; Karas, Vladimir; Krawczynski, Henric; Lisalda, Lindsey; Madejski, Grzegorz; Malacaria, Christian; Marin, Frédéric; Marinucci, Andrea; Massaro, Francesco; Matt, Giorgio; Mitsuishi, Ikuyuki; O'Dell, Stephen L.; Paggi, Alessandro; Peirson, Abel L.; Petrucci, Pierre-Olivier; Ramsey, Brian D.; Tennant, Allyn F.; Wu, Kinwah; Bachetti, Matteo; Baldini, Luca; Baumgartner, Wayne H.; Bellazzini, Ronaldo; Bianchi, Stefano; Bongiorno, Stephen D.; Brez, Alessandro; Bucciantini, Niccolò; Capitanio, Fiamma; Castellano, Simone; Ciprini, Stefano; Costa, Enrico; Del Monte, Ettore; Di Lalla, Niccolò; Doroshenko, Victor; Dovčiak, Michal; Enoto, Teruaki; Evangelista, Yuri; Fabiani, Sergio; Ferrazzoli, Riccardo; Garcia, Javier A.; Gunji, Shuichi; Hayashida, Kiyoshi; Heyl, Jeremy; Iwakiri, Wataru; Kislat, Fabian; Kitaguchi, Takao; Kolodziejczak, Jeffery J.; La Monaca, Fabio; Latronico, Luca; Maldera, Simone; Manfreda, Alberto; Ng, C. -Y.; Omodei, Nicola; Oppedisano, Chiara; Papitto, Alessandro; Pavlov, George G.; Pesce-Rollins, Melissa; Pilia, Maura; Possenti, Andrea; Poutanen, Juri; Rankin, John; Ratheesh, Ajay; Roberts, Oliver J.; Sgrò, Carmelo; Slane, Patrick; Soffitta, Paolo; Spandre, Gloria; Swartz, Douglas A.; Tamagawa, Toru; Taverna, Roberto; Tawara, Yuzuru; Thomas, Nicholas E.; Tombesi, Francesco; Trois, Alessio; Tsygankov, Sergey S.; Turolla, Roberto; Vink, Jacco; Weisskopf, Martin C.; Xie, Fei; Zane, Silvia

The magnetic-field conditions in astrophysical relativistic jets can be probed by multiwavelength polarimetry, which has been recently extended to X-rays. For example, one can track how the magnetic field changes in the flow of the radiating particles by observing rotations of the electric vector position angle ¿. Here we report the discovery of a ¿X rotation in the X-ray band in the blazar Markarian¿421 at an average flux state. Across the 5¿days of Imaging X-ray Polarimetry Explorer observations on 4¿6 and 7¿9 June 2022, ¿X rotated in total by ¿360°. Over the two respective date ranges, we find constant, within uncertainties, rotation rates (80¿±¿9° per day and 91¿±¿8° per day) and polarization degrees (¿X¿=¿10%¿±¿1%). Simulations of a random walk of the polarization vector indicate that it is unlikely that such rotation(s) are produced by a stochastic process. The X-ray-emitting site does not completely overlap the radio, infrared and optical emission sites, as no similar rotation of ¿ was observed in quasi-simultaneous data at longer wavelengths. We propose that the observed rotation was caused by a helical magnetic structure in the jet, illuminated in the X-rays by a localized shock propagating along this helix. The optically emitting region probably lies in a sheath surrounding an inner spine where the X-ray radiation is released. © 2023, The Author(s), under exclusive licence to Springer Nature Limited.