Di Gesu, LauraDonnarumma, ImmacolataTavecchio, FabrizioAgudo, IvánBarnouin, ThibaultCibrario, NicolòDi Lalla, NiccolòDi Marco, AlessandroEscudero, JuanErrando, ManelJorstad, Svetlana G.Kim, Dawoon E.Kouch, Pouya M.Liodakis, IoannisLindfors, ElinaMadejski, GrzegorzMarshall, Herman L.Marscher, Alan P.Middei, RiccardoMuleri, FabioMyserlis, IoannisNegro, MichelaOmodei, NicolaPacciani, LuigiPaggi, AlessandroPerri, MatteoPuccetti, SimonettaAntonelli, L.A.Bachetti, MatteoBaldini, LucaBaumgartner, Wayne H.Bellazzini, RonaldoBianchi, StefanoBongiorno, Stephen D.Bonino, RaffaellaBrez, AlessandroBucciantini, NiccolòCapitanio, FiammaCastellano, SimoneCavazzuti, ElisabettaCiprini, StefanoCosta, EnricoDe Rosa, AlessandraDel Monte, EttoreDoroshenko, VictorDovčiak, MichalEhlert, Steven R.Enoto, TeruakiEvangelista, YuriFabiani, SergioFerrazzoli, RiccardoGarcia, Javier A.Gunji, ShuichiHayashida, KiyoshiHeyl, JeremyIwakiri, WataruKaras, VladimirKitaguchi, TakaoKolodziejczak, Jeffery J.Krawczynski, HenricLa Monaca, FabioLatronico, LucaMaldera, SimoneManfreda, AlbertoMarin, FrédéricMarinucci, AndreaMassaro, FrancescoMatt, GiorgioMitsuishi, IkuyukiMizuno, TsunefumiNg, C.-Y.O’Dell, Stephen L.Oppedisano, ChiaraPapitto, AlessandroPavlov, George G.Peirson, Abel L.Pesce-Rollins, MelissaPetrucci, Pierre-OlivierPilia, MauraPossenti, AndreaPoutanen, JuriRamsey, Brian D.Rankin, JohnRatheesh, AjayRomani, Roger W.Sgrò, CarmeloSlane, PatrickSoffitta, PaoloSpandre, GloriaTamagawa, ToruTaverna, RobertoTawara, YuzuruTennant, Allyn F.Thomas, Nicolas E.Tombesi, FrancescoTrois, AlessioTsygankov, SergeyTurolla, RobertoVink, JaccoWeisskopf, Martin C.Wu, KinwahXie, FeiZane, Silvia2023-07-032023-07-032022-10-01L. Di Gesu, I. Donnarumma, F. Tavecchio, I. Agudo, T. Barnounin, N. Cibrario, N. Di Lalla, A. Di Marco, J. Escudero, M. Errando, S.G. Jorstad, D.E. Kim, P.M. Kouch, I. Liodakis, E. Lindfors, G. Madejski, H.L. Marshall, A.P. Marscher, R. Middei, F. Muleri, I. Myserlis, M. Negro, N. Omodei, L. Pacciani, A. Paggi, M. Perri, S. Puccetti, L.A. Antonelli, M. Bachetti, L. Baldini, W.H. Baumgartner, R. Bellazzini, S. Bianchi, S.D. Bongiorno, R. Bonino, A. Brez, N. Bucciantini, F. Capitanio, S. Castellano, E. Cavazzuti, S. Ciprini, E. Costa, A. De Rosa, E. Del Monte, V. Doroshenko, M. Dovčiak, S.R. Ehlert, T. Enoto, Y. Evangelista, S. Fabiani, R. Ferrazzoli, J.A. Garcia, S. Gunji, K. Hayashida, J. Heyl, W. Iwakiri, V. Karas, T. Kitaguchi, J.J. Kolodziejczak, H. Krawczynski, F. La Monaca, L. Latronico, S. Maldera, A. Manfreda, F. Marin, A. Marinucci, F. Massaro, G. Matt, I. Mitsuishi, T. Mizuno, C.-Y. Ng, S.L. O’Dell, C. Oppedisano, A. Papitto, G.G. Pavlov, A.L. Peirson, M. Pesce-Rollins, P.-O. Petrucci, M. Pilia, A. Possenti, J. Poutanen, B.D. Ramsey, J. Rankin, A. Ratheesh, R.W. Romani, C. Sgrò, P. Slane, P. Soffitta, G. Spandre, T. Tamagawa, R. Taverna, Y. Tawara, A.F. Tennant, N.E. Thomas, F. Tombesi, A. Trois, S. Tsygankov, R. Turolla, J. Vink, M.C. Weisskopf, K. Wu, F. Xie, S. Zane. 2022. "The X-Ray Polarization View of Mrk 421 in an Average Flux State as Observed by the Imaging X-Ray Polarimetry Explorer" Letters of the Astrophysical Journal, Volume 938, Issue 1, pp.L7-L7. https://doi.org/10.3847/2041-8213/ac913a2041-82052041-8213https://hdl.handle.net/2144/46396Particle acceleration mechanisms in supermassive black hole jets, such as shock acceleration, magnetic reconnection, and turbulence, are expected to have observable signatures in the multiwavelength polarization properties of blazars. The recent launch of the Imaging X-Ray Polarimetry Explorer (IXPE) enables us, for the first time, to use polarization in the X-ray band (2–8 keV) to probe the properties of the jet synchrotron emission in high-synchrotron-peaked BL Lac objects (HSPs). We report the discovery of X-ray linear polarization (degree Πx = 15% ± 2% and electric vector position angle ψ x = 35° ± 4°) from the jet of the HSP Mrk 421 in an average X-ray flux state. At the same time, the degree of polarization at optical, infrared, and millimeter wavelengths was found to be lower by at least a factor of 3. During the IXPE pointing, the X-ray flux of the source increased by a factor of 2.2, while the polarization behavior was consistent with no variability. The higher level of Πx compared to longer wavelengths, and the absence of significant polarization variability, suggest a shock is the most likely X-ray emission site in the jet of Mrk 421 during the observation. The multiwavelength polarization properties are consistent with an energy-stratified electron population, where the particles emitting at longer wavelengths are located farther from the acceleration site, where they experience a more disordered magnetic field.p. L7en-US© 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.https://creativecommons.org/licenses/by/4.0/Astronomical and space sciencesAstronomy & astrophysicsAstronomical sciencesSpace sciencesArticle2023-02-0710.3847/2041-8213/ac913a0000-0002-4700-4549 (Donnarumma, Immacolata)0000-0003-0256-0995 (Tavecchio, Fabrizio)0000-0002-3777-6182 (Agudo, Iván)0000-0002-7574-1298 (Di Lalla, Niccolò)0000-0003-0331-3259 (Di Marco, Alessandro)0000-0001-6158-1708 (Jorstad, Svetlana G)0000-0002-6492-1293 (Marshall, Herman L)0000-0001-7396-3332 (Marscher, Alan P)0000-0003-3331-3794 (Muleri, Fabio)0000-0003-3025-9497 (Myserlis, Ioannis)0000-0002-6548-5622 (Negro, Michela)0000-0002-5448-7577 (Omodei, Nicola)0000-0002-5646-2410 (Paggi, Alessandro)0000-0003-3613-4409 (Perri, Matteo)0000-0002-2734-7835 (Puccetti, Simonetta)0000-0002-4576-9337 (Bachetti, Matteo)0000-0002-9785-7726 (Baldini, Luca)0000-0002-2469-7063 (Bellazzini, Ronaldo)0000-0002-4622-4240 (Bianchi, Stefano)0000-0002-4264-1215 (Bonino, Raffaella)0000-0002-8848-1392 (Bucciantini, Niccolò)0000-0001-7150-9638 (Cavazzuti, Elisabetta)0000-0002-0712-2479 (Ciprini, Stefano)0000-0003-4925-8523 (Costa, Enrico)0000-0001-5668-6863 (De Rosa, Alessandra)0000-0002-3013-6334 (Del Monte, Ettore)0000-0001-8162-1105 (Doroshenko, Victor)0000-0003-0079-1239 (Dovčiak, Michal)0000-0003-4420-2838 (Ehlert, Steven R)0000-0003-1244-3100 (Enoto, Teruaki)0000-0001-6096-6710 (Evangelista, Yuri)0000-0003-1533-0283 (Fabiani, Sergio)0000-0003-1074-8605 (Ferrazzoli, Riccardo)0000-0003-3828-2448 (Garcia, Javier A)0000-0002-5881-2445 (Gunji, Shuichi)0000-0001-9739-367X (Heyl, Jeremy)0000-0002-0207-9010 (Iwakiri, Wataru)0000-0002-5760-0459 (Karas, Vladimir)0000-0002-1084-6507 (Krawczynski, Henric)0000-0001-8916-4156 (La Monaca, Fabio)0000-0002-0984-1856 (Latronico, Luca)0000-0002-0698-4421 (Maldera, Simone)0000-0002-0998-4953 (Manfreda, Alberto)0000-0003-4952-0835 (Marin, Frédéric)0000-0002-2055-4946 (Marinucci, Andrea)0000-0002-1704-9850 (Massaro, Francesco)0000-0002-2152-0916 (Matt, Giorgio)0000-0001-7263-0296 (Mizuno, Tsunefumi)0000-0002-5847-2612 (Ng, C-Y)0000-0001-6194-4601 (Oppedisano, Chiara)0000-0001-6289-7413 (Papitto, Alessandro)0000-0002-7481-5259 (Pavlov, George G)0000-0001-6292-1911 (Peirson, Abel L)0000-0003-1790-8018 (Pesce-Rollins, Melissa)0000-0001-7397-8091 (Pilia, Maura)0000-0001-5902-3731 (Possenti, Andrea)0000-0002-0983-0049 (Poutanen, Juri)0000-0002-9774-0560 (Rankin, John)0000-0001-6711-3286 (Romani, Roger W)0000-0001-5676-6214 (Sgrò, Carmelo)0000-0002-6986-6756 (Slane, Patrick)0000-0002-7781-4104 (Soffitta, Paolo)0000-0002-8801-6263 (Tamagawa, Toru)0000-0002-1768-618X (Taverna, Roberto)0000-0002-3180-6002 (Trois, Alessio)0000-0002-9679-0793 (Tsygankov, Sergey)0000-0003-3977-8760 (Turolla, Roberto)0000-0002-4708-4219 (Vink, Jacco)0000-0002-5270-4240 (Weisskopf, Martin C)0000-0002-0105-5826 (Xie, Fei)767359