The X-ray Polarization View of Mrk~421 in an Average Flux State as Observed by the Imaging X-ray Polarimetry Explorer
Creators: name: Di Gesu, Laura
name: Donnarumma, Immacolata
name: Tavecchio, Fabrizio
name: Agudo, Ivan
name: Barnounin, Thibault
name: Cibrario, Nicolò
name: Di Lalla, Niccolò
name: Di Marco, Alessandro
name: Escudero, Juan
name: Errando, Manel
name: Jorstad, Svetlana G.
name: Kim, Dawoon
name: Kouch, Pouya M.
name: Lindfors, Elina
name: Liodakis, Ioannis
name: Madejski, Grzegorz
name: Marshall, Herman L.
name: Marscher, Alan P.
name: Middei, Riccardo
name: Muleri, Fabio
name: Myserlis, Ioannis
name: Negro, Michela
name: Omodei, Nicola
name: Pacciani, Luigi
name: Paggi, Alessandro
name: Perri, Matteo
name: Puccetti, Simonetta
name: Antonelli, Lucio A.
name: Bachetti, Matteo
name: Baldini, Luca
name: Baumgartner, Wayne H.
name: Bellazzini, Ronaldo
name: Bianchi, Stefano
name: Bongiorno, Stephen D.
name: Bonino, Raffaella
name: Brez, Alessandro
name: Bucciantini, Niccolò
name: Capitanio, Fiamma
name: Castellano, Simone
name: Cavazzuti, Elisabetta
name: Ciprini, Stefano
name: Costa, Enrico
name: De Rosa, Alessandra
name: Del Monte, Ettore
name: Doroshenko, Victor
name: Dovčiak, Michal
name: Ehlert, Steven R.
name: Enoto, Teruaki
name: Evangelista, Yuri
name: Fabiani, Sergio
name: Ferrazzoli, Riccardo
name: Garcia, Javier A.
name: Gunji, Shuichi
name: Hayashida, Kiyoshi
name: Heyl, Jeremy
name: Iwakiri, Wataru
name: Karas, Vladimir
name: Kitaguchi, Takao
name: Kolodziejczak, Jeffery J.
name: Krawczynski, Henric
name: La Monaca, Fabio
name: Latronico, Luca
name: Maldera, Simone
name: Manfreda, Alberto
name: Marin, Frédéric
name: Marinucci, Andrea
name: Massaro, Francesco
name: Matt, Giorgio
name: Mitsuishi, Ikuyuki
name: Mizuno, Tsunefumi
name: Ng, C. -Y.
name: O'Dell, Stephen L.
name: Oppedisano, Chiara
name: Papitto, Alessandro
name: Pavlov, George G.
name: Peirson, Abel L.
name: Pesce-Rollins, Melissa
name: Petrucci, Pierre-Olivier
name: Pilia, Maura
name: Possenti, Andrea
name: Poutanen, Juri
name: Ramsey, Brian D.
name: Rankin, John
name: Ratheesh, Ajay
name: Romani, Roger W.
name: Sgrò, Carmelo
name: Slane, Patrick
name: Soffitta, Paolo
name: Spandre, Gloria
name: Tamagawa, Toru
name: Taverna, Roberto
name: Tawara, Yuzuru
name: Tennant, Allyn F.
name: Thomas, Nicolas E.
name: Tombesi, Francesco
name: Trois, Alessio
name: Tsygankov, Sergey
name: Turolla, Roberto
name: Vink, Jacco
name: Weisskopf, Martin C.
name: Wu, Kinwah
name: Xie, Fei
name: Zane, Silvia
Publisher: name: arXiv
Year: 2022
Subjects: subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: Particle acceleration mechanisms in supermassive black hole jets, such as shock acceleration, magnetic reconnection, and turbulence, are expected to have observable signatures in the multi-wavelength 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-frequency-peaked BL Lac objects (HSPs). We report the discovery of X-ray linear polarization (degree $Π_{\rm x}=15\pm$2\% and electric-vector position angle $Ψ_{\rm x}=35^\circ\pm4^\circ$) 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 $Π_{\rm x}$ compared to longer wavelengths, and the absence of significant polarization variability, suggest a shock as 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.
description: accepted for ApJ Letters