The X-Ray Polarization View of Mrk 421 in an Average Flux State as Observed by the Imaging X-Ray Polarimetry Explorer
Laura Di Gesu, I. Donnarumma, F. Tavecchio, I. Agudo, Thibault Barnounin, Nicolò Cibrario, Niccolò Di Lalla, Alessandro Di Marco, Juan Escudero, M. Errando, Svetlana G. Jorstad, Dawoon E. Kim, Pouya M. Kouch, Ioannis Liodakis, E. Lindfors, G. Madejski, Herman L. Marshall, Alan P. Marscher, R. Middei, Fabio Muleri, I. Myserlis, Michela Negro, N. Omodei, L. Pacciani, A. Paggi, M. Perri, Simonetta Puccetti, L. A. Antonelli, Matteo Bachetti, L. Baldini, W. H. Baumgartner, R. Bellazzini, S. Bianchi, Stephen D. Bongiorno, R. Bonino, A. Brez, N. Bucciantini, Fiamma Capitanio, Simone Castellano, E. Cavazzuti, S. Ciprini, E. Costa, Alessandra De Rosa, E. Del Monte, Victor Doroshenko, Michal Dovčiak, Steven R. Ehlert, Teruaki Enoto, Y. Evangelista, Sergio Fabiani, Riccardo Ferrazzoli, Javier A. García, Shuichi Gunji, Kiyoshi Hayashida, Jeremy Heyl, W. Iwakiri, V. Karas, Takao Kitaguchi, Jeffery J. Kolodziejczak, H. Krawczynski, Fabio La Monaca, L. Latronico, S. Maldera, Alberto Manfreda, Frédéric Marin, Andrea Marinucci, F. Massaro, G. Matt, Ikuyuki Mitsuishi, Tsunefumi Mizuno, C.‐Y. Ng, Stephen L. O’Dell, C. Oppedisano, Alessandro Papitto, George G. Pavlov, Abel L. Peirson, M. Pesce-Rollins, Pierre-Olivier Petrucci, M. Pilia, Andrea Possenti, Juri Poutanen, Brian D. Ramsey, John Rankin, Ajay Ratheesh, Roger W. Romani, C. Sgró, Patrick Slane, P. Soffitta, G. Spandre, Toru Tamagawa, Roberto Taverna, Yuzuru Tawara, Allyn F. Tennant, Nicolas E. Thomas, Francesco Tombesi, A. Trois, Sergey S. Tsygankov, Roberto Turolla, Jacco Vink, Martin C. Weisskopf
Abstract
Abstract Particle 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 Π x = 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 Π x 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.