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 , Immacolata Donnarumma , Fabrizio Tavecchio , Iván Agudo , Thibault Barnounin (1) , Nicolò Cibrario , Niccolò Di Lalla , Alessandro Di Marco , Juan Escudero , Manel Errando , Svetlana G. Jorstad , Dawoon E. Kim , Pouya M. Kouch , Ioannis Liodakis , Elina Lindfors , Grzegorz Madejski , Herman L. Marshall , Alan P. Marscher , Riccardo Middei , Fabio Muleri , Ioannis Myserlis , Michela Negro , Nicola Omodei , Luigi Pacciani , Alessandro Paggi , Matteo Perri , Simonetta Puccetti , Lucio A. Antonelli , Matteo Bachetti , Luca Baldini , Wayne H. Baumgartner , Ronaldo Bellazzini , Stefano Bianchi , Stephen D. Bongiorno , Raffaella Bonino , Alessandro Brez , Niccolò Bucciantini , Fiamma Capitanio , Simone Castellano , Elisabetta Cavazzuti , Stefano Ciprini , Enrico Costa , Alessandra de Rosa , Ettore del Monte , Victor Doroshenko , Michal Dovčiak , Steven R. Ehlert , Teruaki Enoto , Yuri Evangelista , Sergio Fabiani , Riccardo Ferrazzoli , Javier A. Garcia , Shuichi Gunji , Kiyoshi Hayashida , Jeremy Heyl , Wataru Iwakiri , Vladimir Karas , Takao Kitaguchi , Jeffery J. Kolodziejczak , Henric Krawczynski , Fabio La Monaca , Luca Latronico , Simone Maldera , Alberto Manfreda , Frédéric Marin (1) , Andrea Marinucci , Francesco Massaro , Giorgio Matt , Ikuyuki Mitsuishi , Tsunefumi Mizuno , C. -Y. Ng , Stephen L. O'Dell , Chiara Oppedisano , Alessandro Papitto , George G. Pavlov , Abel L. Peirson , Melissa Pesce-Rollins , Pierre-Olivier Petrucci (2) , Maura Pilia , Andrea Possenti , Juri Poutanen , Brian D. Ramsey , John Rankin , Ajay Ratheesh , Roger W. Romani , Carmelo Sgrò , Patrick Slane , Paolo Soffitta , Gloria Spandre , Toru Tamagawa , Roberto Taverna , Yuzuru Tawara , Allyn F. Tennant , Nicolas E. Thomas , Francesco Tombesi , Alessio Trois , Sergey Tsygankov , Roberto Turolla , Jacco Vink , Martin C. Weisskopf , Kinwah Wu , Fei Xie , Silvia Zane

1 ObAS - Observatoire astronomique de Strasbourg
2 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble

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.

Keywords: Polarimetry; X-ray active galactic nuclei; Astrophysics - High Energy Astrophysical Phenomena; Active galactic nuclei; Blazars

Domains: Sciences of the Universe [physics]

Laura Di Gesu, Immacolata Donnarumma, Fabrizio Tavecchio, Iván Agudo, Thibault Barnounin, et al.. The X-Ray Polarization View of Mrk 421 in an Average Flux State as Observed by the Imaging X-Ray Polarimetry Explorer. The Astrophysical Journal Letters, 2022, 938, ⟨10.3847/2041-8213/ac913a⟩. ⟨insu-03828205⟩