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, Ivan Agudo, Thibault Barnounin, Nicolò Cibrario, Niccolò Di Lalla, Alessandro Di Marco, Juan Escudero, Manel Errando, Svetlana G. Jorstad, Dawoon Kim, Pouya M. Kouch, Elina Lindfors, Ioannis Liodakis, 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 et al.

Submitted on 15 September 2022


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 Πx=15±2\% and electric-vector position angle Ψx=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 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.


Comment: accepted for ApJ Letters

Subject: Astrophysics - High Energy Astrophysical Phenomena