Blazars from the new window of X-ray polarimetry: IXPE observations of Mrk 421

Abstract

The NASA/ASI Imaging X-ray Polarimetry Explorer (IXPE) was launched on December 9, 2021 thereby opening the new observational window of X-ray polarimetry. Blazars, which are active galactic nuclei (AGN) where the jet is oriented towards the observer, are prime candidates for X-ray polarization observations. For instance, a high degree of X-ray polarization is expected from High synchrotron Peak (HSP) blazars in the case of synchrotron radiation produced in an ordered magnetic field. Moreover, we expect different time variability patterns of the X-ray and optical polarization properties depending on which physical mechanism energizes the particles in the jets, e.g. shock acceleration, magnetic reconnection, and turbulence. We report on the first year IXPE observations of the archetypical HSP Mrk 421 and we discuss what X-ray polarimetry is teaching us about the inner physics of jets.