Abstract
X-ray polarimetry promises us an unprecedented look at the structure of magnetic fields and on the processes at the base of acceleration of particles up to ultrarelativistic energies in relativistic jets. Crucial pieces of information are expected from observations of blazars (that are characterized by the presence of a jet pointing close to the Earth), in particular of the subclass defined by a synchrotron emission extending to the X-ray band (so-called high synchrotron peak blazars, HSP). In this review, I give an account of some of the models and numerical simulations developed to predict the polarimetric properties of HSP at high energy, contrasting the predictions of scenarios assuming particle acceleration at shock fronts with those that are based on magnetic reconnection, and I discuss the prospects for the observations of the upcoming Imaging X-ray Polarimetry Explorer (IXPE) satellite.
Highlights
The comprehension of the physical processes behind the acceleration of relativistic particles in the relativistic collimated outflows that are associated with Active Galactic Nuclei (AGN) is one of the most intriguing challenges of modern astrophysics
Current studies converge to support two main general paths by which the energy that is carried by a relativistic jet can be dissipated and made available for the acceleration of relativistic particles: (a) for magnetically dominated jets simulations show that a sizable part of the initial magnetic energy can be dissipated through reconnection, being triggered during the non-linear stages of jet instabilities (e.g., Kelvin–Helmholtz or current-driven kink instabilities, (e.g., [6]))
Particle-in-cell (PIC) simulations show that, in current sheets associated with magnetic reconnection, particles can be efficiently accelerated forming non-thermal energy distributions (e.g., [7,8,9,10]); (b) for weakly magnetized flows, instead, the most likely dissipation sites are shocks (e.g., [11,12,13]), where the formation of non-thermal populations occurs through the classical diffusive shock acceleration (DSA) mechanism (e.g., [14,15,16])
Summary
The comprehension of the physical processes behind the acceleration of relativistic particles (electrons, possibly nuclei) in the relativistic collimated outflows (jets) that are associated with Active Galactic Nuclei (AGN) is one of the most intriguing challenges of modern astrophysics. While simulations and theoretical studies can delineate the landscape of the potential physical processes at action, only the confrontation with the observational evidence can decide which mechanism(s) is responsible for particle acceleration in jets In this respect, for a long time, multiband polarimetric measurements have been considered to be a powerful tool for investigating structure and dynamics of relativistic jets, magnetic field geometries and particle acceleration (e.g., [4,17]). We will focus our attention to the subclass of blazars characterized by the synchrotron component peaking in the X-ray band, the so-called high synchrotron peak (HSP) Polarization measurements of these sources in the X-rays, exploring the most energetic, freshly accelerated, electrons, can provide unique in situ information on magnetic field geometry, turbulence, and particle distribution inside the jets, key inputs to test and improve our models.
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