Probing dissipation mechanisms in BL Lac jets through X-ray polarimetry

Abstract

The dissipation of energy flux in blazar jets plays a key role in the acceleration of relativistic particles. Two possibilities are commonly considered for the dissipation processes, magnetic reconnection -possibly triggered by instabilities in magnetically-dominated jets -, or shocks -for weakly magnetized flows. We consider the polarimetric features expected for the two scenarios analyzing the results of state-of-the-art simulations. For the magnetic reconnection scenario we conclude, using results from global relativistic MHD simulations, that the emission likely occurs in turbulent regions with unstructured magnetic fields, although the simulations do not allow us to draw firm conclusions. On the other hand, with local particle-in-cell simulations we show that, for shocks with a magnetic field geometry suitable for particle acceleration, the self-generated magnetic field at the shock front is predominantly orthogonal to the shock normal and becomes quasi-parallel downstream. Based on this result we develop a simplified model to calculate the frequency-dependent degree of polarization, assuming that high-energy particles are injected at the shock and cool downstream. We apply our results to HBLs, blazars with the maximum of their synchrotron output at UV-soft X-ray energies. While in the optical band the predicted degree of polarization is low, in the X-ray emission it can ideally reach 50%, especially during active/flaring states. The comparison between measurements in the optical and in the X-ray band made during active states (feasible with the planned IXPE satellite) are expected to provide valuable constraints on the dissipation and acceleration processes.

Probing Magnetic Fields and Acceleration Mechanisms in Blazar Jets with X-ray Polarimetry

“…(1) For acceleration by shocks that are characterized by a magnetic field with a strong selfproduced component, one anticipates a quite large (around 40% for the parameters assumed in [33,54]) and stable degree of polarization of the X-ray emission. Moreover, since the emission occurs in a region that is characterized by a well defined orientation of the magnetic field, the angle of polarization is not expected to display large changes during the evolution of a flare.…”

“…Being informed by these results, References [33,54] presented a model for the polarization of the synchrotron radiation produced by high-energy particles accelerated at a shock-being particularly suitable for describing the expected properties of the X-ray emission. The adopted set-up is simple: the jet is modeled as a cylinder, and it is assumed that the observer lies at an angle θ v = 1/Γ d , where Γ d is the bulk Lorentz factor of the downstream plasma.…”

“…Figure 4. The spatial profiles of the ratio between the averaged perpendicular component of the magnetic field projected onto the plane of the sky and the parallel magnetic field component as a function of the distance from the shock front for the model that si assumed in [33,54]. The profiles correspond to m = 3 (red) and m = 5 (blue).…”

“…The second panel clearly shows the generation of a strong magnetic field at the shock parallel to the front (third panel). From[54]. Reproduced with kind permission from Oxford University Press and the Royal Astronomical Society.…”

Relativistic Jets from Active Galactic Nuclei

“…In particular the knots in sources like M87 and the hot spots in FR-II lobes are plausibly shocks, while reconnection should be endemic in boundary layers. Shocks and reconnection should be distinguishable through X-ray polarimetry (Tavecchio et al 2018) 4.1.4. Stochastic Acceleration.…”