Temporal variability of polarization in blazars

TL;DR

This work addresses how blazar polarization can vary in time, including large EVPA rotations and irregular patterns, by proposing a deterministic geometric model with off-axis blobs propagating along axisymmetric, magnetically dominated jets. The jet fields are computed self-consistently within a Lyubarsky-based framework, and blob dynamics follow the local flow via E×B/B^2, with polarization computed from synchrotron emission of a power-law electron population. The study demonstrates that complex polarization signatures, including rotations exceeding 180 degrees and abrupt EVPA jumps, can arise without turbulence, and that the observed patterns depend sensitively on jet shape and the initial blob configurations, sometimes correlating with luminosity peaks but not universally. These results have implications for interpreting multifrequency polarimetry, such as IXPE observations, and highlight the challenge of constraining particle acceleration mechanisms from polarization data alone, since geometric effects can mimic both deterministic and seemingly stochastic variability.

Abstract

We investigate the temporal variability of polarization of synchrotron radiation from blazar jets. Multiwavelength observations revealed high-amplitude rotations of the electric vector position angle (EVPA), both in the optical and in the X-rays. More often, the polarization degree and the EVPA show a seemingly erratic variability. To interpret these observations, we present a geometric and deterministic model in which off-axis, compact emitting features (i.e.,~blobs) propagate along the jet with the local velocity of the flow. The dynamics of the blobs is determined by the jet electromagnetic fields, which are calculated self-consistently using an analytical model of magnetically dominated outflows. The jet is axisymmetric, and its electromagnetic fields do not have a turbulent component. We show that the observed polarization is sensitive to the initial spatial configurations of the blobs. For the same jet structure, we observe several remarkably complex polarization patterns, including large EVPA rotations of $180^{\circ}$ or more in both directions and more erratic fluctuations. Simultaneous high-amplitude variations of the polarization degree and the EVPA can coincide with peaks of the observed luminosity. However, seemingly uncorrelated variations are also possible. We discuss the feasibility of constraining the particle acceleration mechanism from multifrequency polarimetric observations.

Figures (6)

• Figure 1: Cartoon of our jet model. The equilibrium boundary radius of the jet, $R$, is proportional to $z^q$, where $z$ is the distance from the supermassive black hole. The value of the parameter $q<1$ depends on the pressure profile of the external medium that collimates the jet. We consider a cylindrical jet ($q=0$), a "sausage-like" jet ($q=0.3$), and a nearly parabolic jet ($q=0.4$). In the "sausage-like" jet, the boundary oscillates about the equilibrium value. The emission is produced by an ensemble of compact off-axis emission features (yellow blobs) that propagate along the jet with the local velocity of the flow. The line of sight makes an angle $\theta_{\rm obs}$ with respect to the direction of the jet axis.
• Figure 2: Jet transverse radius normalized to its minimum value, $R / R_{ \mathrm{min}}$ (left panel), bulk Lorentz factor in the proper frame normalized to its minimum value, $\Gamma / \Gamma_{\mathrm{min}}$ (middle panel), and poloidal magnetic field magnitude normalized to its minimum value, $B_{\rm p}/ B_{\rm{p,\, min}}$ (right panel), as functions of the distance from the black hole expressed in units of $c/\Omega$, for different jet shapes.
• Figure 3: Polarization degree $\Pi$ (left panels), electric vector position angle (EVPA) $\Psi$ (middle panels), and intensity normalized to its maximum value $I/I_{\rm max}$ (right panels) as a function of time in the observed frame, $t_{\rm obs}$, measured in days. The shape of the jet is cylindrical. Top panels: emission from a single blob ($N_{\rm blobs}=1$ and $\eta_{\rm jet}=0$; dashed line) and from a single blob plus a nearly axisymmetric jet ($N_{\rm blobs}=1$ and $\eta_{\rm jet}=1$; solid line). Bottom panels: emission from multiple blobs ($N_{\rm blobs}=10$ and $\eta_{\rm jet}=0$; dashed line) and from multiple blobs plus a nearly axisymmetric jet ($N_{\rm blobs}=10$ and $\eta_{\rm jet}=1$; solid line).
• Figure 4: Same as Fig. \ref{['fig:cilinder']}, but the shape of the jet is "sausage-like" with $C_2=0.8$.
• Figure 5: Same as Fig. \ref{['fig:cilinder']}, but the shape of the jet is "sausage-like" with $C_2=2$.