Multiwavelength polarization as a diagnostic of blazar physics

Abstract

The nature of the high-energy emission in jet-dominated AGN (especially blazars) as well as the location and mode of acceleration of particles to ultra- relativistic energies are a matter of intensive debate. Both leptonic and hadronic origins of the high-energy emission are still possible, and spectral energy distribution (SED) modeling alone is not sufficient to distinguish between leptonic and hadronic models. We will discuss high-energy (X-ray and γ-ray) polarization as a possible diagnostic to distinguish leptonic from hadronic models, showing that substantial Xray and gamma-ray polarization would be a tell-tale signature of hadronic high-energy emission. Motivated by these results, the potential impact of γ-ray polarization on the evaluation of γγ absorption of high-energy γ-rays from relativistic jets will be discussed. Finally, we turn to the occasionally observed feature of large (≥ 180◦ ) swings of the optical polarization angle correlated with multi-wavelength flaring activity in several blazars. We suggest an interpretation not requiring any helical pattern motions or bent jets, simply based on a helical magnetic field pervading a straight jet. We present simultaneous fits to the SEDs, multi-wavelength light curves and time-dependent optical polarization features (degree and angle) in one coherent model for the prominent polarization-angle swing + γ-ray flaring event observed in 3C279. Such an interpretation requires a mode of conversion of magnetic energy to particle energy (e.g., magnetic reconnection) as the driving mechanism of such flaring activity