Abstract
Relativistic shocks are one of the most plausible sites of the emission of strongly variable, polarized multi-wavelength emission from relativistic jet sources such as blazars, via the diffusive shock acceleration (DSA) of relativistic particles. This paper summarizes recent results on a self-consistent coupling of diffusive shock acceleration and radiation transfer in blazar jets. We demonstrate that the observed spectral energy distributions (SEDs) of blazars strongly constrain the nature of hydromagnetic turbulence responsible for pitch-angle scattering by requiring a strongly energy-dependent pitch-angle mean free path. The prominent soft X-ray excess (“Big Blue Bump”) in the SED of the BL Lac object AO 0235+164 can be modelled as the signature of bulk Compton scattering of external radiation fields by the thermal electron population, which places additional constraints on the level of hydromagnetic turbulence. It has further been demonstrated that internal shocks propagating in a jet pervaded by a helical magnetic field naturally produce polarization-angle swings by 180 ∘ , in tandem with multi-wavelength flaring activity, without requiring any helical motion paths or other asymmetric jet structures. The specific application of this model to 3C279 presents the first consistent simultaneous modeling of snap-shot SEDs, multi-wavelength light curves, and time-dependent polarization signatures of a blazar during a polarization-angle (PA) rotation. This model has recently been generalized to a lepto-hadronic model, in which the high-energy emission is dominated by proton synchrotron radiation. It is shown that in this case, the high-energy (X-ray and γ-ray) polarization signatures are expected to be significantly more stable (not showing PA rotations) than the low-energy (electron-synchrotron) signatures.
Highlights
Ever since the pioneering work of Marscher & Gear [1], relativistic shocks have been considered one of the leading contenders for the location of relativistic particle acceleration, resulting in the observed rapidly variable, often highly polarized multi-wavelength emission from blazars
This model has recently been generalized to a lepto-hadronic model, in which the high-energy emission is dominated by proton synchrotron radiation
Models focusing on the multi-zone radiative transfer problem(given efficient relativistic shock acceleration) have reached an increasing level of sophistication (e.g., [2,3,4,5,6,7,8,9,10]), resulting in successful fits to multi-wavelength spectral energy distributions (SEDs) and light curves of individual flares in blazars
Summary
Ever since the pioneering work of Marscher & Gear [1], relativistic shocks have been considered one of the leading contenders for the location of relativistic particle acceleration, resulting in the observed rapidly variable, often highly polarized multi-wavelength emission from blazars. With very few exceptions (e.g., [16,17]), theoretical models for synchrotron polarization variations are purely geometrical, often with an ad hoc assumption of helical pattern motions guided by helical magnetic fields, without consistent considerations of radiation physics or particle dynamics (e.g., [18]) In this context, it should be pointed out that most works modeling the SEDs of blazars in detail (e.g., [13,19]) indicate that in the high-energy emission region, blazar jets are particle dominated (i.e., with sub-equipartition magnetic fields), in which case the relativistic plasma is not expected to be guided by the magnetic fields, but instead the magnetic fields will follow the plasma motion. There is a significant number of events in which PA rotations do occur in clear correlation with multi-wavelength flares (e.g., [14,15]) For this latter case, Zhang et al [17,21] have developed a model self-consistently tracing particle dynamics, radiation transfer, and resulting polarization characteristics in a shock-in-jet model with an ordered, helical magnetic field.
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