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Register a new userTotal and linearly polarized synchrotron emission from overpressured magnetized relativistic jets
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We present relativistic magnetohydrodynamic (RMHD) simulations of stationary overpressured magnetized relativistic jets which are characterized by their dominant type of energy, namely internal, kinetic, or magnetic. Each model is threaded by a helical magnetic field with a pitch angle of $45^circ$ and features a series of recollimation shocks produced by the initial pressure mismatch, whose strength and number varies as a function of the dominant type of energy. We perform a study of the polarization signatures from these models by integrating the radiative transfer equations for synchrotron radiation using as inputs the RMHD solutions. These simulations show a top-down emission asymmetry produced by the helical magnetic field and a progressive confinement of the emission into a jet spine as the magnetization increases and the internal energy of the non-thermal population is considered to be a constant fraction of the thermal one. Bright stationary components associated with the recollimation shocks appear presenting a relative intensity modulated by the Doppler boosting ratio between the pre-shock and post-shock states. Small viewing angles show a roughly bimodal distribution in the polarization angle due to the helical structure of the magnetic field, which is also responsible for the highly stratified degree of linear polarization across the jet width. In addition, small variations of the order of $26^circ$ are observed in the polarization angle of the stationary components, which can be used to identify recollimation shocks in astrophysical jets.
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The class of Double-Double Radio Galaxies (DDRGs) relates to episodic jet outbursts. How various regions and components add to the total intensity in radio images is less well known. In this paper we synthesize synchrotron images for DDRGs based on special relativistic hydrodynamic simulations, making advanced approximations for the magnetic fields. We study the synchrotron images for: Three different radial jet profiles; Ordered, entangled or mixed magnetic fields; Spectral ageing from synchrotron cooling; The contribution from different jet components; The viewing angle and Doppler (de-)boosting; The various epochs of the evolution of the DDRG. To link our results to observational data, we adopt to J1835+6204 as a reference source. In all cases the synthesized synchrotron images show two clear pairs of hotspots, in the inner and outer lobes. The best resemblance is obtained for the piecewise isochoric jet model, for a viewing angle of approximately $vartheta sim -71^{circ}$, i.e. inclined with the lower jet towards the observer, with predominantly entangled ($gtrsim 70$ per cent of the magnetic pressure) in turbulent, rather than ordered fields. The effects of spectral ageing become significant when the ratio of observation frequencies and cut-off frequency $ u_{rm obs}/ u_{infty,0} gtrsim 10^{-3}$, corresponding to $sim 3 cdot 10^2$ MHz. For viewing angles $vartheta lesssim -30^{circ}$, a DDRG morphology can no longer be recognized. The second jets must be injected within $lesssim$ 4 per cent of the lifetime of the first jets for a DDRG structure to emerge, which is relevant for Active Galactic Nuclei feedback constraints.
Relativistic magnetized shocks are a natural source of coherent emission, offering a plausible radiative mechanism for Fast Radio Bursts (FRBs). We present first-principles 3D simulations that provide essential information for the FRB models based on shocks: the emission efficiency, spectrum, and polarization. The simulated shock propagates in an $e^pm$ plasma with magnetization $sigma>1$. The measured fraction of shock energy converted to coherent radiation is $simeq 10^{-3} , sigma^{-1}$, and the energy-carrying wavenumber of the wave spectrum is $simeq 4 ,omega_{rm c}/c$, where $omega_{rm c}$ is the upstream gyrofrequency. The ratio of the O-mode and X-mode energy fluxes emitted by the shock is $simeq 0.4,sigma^{-1}$. The dominance of the X-mode at $sigmagg 1$ is particularly strong, approaching 100% in the spectral band around $2,omega_{rm c}$. We also provide a detailed description of the emission mechanism for both X- and O-modes.
We have performed two-dimensional special-relativistic magnetohydrodynamic simulations of non-equilibrium over-pressured relativistic jets in cylindrical geometry. Multiple stationary recollimation shock and rarefaction structures are produced along the jet by the nonlinear interaction of shocks and rarefaction waves excited at the interface between the jet and the surrounding ambient medium. Although initially the jet is kinematically dominated, we have considered axial, toroidal and helical magnetic fields to investigate the effects of different magnetic-field topologies and strengths on the recollimation structures. We find that an axial field introduces a larger effective gas-pressure and leads to stronger recollimation shocks and rarefactions, resulting in larger flow variations. The jet boost grows quadratically with the initial magnetic field. On the other hand, a toroidal field leads to weaker recollimation shocks and rarefactions, modifying significantly the jet structure after the first recollimation rarefaction and shock. The jet boost decreases systematically. For a helical field, instead, the behaviour depends on the magnetic pitch, with a phenomenology that ranges between the one seen for axial and toroidal magnetic fields, respectively. In general, however, a helical magnetic field yields a more complex shock and rarefaction substructure close to the inlet that significantly modifies the jet structure. The differences in shock structure resulting from different field configurations and strengths may have observable consequences for disturbances propagating through a stationary recollimation shock.
We study the conditions required for the production of the synchrotron maser emission downstream of a relativistic shock. We show that for weakly magnetized shocks, synchrotron maser emission can be generated at frequencies significantly exceeding the relativistic gyrofrequency. This high-frequency maser emission seems to be the most suitable for interpreting peculiar GHz radio sources. To illustrate this, we consider a magnetar flare model for FRBs. Our analysis shows that the maser emission is radiated away from the central magnetar, which guarantees a short duration of bursts independently of the shock wave radius. If FRBs are produced by the high-frequency maser emission then one can significantly relax the requirements for several key parameters: the magnetic field strength at the production site, luminosity of the flare, and the production site bulk Lorentz factor. To check the feasibility of this model, we study the statistical relation between powerful magnetar flares and the rate of FRBs. The expected ratio is derived by convoluting the redshift-dependent magnetar density with their flare luminosity function above the energy limit determined by the FRB detection threshold. We obtain that only a small fraction, (sim10^{-5}), of powerful magnetar flares trigger FRBs. This ratio agrees surprisingly well with our estimates: we obtained that (10%) of magnetars should be in the evolutionary phase suitable for the production of FRBs, and only (10^{-4}) of all flares are expected to be weakly magnetized, which is a necessary condition for the high-frequency maser emission.
Relativistic jets launched by rotating black holes are powerful emitters of non-thermal radiation. Extraction of the rotational energy via electromagnetic stresses produces magnetically-dominated jets, which may become turbulent. Studies of magnetically-dominated plasma turbulence from first principles show that most of the accelerated particles have small pitch angles, i.e. the particle velocity is nearly aligned with the local magnetic field. We examine synchrotron-self-Compton radiation from anisotropic particles in the fast cooling regime. The small pitch angles reduce the synchrotron cooling rate and promote the role of inverse Compton (IC) cooling, which can occur in two different regimes. In the Thomson regime, both synchrotron and IC components have soft spectra, $ u F_ upropto u^{1/2}$. In the Klein-Nishina regime, synchrotron radiation has a hard spectrum, typically $ u F_ upropto u$, over a broad range of frequencies. Our results have implications for the modelling of BL Lacs and Gamma-Ray Bursts (GRBs). BL Lacs produce soft synchrotron and IC spectra, as expected when Klein-Nishina effects are minor. The observed synchrotron and IC luminosities are typically comparable, which indicates a moderate anisotropy with pitch angles $thetagtrsim0.1$. Rare orphan gamma-ray flares may be produced when $thetall0.1$. The hard spectra of GRBs may be consistent with synchrotron radiation when the emitting particles are IC cooling in the Klein-Nishina regime, as expected for pitch angles $thetasim0.1$. Blazar and GRB spectra can be explained by turbulent jets with a similar electron plasma magnetisation parameter, $sigma_{rm e}sim10^4$, which for electron-proton plasmas corresponds to an overall magnetisation $sigma=(m_{rm e}/m_{rm p})sigma_{rm e}sim10$.
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