Relativistic Plasma Outflows Research Articles

Relativistic shocks in conductive media

Context. Relativistic shocks are present in all high-energy astrophysical processes involving relativistic plasma outflows interacting with their ambient medium. While they are well understood in the context of relativistic hydrodynamics and ideal magnetohydrodynamics (MHD), there is a limited understanding of the properties related to their propagation in media characterized by finite electrical conductivity. Aims. This work presents a systematic method for the derivation and solution of the jump conditions for relativistic shocks propagating in MHD media with finite electrical conductivity. This method is applied to the numerical solution of the Riemann problem and the determination of the conditions inside the blastwave that is formed when ultrarelativistic magnetized ejecta interact with the circumburst medium during a gamma-ray burst. Methods. We derived the covariant relations expressing the jump conditions in a frame-independent manner. The resulting algebraic equations expressing the Rankine-Hugoniot conditions in the propagation medium’s frame were then solved numerically. A variable adiabatic index equation of state was used in order to obtain a realistic description of the post-shock fluid’s thermodynamics. This method was then employed for the solution of the Riemann problem for the case of a forward and a reverse shock, both of which form during the interaction of a gamma-ray burst ejecta with the circumburst medium. This allowed us to determine the kinematics of the resulting blastwave and the dynamical conditions in its interior. Results. Our solutions clearly depict the impact of the plasma’s electrical conductivity in the properties of the post-shock medium. Two characteristic regimes are identified with respect to the value of a dimensionless parameter that has a linear dependence on the conductivity. For small values of this parameter, the shock affects only the hydrodynamic properties of the propagation medium and leaves its electromagnetic field unaffected. No current layer forms in the shock front; thus, we refer to this as the current-free regime. For large values of this parameter, the ideal MHD regime has been retrieved. We also show that the assumption of a finite electrical conductivity can lead to higher efficiencies in the conversion of the ejecta energy into thermal energy of the blastwave through the reverse shock. The theory developed in this work can be applied to the construction of Riemann solvers for resistive relativistic MHD (RR4MHD).

Evaluating MHD parameters of relativistic shock waves with particle-in-cell modeling

Relativistic plasma outflows are observed in gamma-ray burst sources, jets of active galactic nuclei, pulsar wind nebulae and supernovae explosions. Magnetohydrodynamical (MHD) shock waves inevitably result from interactions of such relativistic outflows with the ambient interstellar matter. The widely used single-fluid MHD description of relativistic shock waves is the main tool to study the global structure of such objects. However, to justify the validity of the global MHD models and to interpret the observed emission spectra of space objects with relativistic shocks, a kinetic description of electrons, positrons, and ions at microscales is needed. We model a plane relativistic shock propagating transverse to a regular magnetic field in the electron-ion plasmas with imposed turbulent fluctuations in the shock upstream. Namely we study the effect of the micro-scale plasma processes on macroscopic parameters of the mildly-relativistic shocks as the adiabatic index of the relativistic fluid in the shock downstream. The adiabatic index is a macroscopic parameter of the single-fluid MHD models commonly used for shock modeling at much longer hydrodynamical scales and it is especially important for the MHD modeling of the mildly-relativistic shocks.

Multiwavelength and parsec‐scale properties of extragalactic jets

AbstractExtragalactic jets originating from the central supermassive black holes of active galaxies are powerful, highly relativistic plasma outflows, emitting light from the radio up to the y‐ray regime. The details of their formation, composition and emission mechanisms are still not completely clear. The combination of high‐resolution observations using very long baseline interferometry (VLBI) and multiwavelength monitoring provides the best insight into these objects. Here, such a combined study of sources of the TANAMI sample is presented, investigating the parsec‐scale and high‐energy properties. The TANAMI program is a multiwavelength monitoring program of a sample of the radio and y‐ray brightest extragalactic jets in the southern sky, below –30° declination. We obtain the first‐ever VLBI images for most of the sources, providing crucial information on the jet kinematics and brightness distribution at milliarcsecond resolution. Two particular sources are discussed in detail: PMN J1603–4904, which can be classified either as an atypical blazar or a y‐ray loud (young) radio galaxy, and Centaurus A, the nearest radio‐loud active galaxy. The VLBI kinematics of the innermost parsec of Centaurus A's jet result in a consistent picture of an accelerated jet flow with a spine‐sheath like structure. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Physical properties of the gamma-ray binary LS 5039 through low- and high-frequency radio observations

We have studied in detail the 0.15-15 GHz radio spectrum of the gamma-ray binary LS 5039 to look for a possible turnover and absorption mechanisms at low frequencies, and to constrain the physical properties of its emission. We have analysed two archival VLA monitorings, all the available archival GMRT data and a coordinated quasi-simultaneous observational campaign conducted in 2013 with GMRT and WSRT. The data show that the radio emission of LS 5039 is persistent on day, week and year timescales, with a variability $\lesssim 25~\%$ at all frequencies, and no signature of orbital modulation. The obtained spectra reveal a power-law shape with a curvature below 5 GHz and a turnover at $\sim0.5$ GHz, which can be reproduced by a one-zone model with synchrotron self-absorption plus Razin effect. We obtain a coherent picture for a size of the emitting region of $\sim0.85~\mathrm{mas}$, setting a magnetic field of $B\sim20~\mathrm{mG}$, an electron density of $n_{\rm e}\sim4\times10^5~{\rm cm^{-3}}$ and a mass-loss rate of $\dot M\sim5\times10^{-8}~{\rm M_{\odot} yr^{-1}}$. These values imply a significant mixing of the stellar wind with the relativistic plasma outflow from the compact companion. At particular epochs the Razin effect is negligible, implying changes in the injection and the electron density or magnetic field. The Razin effect is reported for first time in a gamma-ray binary, giving further support to the young non-accreting pulsar scenario.

Radiative transfer in relativistic plasma outflows and comptonization of photons near the photosphere

We study radiative transfer in plasma by numerical solution of kinetic Boltzmann equations for all particles. We are interested in the thermalization of photons. We considered three cases: 1. The calculations of the timescales of the thermalization in the uniform isotropic plasma. 2. The expansion of the mildly relativistic pair plasma for the mini fireball in the frame of the kinetic approach. 3. The case of ultra relativistically expanding outflow from the surface of the compact object with the Fokker-Planck approximation to the Boltzmann equation for photons. The last case gives the generalized Kompaneets equation which takes into account anisotropic distribution of photons developed near the photosphere. For the electron temperature dependence from radius T ∝ r−2 and thermal electrons spectrum we found the low-energy photon index can be ≲0.5 as typically observed in GRB.

Radio emission of the cosmic gamma-ray burst GRB 080319B: Observations of the afterglow using telescopes of the Institute of Applied Astronomy of the Russian Academy of Sciences

Results of radio observations of the cosmic gamma-ray burst GRB 080319B at 8.45 GHz during the afterglow are reported. The observations were carried out on telescopes of the Zelenchukskaya and Svetloe Observatories of the Institute of Applied Astronomy, Russian Academy of Sciences. Two outbursts in the radio brightness were detected in the afterglow of GRB 080319B. A total of 148 radio observations were performed at 3.5, 6.2, and 13 cm. The observations were conducted in a mode with smooth scanning in elevation, which was also used to update the flux densities of the primary reference sources. The first powerful radio outburst was recorded on March 28, 2008, 6.86d after the gamma-ray burst, when the maximum flux density was F8.45 GHz = 44 ± 12 mJy. Almost two months later, a second increase in the radio brightness was observed. The flux density monotonically increased from 19 mJy (59.55d) to 34mJy (59.79d) over 6.5 h; 1.17 d later, the flux density fell to 12mJy.At this last epoch, the radio flux demonstrated variability within 3σ on timescales of 9d−10d. The detected radio brightness increases are interpreted in terms of MHD interactions of a fast plasma outflow with a cloud of inhomogeneous surrounding medium. This interaction is accompanied by restructuring of the relativistic plasma outflow; the analysis of this process has been carried out.

Statistics of Cosmological Black Hole Jet Sources: Blazar Predictions for theGamma-Ray Large Area Space Telescope

A study of the statistics of cosmological black hole jet sources is applied to EGRET blazar data and predictions are made for GLAST. Black hole jet sources are modeled as collimated relativistic plasma outflows with radiation beamed along the jet axis due to strong Doppler boosting. The comoving rate density of blazar flares is assumed to follow a blazar formation rate (BFR), modeled by analytic functions based on astronomical observations and fits to EGRET data. The redshift and size distributions of γ-ray blazars observed with EGRET, separated into BL Lac objects (BLs) and flat spectrum radio quasar (FSRQ) distributions, are fit with monoparametric functions for the distributions of the jet Lorentz factor Γ, comoving directional power l, and spectral slope. A BFR factor ≈10× greater at z ≳ 1 than at present is found to fit the FSRQ data. A smaller comoving rate density and greater luminosity of BL flares at early times compared to the present epoch fits the BL data. Based on the EGRET observations, ≈1000 blazars consisting of ≈800 FSRQs and FR 2 radio galaxies and ≈200 BL Lac objects and FR 1 radio galaxies will be detected with GLAST during the first year of the mission. Additional AGN classes, such as hard-spectrum BL Lac objects that were mostly missed with EGRET, could add more GLAST sources. The FSRQ and BL contributions to the EGRET γ-ray background at 1 GeV are estimated at the level of ≈10%-15% and ≈2%-4%, respectively. EGRET and GLAST sensitivities to blazar flares are considered in the optimal case, and a GLAST analysis method for blazar detection is outlined.

The example of effective plasma acceleration in a magnetosphere

The problem of effective transform of Poynting flux energy into the kinetic energy of relativistic plasma outflow in a magnetosphere is considered. In this article we present an example of such acceleration. In order to perform it, we use the approach of ideal axisymmetric magnetohydrodynamics (MHD). For highly magnetized plasma outflow we show that a linear growth of Lorentz factor with a cylindrical distance from the rotational axis is a general result for any field configuration in the sub-magnetosonic flow. In the far region the full magnetohydrodynamics problem for one-dimensional flow is considered. It turns out that the effective plasma outflow acceleration is possible in the paraboloidal magnetic field. It is shown that such an acceleration is due to the drift of charged particles in the crossed electric and magnetic field. The clear explanation of the absence of acceleration in the monopole magnetic field if given.

Shock formation at the magnetic collimation of relativistic jets

If the observed relativistic plasma outflows in astrophysical jets are magnetically collimated and a single-component model is adopted, consisting of a wind-type outflow from a central object, then a problem arises with the inefficiency of magnetic self-collimation to collimate a sizeable portion of the mass and magnetic fluxes in the relativistic outflow from the central object. To solve this dilemma, we have applied the mechanism of magnetic collimation to a two-component model consisting of a relativistic wind-type outflow from a central source and a non-relativistic wind from a surrounding disc. By employing a numerical code for a direct numerical solution of the steady-state problem in the zone of super-fast magnetized flow, which allows us to perform a determination of the flow with shocks, it is shown that in this two-component model it is possible to collimate into cylindrical jets all the mass and magnetic fluxes that are available from the central source. In addition, it is shown that the collimation of the plasma in this system is usually accompanied by the formation of oblique shock fronts. The non-relativistic disc-wind not only plays the role of the jet collimator, but it also induces the formation of shocks as it collides with the initially radial inner relativistic wind and also as the outflow is reflected by the system axis. Another interesting feature of this process of magnetic collimation is a sequence of damped oscillations in the width of the jet.

On Spectral and Temporal Variability in Blazars and Gamma-Ray Bursts

A simple model for variability in relativistic plasma outflows is studied, in which nonthermal electrons are continuously and uniformly injected in the comoving frame over a time interval dt. The evolution of the electron distribution is assumed to be dominated by synchrotron losses, and the energy- and time-dependence of the synchrotron and synchrotron self-Compton (SSC) fluxes are calculated for a power-law electron injection function with index s = 2. The mean time of a flare or pulse measured at photon energy E with respect to the onset of the injection event varies as E^{-1/2} and E^{-1/4} for synchrotron and SSC processes, respectively, until the time approaches the limiting intrinsic mean time (1+z)dt/(2 D), where z is the redshift and D is the Doppler factor. This dependence is in accord with recent analyses of blazar and GRB emissions, and suggests a method to discriminate between external Compton and SSC models of high-energy gamma radiation from blazars and GRBs. The qualititative behavior of the X-ray spectral index/flux relation observed from BL Lac objects can be explained with this model. This demonstrates that synchrotron losses are primarily responsible for the X-ray variability behavior and strengthens a new test for beaming from correlated hard X-ray/TeV observations.

Deterministic Chaos in the Microstructure of Radio Pulses of PSR 0809+74

Abstract480 single pulses from PSR 0809+74, recorded at 102.5 MHz with a time resolution of 10μs, have been analyzed by the time delay technique in order to look for the parameters of deterministic chaos in the microstructure of radio pulses. The correlation dimension n was shown to be less than 5 in more than 20% of the analyzed pulses. This means that on such occasions the microstructure of pulsar radio emission with the time scales 10 to 100μs may be determined by the behavior of a nonlinear dynamical system with comparatively small numbers of independent parameters. For example, the observed low dimensional chaos may result from a turbulence process associated with the outflow of plasma—as in versions of the polar cap model where microstructure can be interpreted as reflecting the spatial structure of relativistic plasma outflow in the radio emission region.However, the correlation-time distribution demonstrates the tendency for microstructure to consist of a random sequence of unresolved micropulses in the majority of cases, which means in the framework of polar cap models the presence of well developed turbulence in the relativistic plasma outflow.

Propagation of Large Amplitude Pulsar Waves

Large amplitude waves (hereafter l.a.w.) have been studied mainly in connection with pulsars. A rotating neutron star, with an intense non-aligned magnetic dipole field, surrounded by a vacuum, radiates beyond the light cylinder distance, large amplitude electromagnetic vacuum waves of very low frequency (Ostriker and Gunn, 1969). In such a rotating configuration electromagnetic effects completely dominate and imply the existence of a relativistic plasma in the pulsar magnetosphere (Goldreich and Julian, 1969). Because the oblique vacuum model predicts a value of 3 for the braking index of the pulsar whereas the observed or computed values are different, a realistic model has to include both the relativistic plasma outflow and the electromagnetic wave emission. The vacuum wave model has been changed to include self-consistent plasma effects (Asséo et al., 1975; Asséo et al., 1978) in plane geometry and inhomogeneities linked to spherical geometry (Asséo et al., 1981). This results in very restrictive conditions for the possibility of propagation of the l.a.w.