THE VELOCITY OF THE EMITTING PLASMA OF THE SUPERLUMINAL GALACTIC SOURCE GRS 1915+105

We calculate the flux and size ratio of radio emitting features moving in antiparallel directions at relativistic speeds, taking into account that the pattern we observe may move at a velocity different from the one of the emitting plasma. Both velocities can be determined in sources in which both the approaching and the receding features are observed. This is the case of the galactic superluminal source GRS 1915+105, for which the pattern speed (responsible for the apparent superluminal velocity) was found to be 0.92$c$. For this source we find that the velocity of the plasma is 0.73$c$. The found velocity helps decreasing the estimate of the associated kinetic power. Since intrinsically identical knots will be observed to have different sizes, high resolution radio observations of GRS 1915+105 will be a test for the proposed difference of the pattern and plasma velocities.Comment: 8 pages, tex. Accepted for ApJ Letter

Linear stability analysis of magnetized relativistic rotating jets

We carry out a linear stability analysis of a magnetized relativistic rotating cylindrical jet flow using the approximation of zero thermal pressure. We identify several modes of instability in the jet: Kelvin-Helmholtz, current driven and two kinds of centrifugal-buoyancy modes -- toroidal and poloidal. The Kelvin-Helmholtz mode is found at low magnetization and its growth rate depends very weakly on the pitch parameter of the background magnetic field and on rotation. The current driven mode is found at high magnetization, the values of its growth rate and the wavenumber, corresponding to the maximum growth, increase as we decrease the pitch parameter of the background magnetic field. This mode is stabilized by rotation, especially, at high magnetization. The centrifugal-buoyancy modes, arising due to rotation, tend also to be more stable when magnetization is increased. Overall, relativistic jet flows appear to be more stable with respect to their non-relativistic counterpart.Comment: 15 pages, 15 figures, accepted for pubblication in MNRA

MHD simulations of three-dimensional Resistive Reconnection in a cylindrical plasma column

Magnetic reconnection is a plasma phenomenon where a topological rearrangement of magnetic field lines with opposite polarity results in dissipation of magnetic energy into heat, kinetic energy and particle acceleration. Such a phenomenon is considered as an efficient mechanism for energy release in laboratory and astrophysical plasmas. An important question is how to make the process fast enough to account for observed explosive energy releases. The classical model for steady state magnetic reconnection predicts reconnection times scaling as $S^{1/2}$ (where $S$ is the Lundquist number) and yields times scales several order of magnitude larger than the observed ones. Earlier two-dimensional MHD simulations showed that for large Lundquist number the reconnection time becomes independent of $S$ ("fast reconnection" regime) due to the presence of the secondary tearing instability that takes place for $S \gtrsim 1 \times 10^4$. We report on our 3D MHD simulations of magnetic reconnection in a magnetically confined cylindrical plasma column under either a pressure balanced or a force-free equilibrium and compare the results with 2D simulations of a circular current sheet. We find that the 3D instabilities acting on these configurations result in a fragmentation of the initial current sheet in small filaments, leading to enhanced dissipation rate that becomes independent of the Lundquist number already at $S \simeq 1\times 10^3$.Comment: 11 pages, 11 figures, accepted for publication in MNRA

A Particle Module for the PLUTO code: II - Hybrid Framework for Modeling Non-thermal emission from Relativistic Magnetized flows

We describe a new hybrid framework to model non-thermal spectral signatures from highly energetic particles embedded in a large-scale classical or relativistic MHD flow. Our method makes use of \textit{Lagrangian} particles moving through an Eulerian grid where the (relativistic) MHD equations are solved concurrently. Lagrangian particles follow fluid streamlines and represent ensembles of (real) relativistic particles with a finite energy distribution. The spectral distribution of each particle is updated in time by solving the relativistic cosmic ray transport equation based on local fluid conditions. This enables us to account for a number of physical processes, such as adiabatic expansion, synchrotron and inverse Compton emission. An accurate semi-analytically numerical scheme that combines the method of characteristics with a Lagrangian discretization in the energy coordinate is described. In presence of (relativistic) magnetized shocks, a novel approach to consistently model particle energization due to diffusive shock acceleration has been presented. Our approach relies on a refined shock-detection algorithm and updates the particle energy distribution based on the shock compression ratio, magnetic field orientation and amount of (parameterized) turbulence. The evolved distribution from each \textit{Lagrangian} particle is further used to produce observational signatures like emission maps and polarization signals accounting for proper relativistic corrections. We further demonstrate the validity of this hybrid framework using standard numerical benchmarks and evaluate the applicability of such a tool to study high energy emission from extra-galactic jets.Comment: 23 pages, 14 figures, Accepted for publication in The Astrophysical Journa

Magnetocentrifugal mechanism of pair creation in AGN

In the manuscript, we study the efficiency of pair creation by means of the centrifugal mechanism. The strong magnetic field and the effects of rotation, which always take place in Kerr-type black holes, guarantee the frozen-in condition, leading to the generation of an exponentially amplifying electrostatic field. This field, when reaching the Schwinger threshold, leads to efficient pair production. The process has been studied for a wide range of AGN luminosities and black hole masses, and it was found that the mechanism is very efficient, indicating that for AGNs where centrifugal effects are significant, the annihilation lines in the MeV range will be very strong.Comment: 15 pages, 5 figure

The Piecewise Parabolic Method for Multidimensional Relativistic Fluid Dynamics

We present an extension of the Piecewise Parabolic Method to special relativistic fluid dynamics in multidimensions. The scheme is conservative, dimensionally unsplit, and suitable for a general equation of state. Temporal evolution is second-order accurate and employs characteristic projection operators; spatial interpolation is piece-wise parabolic making the scheme third-order accurate in smooth regions of the flow away from discontinuities. The algorithm is written for a general system of orthogonal curvilinear coordinates and can be used for computations in non-cartesian geometries. A non-linear iterative Riemann solver based on the two-shock approximation is used in flux calculation. In this approximation, an initial discontinuity decays into a set of discontinuous waves only implying that, in particular, rarefaction waves are treated as flow discontinuities. We also present a new and simple equation of state which approximates the exact result for the relativistic perfect gas with high accuracy. The strength of the new method is demonstrated in a series of numerical tests and more complex simulations in one, two and three dimensions

Simulating the dynamics and non-thermal emission of relativistic magnetised jets I. Dynamics

We have performed magneto-hydrodynamic simulations of relativistic jets from supermassive blackholes over a few tens of kpc for a range of jet parameters. One of the primary aims were to investigate the effect of different MHD instabilities on the jet dynamics and their dependence on the choice of jet parameters. We find that two dominant MHD instabilities affect the dynamics of the jet, small scale Kelvin- Helmholtz (KH) modes and large scale kink modes, whose evolution depend on internal jet parameters like the Lorentz factor, the ratio of the density and pressure to the external medium and the magnetisation and hence consequently on the jet power. Low power jets are susceptible to both instabilities, kink modes for jets with higher central magnetic field and KH modes for lower magnetisation. Moderate power jets do not show appreciable growth of kink modes, but KH modes develop for lower magnetisation. Higher power jets are generally stable to both instabilities. Such instabilities decelerate and decollimate the jet while inducing turbulence in the cocoon, with consequences on the magnetic field structure. We model the dynamics of the jets following a generalised treatment of the Begelman-Cioffi relations which we present here. We find that the dynamics of stable jets match well with simplified analytic models of expansion of non self-similar FRII jets, whereas jets with prominent MHD instabilities show a nearly self-similar evolution of the morphology as the energy is more evenly distributed between the jet head and the cocoon.Comment: Accepted for publication in MNRA

Linear Wave Propagation for Resistive Relativistic Magnetohydrodynamics

We present a linear mode analysis of the relativistic MHD equations in the presence of finite electrical conductivity. Starting from the fully relativistic covariant formulation, we derive the dispersion relation in the limit of small linear perturbations. It is found that the system supports ten wave modes which can be easily identified in the limits of small or large conductivities. In the resistive limit, matter and electromagnetic fields decouple and solution modes approach pairs of light and acoustic waves as well as a number of purely damped (non-propagating) modes. In the opposite (ideal) limit, the frozen-in condition applies and the modes of propagation coincide with a pair of fast magnetosonic, a pair of slow and Alfv\'en modes, as expected. In addition, the contact mode is always present and it is unaffected by the conductivity. For finite values of the conductivity, the dispersion relation gives rise to either pairs of opposite complex conjugate roots or purely imaginary (damped) modes. In all cases, the system is dissipative and also dispersive as the phase velocity depends nonlineary on the wavenumber. Occasionally, the group velocity may exceed the speed of light although this does not lead to superluminal signal propagation

Linear analysis of the Kelvin-Helmholtz instability in relativistic magnetized symmetric flows

We study the linear stability of a planar interface separating two fluids in relative motion, focusing on the symmetric configuration where the two fluids have the same properties (density, temperature, magnetic field strength, and direction). We consider the most general case with arbitrary sound speed $c_{\rm s}$, Alfv\'en speed $v_{\rm A}$, and magnetic field orientation. For the instability associated with the fast mode, we find that the lower bound of unstable shear velocities is set by the requirement that the projection of the velocity onto the fluid-frame wavevector is larger than the projection of the Alfv\'en speed onto the same direction, i.e., shear should overcome the effect of magnetic tension. In the frame where the two fluids move in opposite directions with equal speed $v$, the upper bound of unstable velocities corresponds to an effective relativistic Mach number $M_{re} \equiv v/v_{\rm f\perp} \sqrt{(1-v_{\rm f\perp}^2)/(1-v^2)} \cos\theta=\sqrt{2}$, where $v_{rm f\perp}=[v_A^2+c_{\rm s}^2(1-v_A^2)]^{1/2}$ is the fast speed assuming a magnetic field perpendicular to the wavevector (here, all velocities are in units of the speed of light), and $\theta$ is the laboratory-frame angle between the flow velocity and the wavevector projection onto the shear interface. Our results have implications for shear flows in the magnetospheres of neutron stars and black holes -- both for single objects and for merging binaries -- where the Alfv\'en speed may approach the speed of light.Comment: 11 pages, 7 figures, 1 table, Accepted for publication in Monthly Notices of the Royal Astronomical Societ