Manuel Perucho

On the interaction of microquasar jets with stellar winds

Context. Strong interactions between jets and stellar winds at binary-system, spatial-scales could occur in high-mass microquasars. Aims. We study here, mainly from a dynamical but also a radiative point of view, the collision between a dense stellar wind and a mildly relativistic hydrodynamical jet of supersonic nature. Methods. We have performed numerical two-dimensional simulations of jets, with cylindrical and planar (slab) symmetry, crossing the stellar-wind material. From the results of the simulations, we derive estimates of the particle acceleration efficiency, using firstorder, Fermi-acceleration theory, and provide insight into the possible radiative outcomes. Results. We find that, during jet launching, the jet head generates a strong shock in the wind. During and after this process, strong recollimation shocks can occur due to the initial overpressure of the jet with its environment. The conditions in all these shocks are convenient to accelerate particles up to ∼TeV energies, which can lead to leptonic (synchrotron and inverse Compton) and hadronic (proton-proton) radiation. In principle, the cylindrical jet simulations show that the jet is stable, and can escape from the system even at relatively low power. However, when accounting for the wind ram pressure, the jet can be bent and disrupted for power < 10 36 erg s −1 .

Nonlinear stability of relativistic sheared planar jets

The linear and non-linear stability of sheared, relativistic planar jets is studied by means of linear stability analysis and numerical hydrodynamical simulations. Our results extend the previous Kelvin-Hemlholtz stability studies for relativistic, planar jets in the vortex sheet approximation performed by Perucho et al. (2004a,b) by including a shear layer between the jet and the external medium and more general perturbations. The models considered span a wide range of Lorentz factors (2.5 20) and internal energies (0.08c 2 60c 2 ) and are classified into three classes according to the main characteristics of their long-term, non-linear evolution. We observe a clear separation of these three groups in a relativistic Mach-number Lorentz-factor plane. Jets with a low Lorentz factor and small relativistic Mach number are disrupted after saturation. Those with a large Lorentz factor and large relativistic Mach number are the stablest, due to the appearance of short wavelength resonant modes which generate local mixing and heating in the shear layer around a fast, unmixed core, giving a plausible solution for the problem of the long-term stability of relativistic jets. A third group is present between them, including jets with intermediate values of Lorentz factor and relativistic Mach number, which are disrupted by a slow process of mixing favored by an efficient and continuous conversion ofkinetic into internal energy. In the long term, all the models develop a distinct transversal structure (shear/transition layers) as a consequence of KH perturbation growth, depending on the class they belong to. The properties of these shear layers are analyzed in connection with the parameters of the original jet models.

Stability of hydrodynamical relativistic planar jets. I. Linear evolution and saturation of Kelvin-Helmholtz modes

The effects of relativistic dynamics and thermodynamics in the development of Kelvin-Helmholtz instabilities in planar, relativistic jets along the early phases (namely linear and saturation phases) of evolution has been studied by a combi- nation of linear stability analysis and high-resolution numerical simulations for the most unstable first reflection modes in the temporal approach. Three different values of the jet Lorentz factor (5, 10 and 20) and a few different values of specific internal energy of the jet matter (from 0.08 to 60.0c 2 ) have been considered. Figures illustrating the evolution of the perturbations are also shown. Our simulations reproduce the linear regime of evolution of the excited eigenmodes of the different models with a high accuracy. In all the cases the longitudinal velocity perturbation is the first quantity that departs from the linear growth when it reaches a value close to the speed of light in the jet reference frame. The saturation phase extends from the end of the linear phase up to the saturation of the transversal velocity perturbation (at approximately 0.5c in the jet reference frame). The saturation times for the different numerical models are explained from elementary considerations, i.e. from properties of linear modes provided by the linear stability analysis and from the limitation of the transversal perturbation velocity. The limitation of the components of the velocity perturbation at the end of the linear and saturation phases allows us to conclude that the relativistic nature of the flow appears to be responsible for the departure of the system from linear evolution. The high accuracy of our simulations in describing the early stages of evolution of the KH instability (as derived from the agreement between the computed and expected linear growth rates and the consistency of the saturation times) establishes a solid basis to study the fully nonlinear regime, to be done elsewhere. The present paper also sets the theoretical and numerical background for these further studies.

Stability of three-dimensional relativistic jets: implications for jet collimation

Context. The stable propagation of jets in FRII sources is remarkable if one takes into account that large-scale jets are subjected to potentially highly disruptive three-dimensional (3D) Kelvin-Helmholtz instabilities. Aims. Numerical simulations can address this problem and help clarify the causes of this remarkable stability. Following previous studies of the stability of relativistic flows in two dimensions (2D), it is our aim to test and extend the conclusions of such works to three dimensions. Methods. We present numerical simulations for the study of the stability properties of 3D, sheared, relativistic flows. This work uses a fully parallelized code (Ratpenat) that solves equations of relativistic hydrodynamics in 3D. Results. The results of the present simulations confirm those in 2D. We conclude that the growth of resonant modes in sheared relativistic flows could be important in explaining the long-term collimation of extragalactic jets.

Physical Parameters in the Hot Spots and Jets of Compact Symmetric Objects

We present a model to determine the physical parameters of jets and hot spots of a sample of compact symmetric objects (CSOs) under very basic assumptions like synchrotron emission and minimum energy conditions. Based on this model, we propose a simple evolutionary scenario for these sources assuming that they evolve in ram pressure equilibrium with the external medium and constant jet power. The parameters of our model are constrained from fits of observational data (radio luminosity, hot spot radius, and hot spot advance speed) versus projected linear size. From these plots we conclude that CSOs evolve self-similarly and that their radio luminosity increases with linear size along the first kiloparsec. Assuming that the jets feeding CSOs are relativistic from both kinematical and thermodynamical points of view, we use the values of the pressure and particle number density within the hot spots to estimate the fluxes of momentum (thrust), energy, and particles of these relativistic jets. The mean jet power obtained in this way is within an order of magnitude of that inferred for Fanaroff-Riley type 2 sources, which is consistent with CSOs being the possible precursors of large doubles. The inferred flux of particles corresponds to, for a barionic jet, about 10% of the mass accreted by a black hole of 108 M☉ at the Eddington limit, pointing toward a very efficient conversion of accretion flow into ejection or to a leptonic composition of jets. We have considered three different models (namely, models I, IIa, and IIb). Model I, assuming constant hot spot advance speed and increasing luminosity, can be ruled out on the grounds of its energy cost. However, models IIa and IIb seem to describe limiting behaviors of sources evolving at constant advance speed and decreasing luminosity (model IIa) and decreasing hot spot advance speed and increasing luminosity (model IIb). In all our models the slopes of the hot spot luminosity and advance speed with source linear size are governed by only one parameter, namely, the external density gradient. A short discussion on the validity of models IIa and IIb to describe the complete evolution of powerful radio sources from their CSO phase is also included.

Stability of hydrodynamical relativistic planar jets. II. Long-term nonlinear evolution

In this paper we continue our study of the Kelvin-Helmholtz (KH) instability in relativistic planar jets following the long-term evolution of the numerical simulations which were introduced in Paper I. The models have been classified into four classes (I to IV) with regard to their evolution in the nonlinear phase, characterized by the process of jet/ambient mixing and momentum transfer. Models undergoing qualitatively different non-linear evolution are clearly grouped in well-separated regions in a jet Lorentz factor/jet-to-ambient enthalpy diagram. Jets with a low Lorentz factor and small enthalpy ratio are disrupted by a strong shock after saturation. Those with a large Lorentz factor and enthalpy ratio are unstable although the process of mixing and momentum exchange proceeds to a longer time scale due to a steady conversion of kinetic to internal energy in the jet. In these cases, the high value of the initial Lorentz seems to prevent transversal velocity from growing far enough to generate the strong shock that breaks the slower jets. Finally, jets with either high Lorentz factors and small enthalpy ratios or low Lorentz factors and large enthalpy ratios appear as the most stable. In the long term, all the models develop a distinct transversal structure (shear/transition layers) as a consequence of KH pertur- bation growth. The properties of these shear layers are analyzed in connection with the parameters of the original jet models.

Resonant Kelvin-Helmholtz modes in sheared relativistic flows

Certain aspects of the (linear and nonlinear) stability of sheared relativistic (slab) jets are analyzed. The linear problem has been solved for a wide range of jet models well inside the ultrarelativistic domain (flow Lorentz factors up to 20, specific internal energies approximately 60c2). As a distinct feature of our work, we have combined the analytical linear approach with high-resolution relativistic hydrodynamical simulations, which has allowed us (i) to identify, in the linear regime, resonant modes specific to the relativistic shear layer, (ii) to confirm the result of the linear analysis with numerical simulations, and (iii) more interestingly, to follow the instability development through the nonlinear regime. We find that very-high-order reflection modes with dominant growth rates can modify the global, long-term stability of the relativistic flow. We discuss the dependence of these resonant modes on the jet flow Lorentz factor and specific internal energy and on the shear-layer thickness. The results could have potential applications in the field of extragalactic relativistic jets.

Physical properties of the jet in 0836+710 revealed by its transversal structure

Studying the internal structure of extragalactic jets is crucial for understanding their physics. The Japanese-led space VLBI project VSOP has presented an opportunity for such studies, by reaching baseline lengths of up to 36 000 km and resolving structures down to an angular size of ≈0.3 mas at 5 GHz. VSOP observations of the jet in 0836+710 at 1.6 and 5 GHz have enabled tracing of the radial structure of the flow on scales from 2 mas to 200 mas along the jet and determination of the wavelengths of individual oscillatory modes responsible for the formation of the structure observed. We apply linear stability analysis to identify the oscillatory modes with modes of Kelvin-Helmholtz instability that match the wavelengths of the structures observed. We find that the jet structure in 0836+710 can be reproduced by the helical surface mode and a combination of the helical and elliptic body modes of KelvinHelmholtz instability. Our results indicate that the jet is substantially stratified and different modes of the instability grow inside the jet at different distances to the jet axis. The helical surface mode can be driven externally, and we discuss the implications of the driving frequency on the physics of the active nucleus in 0836+710.

On the nature of an ejection event in the jet of 3C 111

We present a possible scenario for the ejection of a superluminal component in the jet of the Broad Line Radio Galaxy 3C 111 in early 1996. VLBI observations at 15 GHz discovered the presence of two jet features on scales smaller than one parsec. The first component evolves downstream, whereas the second one fades out after 1 parsec. We propose the injection of a perturbation of dense material followed by a decrease in the injection rate of material in the jet as a plausible explanation. This scenario is supported by 1D relativistic hydrodynamic and emission simulations. The perturbation is modeled as an increase in the jet density, without modifying the original Lorentz factor in the initial conditions. We show that an increase of the Lorentz factor in the material of the perturbation fails to reproduce the observed evolution of this flare. We are able to estimate the lifetime of the ejection event in 3C 111 to be 36 ± 7 days.

A Dynamical Model for the Evolution of Hot Spots in Powerful Radio Sources

Compact symmetric objects are considered the young counterparts of large doubles according to advance speeds measured or inferred from spectral ageing. Here we present a simple power law model for the CSO/FR II evolution based on the study of sources with well defined hot spots. The luminosity of the hot spots is estimated under minimum energy conditions. The advance of the source is considered to proceed in ram pressure equilibrium with the ambient medium. Finally, we also assume that the jets feeding the hot spots are relativistic and have a time dependent power. Comparison with observational data points to an interpretation of the CSO–FR II evolution in terms of decreasing jet power with time.