arXiv subjects

R. V. E. Lovelace

Publications and source records attributed to R. V. E. Lovelace.

At least 37 records · Page 2Linked to original sources

arXiv2013

Transmission Line Analogy for Relativistic Poynting-Flux Jets

Radio emission, polarization, and Faraday rotation maps of the radio jet of the galaxy 3C 303 have shown that one knot of this jet carries a {\it galactic}-scale electric current and that it is magnetically dominated. We develop the theory of magnetically dominated or Poynting-flux jets by making an analogy of a Poynting jet with a transmission line or waveguide carrying a net current and having a potential drop across it (from the jet's axis to its radius) and a definite impedance which we derive. Time-dependent but not necessarily small perturbations of a Poynting-flux jet are described by the "telegrapher's equations." These predict the propagation speed of disturbances and the effective wave impedance for forward and backward propagating wave components. A localized disturbance of a Poynting jet gives rise to localized dissipation in the jet which may explain the enhanced synchrotron radiation in the knots of the 3C 303 jet, and also in the apparently stationary knot HST-1 in the jet near the nucleus of the nearby galaxy M87. For a relativistic Poynting jet on parsec scales, the reflected voltage wave from an inductive termination or load can lead to a backward propagating wave which breaks down the magnetic insulation of the jet giving $|{\bf E}| /|{\bf B}|\geq 1$. At the threshold for breakdown, $|{\bf E}|/|{\bf B}|=1$, positive and negative particles are directly accelerated in the ${\bf E \times B}$ direction which is approximately along the jet axis. Acceleration can occur up to Lorentz factors $\sim 10^7$. This particle acceleration mechanism is distinct from that in shock waves and that in magnetic field reconnection.

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arXiv2013

How strong are the Rossby vortices?

The Rossby wave instability, associated with density bumps in differentially rotating discs, may arise in several different astrophysical contexts, such as galactic or protoplanetary discs. While the linear phase of the instability has been well studied, the nonlinear evolution and especially the saturation phase remain poorly understood. In this paper, we test the non-linear saturation mechanism analogous to that derived for wave-particle interaction in plasma physics. To this end we perform global numerical simulations of the evolution of the instability in a two-dimensional disc. We confirm the physical mechanism for the instability saturation and show that the maximum amplitude of vorticity can be estimated as twice the linear growth rate of the instability. We provide an empirical fitting formula for this growth rate for various parameters of the density bump. We also investigate the effects of the azimuthal mode number of the instability and the energy leakage in the spiral density waves. Finally, we show that our results can be extrapolated to 3D discs.

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arXiv2012

Rossby Wave Instability with Self-Gravity

The Rossby wave instability (RWI) in non-self-gravitating discs can be triggered by a bump at a radius $r_0$ in the disc surface mass-density (which is proportional to the inverse potential vorticity). It gives rise to a growing non-axisymmetric perturbation [$\propto \exp(imϕ)$, $m=1,2..$] in the vicinity of $r_0$ consisting of anticyclonic vortices which may facilitate planetesimal growth in protoplanetary discs. Here, we analyze a continuum of thin disc models ranging from self-gravitating to non-selfgravitating. The key quantities determining the stability/instability are: (1) the parameters of the bump (or depression) in the disc surface density, (2) the Toomre $Q$ parameter of the disc (a non-self-gravitating disc has $Q\gg1$), and (3) the dimensionless azimuthal wavenumber of the perturbation $\bar{k}_ϕ=mQh/r_0$, where $h$ is the half-thickness of the disc. For discs stable to axisymmetric perturbations ($Q>1$), the self-gravity has a significant role for $\bar{k}_ϕ< π/2$ or $m<(π/2) (r_0/h)Q^{-1}$; instability may occur for a depression or groove in the surface density if $Q\lesssim 2$. For $\bar{k}_ϕ> π/2$ the self-gravity is not important, and instability may occur at a bump in the surface density. Thus, for all mode numbers $m \ge 1$, the self-gravity is unimportant for $Q > (π/2)(r_0/h)$. We suggest that the self-gravity be included in simulations for cases where $Q< (r_0/h)$.

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arXiv2012

Accretion into black holes with magnetic fields, and relativistic jets

We discuss the problem of the formation of a large-scale magnetic field in the accretion disks around black holes, taking into account the non-uniform vertical structure of the disk. The high electrical conductivity of the outer layers of the disk prevents the outward diffusion of the magnetic field. This implies a stationary state with a strong magnetic field in the inner parts of the accretion disk close to the black hole, and zero radial velocity at the surface of the disk. Structure of advective accretion disks is investigated, and conditions for formation of optically thin regions in central parts of the accretion disk are found. The problem of jet collimation by magneto-torsion oscillations is considered.

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arXiv2012

Relativistic Poynting-Flux Jets as Transmission Lines

Recent radio emission, polarization, and Faraday rotation maps of the radio jet of the galaxy 3C 303 have shown that one knot of this jet has a {\it galactic}-scale electric current of $\sim 3\times 10^{18}$ Ampère flowing along the jet axis (Kronberg et al. 2011). We develop the theory of relativistic Poynting-flux jets which are modeled as a transmission line carrying a DC current $I_0$, having a potential drop $V_0$, and a definite impedance ${\cal Z}_0 =90(u_z/c)Ω$, where $u_z$ is the bulk velocity of the jet plasma. The electromagnetic energy flow in the jet is ${\cal Z}_0 I_0^2$. The observed current in 3C 303 can be used to calculate the electromagnetic energy flow in this magnetically dominated jet. Time-dependent but not necessarily small perturbations of a Poynting-flux jet - possibly triggered by a gas cloud penetrating the jet - are described by "telegrapher's equations," which predict the propagation speed of disturbances and the effective wave impedance ${\cal Z}$. The disturbance of a Poynting jet by the cloud gives rise to localized dissipation in the jet which may explain the enhanced synchrotron radiation in the knots of the 3C 303 jet, and in the apparently stationary knot HST-1 in the jet from the nucleus of the galaxy M87 (Biretta et al. 1999).

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arXiv2011

MRI-driven Accretion on to Magnetized stars: Global 3D MHD Simulations of Magnetospheric and Boundary Layer Regimes

We discuss results of global 3D MHD simulations of accretion on to a rotating magnetized star with a tilted dipole magnetic field, where the accretion is driven by the magneto-rotational instability (MRI). The simulations show that MRI-driven turbulence develops in the disc, and angular momentum is transported outwards due primarily to the magnetic stress. The turbulent flow is strongly inhomogeneous and the densest matter is in azimuthally-stretched turbulent cells. We investigate two regimes of accretion: a magnetospheric regime and a boundary layer (BL) regime. In the magnetospheric regime, the accretion disc is truncated by the star's magnetic field within a few stellar radii from the star, and matter flows to the star in funnel streams. The funnel streams flowing towards the south and north magnetic poles but are not equal due to the inhomogeneity of the flow. In the BL regime, matter accretes to the surface of the star through the boundary layer. The magnetic field in the inner disc is strongly amplified by the shear of the accretion flow, and the matter and magnetic stresses become comparable. Accreting matter forms a belt-shaped region on the surface of the star. The belt has inhomogeneous density distribution which varies in time due to variable accretion rate. Results of simulations can be applied to classical T Tauri stars, accreting brown dwarfs, millisecond pulsars, dwarf novae cataclysmic variables, and other stars with magnetospheres smaller than several stellar radii.

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arXiv2011

Bondi-Hoyle Accretion onto Magnetized Neutron Star

Axisymmetric MHD simulations are used to investigate the Bondi-Hoyle accretion onto an isolated magnetized neutron star moving supersonically (with Mach number of 3) through the interstellar medium. The star is assumed to have a dipole magnetic field aligned with its motion and a magnetospheric radius R_m less then the accretion radius R_BH, so that the gravitational focusing is important. We find that the accretion rate to a magnetized star is smaller than that to a non-magnetized star for the parameters considered. Close to the star the accreting matter falls to the star's surface along the magnetic poles with a larger mass flow to the leeward pole of the star. In the case of a relatively large stellar magnetic field, the star's magnetic field is stretched in the direction of the matter flow outside of R_m (towards the windward side of the star). For weaker magnetic fields we observed oscillations of the closed magnetosphere frontward and backward. These are accompanied by strong oscillations of the mass accretion rate which varies by factors ~ 3. Old slowly rotating neutron stars with no radio emission may be visible in the X-ray band due to accretion of interstellar matter. In general, the star's velocity, magnetic moment, and angular velocity vectors may all be in different directions so that the accretion luminosity will be modulated at the star's rotation rate.

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arXiv2011

Vertical Structure of Stationary Accretion Disks with a Large-Scale Magnetic Field

In earlier works we pointed out that the disk's surface layers are non-turbulent and thus highly conducting (or non-diffusive) because the hydrodynamic and/or magnetorotational (MRI) instabilities are suppressed high in the disk where the magnetic and radiation pressures are larger than the plasma thermal pressure. Here, we calculate the vertical profiles of the {\it stationary} accretion flows (with radial and azimuthal components), and the profiles of the large-scale, magnetic field taking into account the turbulent viscosity and diffusivity and the fact that the turbulence vanishes at the surface of the disk. Also, here we require that the radial accretion speed be zero at the disk's surface and we assume that the ratio of the turbulent viscosity to the turbulent magnetic diffusivity is of order unity. Thus at the disk's surface there are three boundary conditions. As a result, for a fixed dimensionless viscosity $α$-value, we find that there is a definite relation between the ratio ${\cal R}$ of the accretion power going into magnetic disk winds to the viscous power dissipation and the midplane plasma-$β$, which is the ratio of the plasma to magnetic pressure in the disk. For a specific disk model with ${\cal R}$ of order unity we find that the critical value required for a stationary solution is $β_c \approx 2.4r/(αh)$, where $h$ the disk's half thickness. For weaker magnetic fields, $β> β_c$, we argue that the poloidal field will advect outward while for $β< β_c$ it will advect inward. Alternatively, if the disk wind is negligible (${\cal R} \ll 1$), there are stationary solutions with $β\gg β_c$.

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arXiv2011

Accretion disks with a large scale magnetic field around black holes, and magnetic jet collimation

We discuss the problem of the formation of a large-scale magnetic field in the accretion disks around black holes, taking into account the nonuniform vertical structure of the disk. The high electrical conductivity of the outer layers of the disk prevents the outward diffusion of the magnetic field. This implies a stationary state with a strong magnetic field in the inner parts of the accretion disk close to the black hole, and zero radial velocity at the surface of the disk. Magnetic jet collimation is considered, when the jet radius is hold due to magneto-torsional oscillations. The range of parameters is found where jet radius is oscillating, regularly or chaotically, within restricted values.

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arXiv2010

Possible Signatures of Magnetospheric Accretion onto Young Giant Planets

Magnetospheric accretion is an important process for a wide range of astrophysical systems, and may play a role in the formation of gas giant planets. Extending the formalism describing stellar magnetospheric accretion into the planetary regime, we demonstrate that magnetospheric processes may govern accretion onto young gas giants in the isolation phase of their development. Planets in the isolation phase have cleared out large gaps in their surrounding circumstellar disks, and settled into a quasi-static equilibrium with radii only modestly larger than their final sizes (i.e., $ r \sim 1.4 r_{\rm final}$). Magnetospheric accretion is less likely to play a role in a young gas giant's main accretion phase, when the planet's envelope is predicted to be much larger than the planet's Alfvén radius. For a fiducial 1 M$_J$ gas giant planet with a remnant isolation phase accretion rate of $\dot{M}_{\odot} =$ 10$^{-10} M_{\odot}{\rm yr}^{-1}=10^{-7}M_{J}{\rm yr}^{-1}$, the disk accretion will be truncated at $\sim 2.7r_J$ (with $r_J$ is Jupiter's radius) and drive the planet to rotate with a period of $\sim$7 hours. Thermal emission from planetary magnetospheric accretion will be difficult to observe; the most promising observational signatures may be non-thermal, such as gyrosynchrotron radiation that is clearly modulated at a period much shorter than the rotation period of the host star.

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arXiv2010

One-sided Outflows/Jets from Rotating Stars with Complex Magnetic Fields

We investigate the generation of intrinsically asymmetric or {\it one-sided} outflows or jets from disk accretion onto rotating stars with complex magnetic fields using axisymmetric (2.5D) magnetohydrodynamic simulations. The intrinsic magnetic field of the star is assumed to consist of a superposition of an aligned dipole and an aligned quadrupole in different proportions. The star is assumed to be rapidly rotating in the sense that the star's magnetosphere is in the propeller regime where strong outflows occur. Our simulations show that for conditions where there is a significant quadrupole component in addition to the dipole component, then a dominantly {\it one-sided} conical wind tends to form on the side of the equatorial plane with the larger value of the intrinsic axial magnetic field at a given distance. For cases where the quadrupole component is absent or very small, we find that dominantly one-sided outflows also form, but the direction of the flow "flip-flops" between upward and downward on a time-scale of $\sim 30$ days for a protostar. The average outflow will thus be symmetrical. In the case of a pure quadrupole field we find symmetric outflows in the upward and downward directions.

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arXiv2009

Ring Formation from an Oscillating Black Hole

Massive black hole (BH) mergers can result in the merger remnant receiving a "kick", of order 200 km s$^{-1}$ or more, which will cause the remnant to oscillate about the galaxy centre. Here we analyze the case where the BH oscillates through the galaxy centre perpendicular or parallel to the plane of the galaxy for a model galaxy consisting of an exponential disk, a Plummer model bulge, and an isothermal dark matter halo. For the perpendicular motion we find that there is a strong resonant forcing of the disk radial motion near but somewhat less than the "resonant radii" $r_R$ where the BH oscillation frequency is equal one-half, one-fourth, (1/6, etc.) of the radial epicyclic frequency in the plane of the disk. Near the resonant radii there can be a strong enhancement of the radial flow and disk density which can lead to shock formation. In turn the shock may trigger the formation of a ring of stars near $r_R$. As an example, for a BH mass of $10^8 M_\odot$ and a kick velocity of 150 km s$^{-1}$, we find that the resonant radii lie between 0.2 and 1 kpc. For BH motion parallel to the plane of the galaxy we find that the BH leaves behind it a supersonic wake where star formation may be triggered. The shape of the wake is calculated as well as the slow-down time of the BH. The differential rotation of the disk stretches the wake into ring-like segments.

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arXiv2009

Kelvin-Helmholtz Instability of the Magnetopause of Disc-Accreting Stars

This work investigates the short wavelength stability of the magnetopause between a rapidly-rotating, supersonic, dense accretion disc and a slowly-rotating low-density magnetosphere of a magnetized star. The magnetopause is a strong shear layer with rapid changes in the azimuthal velocity, the density, and the magnetic field over a short radial distance and thus the Kelvin-Helmholtz (KH) instability may be important. The plasma dynamics is treated using non-relativistic, compressible (isentropic) magnetohydrodynamics. It is necessary to include the displacement current in order that plasma wave velocities remain less than the speed of light. We focus mainly on the case of a star with an aligned dipole magnetic field so that the magnetic field is axial in the disc midplane and perpendicular to the disc flow velocity. However, we also give results for cases where the magnetic field is at an arbitrary angle to the flow velocity. For the aligned dipole case the magnetopause is most unstable for KH waves propagating in the azimuthal direction perpendicular to the magnetic field which tends to stabilize waves propagating parallel to it. The wave phase velocity is that of the disc matter. A quasi-linear theory of the saturation of the instability leads to a wavenumber ($k$) power spectrum $\propto k^{-1}$ of the density and temperature fluctuations of the magnetopause, and it gives the mass accretion and angular momentum inflow rates across the magnetopause. For self-consistent conditions this mass accretion rate will be equal to the disc accretion rate at large distances from the magnetopause.

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arXiv2009

Launching of Conical Winds and Axial Jets from the Disk-Magnetosphere Boundary: Axisymmetric and 3D Simulations

We investigate the launching of outflows from the disk-magnetosphere boundary of slowly and rapidly rotating magnetized stars using axisymmetric and exploratory 3D magnetohydrodynamic (MHD) simulations. We find long-lasting outflows in both cases. (1) In the case of slowly rotating stars, a new type of outflow, a conical wind, is found and studied in simulations. The conical winds appear in cases where the magnetic flux of the star is bunched up by the disk into an X-type configuration. The winds have the shape of a thin conical shell with a half-opening angle 30-40 degrees. The conical winds may be responsible for episodic as well as long-lasting outflows in different types of stars. (2) In the case of rapidly rotating stars (the "propeller regime"), a two-component outflow is observed. One component is similar to the conical winds. A significant fraction of the disk matter may be ejected into the winds. A second component is a high-velocity, low-density magnetically dominated axial jet where matter flows along the opened polar field lines of the star. The jet has a mass flux about 10% that of the conical wind, but its energy flux (dominantly magnetic) can be larger than the energy flux of the conical wind. The jet's angular momentum flux (also dominantly magnetic) causes the star to spin-down rapidly. Propeller-driven outflows may be responsible for the jets in protostars and for their rapid spin-down. The jet is collimated by the magnetic force while the conical winds are only weakly collimated in the simulation region.

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arXiv2009

Advection/Diffusion of Large-Scale B-Field in Accretion Disks

Activity of the nuclei of galaxies and stellar mass systems involving disk accretion to black holes is thought to be due to (1) a small-scale turbulent magnetic field in the disk (due to the magneto-rotational instability or MRI) which gives a large viscosity enhancing accretion, and (2) a large-scale magnetic field which gives rise to matter outflows and/or electromagnetic jets from the disk which also enhances accretion. An important problem with this picture is that the enhanced viscosity is accompanied by an enhanced magnetic diffusivity which acts to prevent the build up of a significant large-scale field. Recent work has pointed out that the disk's surface layers are non-turbulent and thus highly conducting (or non-diffusive) because the MRI is suppressed high in the disk where the magnetic and radiation pressures are larger than the thermal pressure. Here, we calculate the vertical ($z$) profiles of the stationary accretion flows (with radial and azimuthal components), and the profiles of the large-scale, magnetic field taking into account the turbulent viscosity and diffusivity due to the MRI and the fact that the turbulence vanishes at the surface of the disk. The stationary solutions we find indicate that a weak ($β> 1$) large-scale field does not diffuse away as suggested by earlier work.

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arXiv2009

Stability of the Magnetopause of Disk-Accreting Rotating Stars

We discuss three modes of oscillation of accretion disks around rotating magnetized neutron stars which may explain the separations of the kilo-Hertz quasi periodic oscillations (QPO) seen in low mass X-ray binaries. The existence of these compressible, non-barotropic magnetohydrodynamic (MHD) modes requires that there be a maximum in the angular velocity $Ω_ϕ(r)$ of the accreting material larger than the angular velocity of the star $Ω_*$, and that the fluid is in approximately circular motion near this maximum rather than moving rapidly towards the star or out of the disk plane into funnel flows. Our MHD simulations show this type of flow and $Ω_ϕ(r)$ profile. The first mode is a Rossby wave instability (RWI) mode which is radially trapped in the vicinity of the maximum of a key function $g(r){\cal F}(r)$ at $r_{R}$. The real part of the angular frequency of the mode is $ω_r=mΩ_ϕ(r_{R})$, where $m=1,2...$ is the azimuthal mode number. The second mode, is a mode driven by the rotating, non-axisymmetric component of the star's magnetic field. It has an angular frequency equal to the star's angular rotation rate $Ω_*$. This mode is strongly excited near the radius of the Lindblad resonance which is slightly outside of $r_R$. The third mode arises naturally from the interaction of flow perturbation with the rotating non-axisymmetric component of the star's magnetic field. It has an angular frequency $Ω_*/2$. We suggest that the first mode with $m=1$ is associated with the upper QPO frequency, $ν_u$; that the nonlinear interaction of the first and second modes gives the lower QPO frequency, $ν_\ell =ν_u-ν_*$; and that the nonlinear interaction of the first and third modes gives the lower QPO frequency $ν_\ell=ν_u-ν_*/2$, where $ν_*=Ω_*/2π$.

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arXiv2009

Launching of Poynting Jets from Accretion Disks

The jets observed to emanate from many compact accreting objects may arise from the twisting of the magnetic field threading a differentially rotating accretion disk which acts to magnetically extract angular momentum and energy from the disk. Two main regimes have been discussed, hydromagnetic outflows, which have a significant mass flux and have energy and angular momentum carried by both matter and electromagnetic field and, Poynting outflows, where the mass flux is negligible and energy and angular momentum are carried predominantly by the electromagnetic field. We describe recent theoretical work on the formation of relativistic Poynting jets from magnetized accretion disks and new relativistic, fully-electromagnetic, particle-in-cell simulations of the formation of jets from accretion disks.

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arXiv2009

Conical Winds from the Disk-Magnetosphere Boundary

A new type of wind - a conical wind - has been discovered in axisymmetric magnetohydrodynamic simulations of the disk-magnetosphere interaction in cases where the magnetic field of the star is bunched into an X-type configuration. Such a configuration arises if the effective viscosity of the disk is larger than the effective diffusivity, or if the accretion rate in the disk is enhanced. Conical outflows flow from the inner edge of the disk into a narrow shell with half-opening angle of 30-45 degrees. The outflow carries about 0.1-0.3 of the disk mass accretion rate and part of the disk's angular momentum. The conical winds are driven by the gradient of the magnetic pressure which exists above the disk due to the winding of the stellar magnetic field. Exploratory 3D simulations show that conical winds are symmetric about rotation axis of the disk even if the magnetic dipole is significantly misaligned with the disk's rotation axis. Conical winds appear around stars of different periods. However, in the case of a star in the "propeller" regime, an additional - much faster component appears: an axial jet, where matter is accelerated up to very high velocities at small distances from the star by magnetic pressure force above the surface of the star. The simulations are done in dimensionless units and are applicable to a variety of the disk-accreting magnetized stars: young stars, white dwarfs, neutron stars, and possibly black holes. For the case of young stars, conical winds and axial jets may appear in different cases, including Class I young stars, classical T Tauri stars, and EXors. In EXors periods of enhanced accretion may lead to the formation of conical winds which correspond to the outflows observed from these stars.

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