arXiv subjects

R. V. E. Lovelace

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

At least 73 records · Page 4Linked to original sources

arXiv2005

Gamma-ray Flares and VLBI Outbursts of Blazars

A model is developed for the time dependent electromagnetic - radio to gamma-ray - emission of active galactic nuclei, specifically, the blazars, based on the acceleration and creation of leptons at a propagating discontinuity or {\it front} of a Poynting flux jet. The front corresponds to a discrete relativistic jet component as observed with very-long-baseline-interferometry (VLBI). Equations are derived for the number, momentum, and energy of particles in the front taking into account synchrotron, synchrotron-self-Compton (SSC), and inverse-Compton processes as well as photon-photon pair production. The apparent synchrotron, SSC, and inverse-Compton luminosities as functions of time are determined. Predictions of the model are compared with observations in the gamma, optical and radio bands. The delay between the high-energy gamma-ray flare and the onset of the radio is explained by self-absorption and/or free-free absorption by external plasma. Two types of gamma-ray flares are predicted depending on pair creation in the front.

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arXiv2005

Winds, B-Fields, and Magnetotails of Pulsars

We investigate the emission of rotating magnetized neutron stars due to the acceleration and radiation of particles in the relativistic wind and in the magnetotail of the star. We consider that the charged particles are accelerated by driven collisionless reconnection. Outside of the light cylinder, the star's rotation acts to wind up the magnetic field to form a predominantly azimuthal, slowly decreasing with distance, magnetic field of opposite polarity on either side of the equatorial plane normal to the star's rotation axis. The magnetic field annihilates across the equatorial plane with the magnetic energy going to accelerate the charged particles to relativistic energies. For a typical supersonically moving pulsar, the star's wind extends outward to the standoff distance with the interstellar medium. At larger distances, the power output of pulsar's wind $\dot{E}_w$ of electromagnetic field and relativistic particles is {\it redirected and collimated into the magnetotail} of the star. In the magnetotail it is proposed that equipartition is reached between the magnetic energy and the relativistic particle energy. For such conditions, synchrotron radiation from the magnetotails may be a significant fraction of $\dot{E}_w$ for high velocity pulsars. An equation is derived for the radius of the magnetotail $r_m(z^\prime)$ as a function of distance $z^\prime$ from the star. For large distances $z^\prime$, of the order of the distance travelled by the star, we argue that the magnetotail has a `trumpet' shape owing to the slowing down of the magnetotail flow.

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arXiv2005

Confinement of supernova explosions in a collapsing cloud

We analyze the confining effect of cloud collapse on an expanding supernova shockfront. We solve the differential equation for the forces on the shockfront due to ram pressure, supernova energy, and gravity. We find that the expansion of the shockfront is slowed and in fact reversed by the collapsing cloud. Including radiative losses and a potential time lag between supernova explosion and cloud collapse shows that the expansion is reversed at smaller distances as compared to the non-radiative case. We also consider the case of multiple supernova explosions at the center of a collapsing cloud. For instance, if we scale our self-similar solution to a single supernova of energy 10^51 ergs occurring when a cloud of initial density 10^2 H/cm^3 has collapsed by 50%, we find that the shockfront is confined to ~15 pc in ~1 Myrs. Our calculations are pertinent to the observed unusually compact non-thermal radio emission in blue compact dwarf galaxies (BCDs). More generally, we demonstrate the potential of a collapsing cloud to confine supernovae, thereby explaining how dwarf galaxies would exist beyond their first generation of star formation.

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arXiv2004

Relativistic Jets from Accretion Disks

The jets observed to emanate from many compact accreting objects may arise from the twisting of a 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 jets, which have a significant mass flux and have energy and angular momentum carried by both matter and electromagnetic field and, Poynting jets, where the mass flux is small and energy and angular momentum are carried predominantly by the electromagnetic field. Here, we describe recent theoretical work on the formation of relativistic Poynting jets from magnetized accretion disks. Further, we describe new relativistic, fully-electromagnetic, particle-in-cell simulations of the formation of jets from accretion disks. Analog Z-pinch experiments may help to understand the origin of astrophysical jets.

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arXiv2002

Poynting Jets from Accretion Disks

We give further considerations on the problem of the evolution of a coronal, force-free magnetic field which threads a differentially rotating, conducting Keplerian disk, extending the work of Li {\it et al.} (2001). This situation is described by the force-free Grad-Shafranov (GS) equation for the flux function $Ψ(r,z)$ which labels the poloidal field lines (in cylindrical coordinates). The GS equation involves a function $H(Ψ)$ describing the distribution of poloidal current which is determined by the differential rotation or {\it twist} of the disk which increases linearly with time. We numerically solve the GS equation in a sequence of volumes of increasing size corresponding to the expansion of the outer perfectly conducting boundaries at ($R_{m}, Z_{m}$). The outer boundaries model the influence of an external non-magnetized plasma. The sequence of GS solutions provides a model for the dynamical evolution of the magnetic field in response to (1) the increasing twist of the disk and (2) the pressure of external plasma. We find solutions with {\it magnetically collimated} Poynting jets where there is a {\it continuous} outflow of energy, angular momentum, and toroidal magnetic flux from the disk into the external space.

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arXiv2002

Lepton Acceleration by Relativistic Collisionless Magnetic Reconnection

We have calculated self-consistent equilibria of a collisionless relativistic electron-positron gas in the vicinity of a magnetic $X-$point. For the considered conditions, pertinent to extra-galactic jets, we find that leptons are accelerated up to Lorentz factors $Γ_0 = κe B_0L {\cal E}^2 /mc^2 \gg 1$, where $B_0$ is the typical magnetic field strength, ${\cal E}\equiv E_0/B_0$, with $E_0$ the reconnection electric field, $L$ is the length scale of the magnetic field, and $κ\approx 12$. The acceleration is due to the dominance of the electric field over the magnetic field in a region around the $X-$point. The distribution function of the accelerated leptons is found to be approximately $dn/dγ\propto γ^{-1}$ for $γ\lesssim Γ_0$. The apparent distribution function may be steeper than $γ^{-1}$ due to the distribution of $Γ_0$ values and/or the radiative losses. Self-consistent equilibria are found only for plasma inflow rates to the $X-$point less than a critical value.

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arXiv2002

Three Dimensional MHD Simulations of Accretion to an Inclined Rotator: The "Cubed Sphere" Method

We describe a three-dimensional, Godunov-type numerical magnetohydrodynamics (MHD) method designed for studying disk accretion to a rotating magnetized star in the general case where the star's rotation axis, its magnetic moment, and the normal to the disk all have different directions. The numerical method uses a "cubed sphere" coordinate system which has advantages of Cartesian and spherical coordinate systems but does not have the singular axis ofthe spherical system. The grid is formed by a sequence of concentric spheres of radii $R_j \propto q^j$ with $j=1..N_R$ and $q={\rm const}>1$. The grid on the surface of the sphere consists of six sectors with the grid on each sector topologically equivalent to the equidistant grid on the face of a cube. Simulation results are discussed for the funnel flows (FF) to a star with dipole moment $\rvecmu$ at an angle $Θ=30^\circ$ to the star's rotation axis ${\bf Ω}$ which is aligned with the normal to the disk. Two important new 3D features are found in these simulations: (1) The funnel flow to the stellar surface is mainly in two streams which approach the star from opposite directions. (2) In the $x-z$ cross section of the flow containing $\rvecmu$ and ${\bf Ω}$, the funnel flow often takes the longer of the two possible paths along magnetic field lines to the surface of the star. A subsequent paper will give a detailed description of the method and results on 3D funnel flows at different inclination angles $Θ$.

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arXiv2002

MHD Simulations of Bondi Accretion to a Star in the "Propeller" Regime

This work investigates Bondi accretion to a rotating magnetized star in the "propeller" regime using axisymmetric resistive, magnetohydrodynamic simulations. In this regime accreting matter tends to be expelled from the equatorial region of the magnetosphere where the centrifugal force on matter rotating with the star exceeds the gravitational force. The regime is predicted to occur if the magnetospheric radius larger than the corotation radius and less than the light cylinder radius. The simulations show that accreting matters is expelled from the equatorial region of the magnetosphere and that it moves away from the star in a supersonic, disk-shaped outflow. At larger radial distances the outflow slows down and becomes subsonic. The equatorial matter outflow is initially driven by the centrifugal force, but at larger distances the pressure gradient becomes significant. We find that the star is spun-down mainly by the magnetic torques at its surface with the rate of loss of angular momentum $\dot{L}$ proportional to $-Ω_*^{1.3}μ^{0.8}$, where $Ω_*$ is the star's rotation rate and $μ$ is its magnetic moment. Further, we find that $\dot{L}$ is approximately independent of the magnetic diffusivity of the plasma $η_m$. The fraction of the Bondi accretion rate which accretes to the surface of the star is found to be $\proptoΩ_*^{-1.0}μ^{-1.7}η_m^{0.4}$. Predictions of this work are important for the observability of isolated old neutron stars and for wind fed pulsars in X-ray binaries.

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arXiv2002

MHD Simulations of Disk-Magnetized Star Interactions in Quiescent Regime: Funnel Flows and Angular Momentum Transport

Magnetohydrodynamic (MHD) simulations have been used to study disk accretion to a rotating magnetized star with an aligned dipole moment. Quiescent initial conditions were developed in order to avoid the fast initial evolution seen in earlier studies. A set of simulations was performed for different stellar magnetic moments and rotation rates. Simulations have shown that the disk structure is significantly changed insidea radius r_{br} where magnetic braking is significant. In this region the disk is strongly inhomogeneous. Radial accretion of matter slows as it approaches the area of strong magnetic field and a dense ring and funnel flow form at the magnetospheric radius r_m where the magnetic pressure is equal to the total, kinetic plus thermal, pressure of the matter.

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arXiv2002

Advective Accretion Disks and Related Problems Including Magnetic Fields

Accretion disk theory was first developed as a theory with the local heat balance, where the whole energy produced by a viscous heating was emitted to the sides of the disk. One of the most important new invention of this theory was the phenomenological treatment of the turbulent viscosity, known the `` alpha'' prescription, where the (r phi) component of the stress tensor was approximated by (alpha p) with a unknown constant alpha. This prescription played the role in the accretion disk theory as well important as the mixing-length theory of convection for stellar evolution. Sources of turbulence in the accretion disk are discussed, including hydrodynamical turbulence, convection and magnetic field role. In parallel to the optically thick geometrically thin accretion disk models, a new branch of the optically thin accretion disk models was discovered, with a larger thickness for the same total luminosity. The choice between these solutions should be done of the base of a stability analysis. The ideas underlying the necessity to include advection into the accretion disk theory are presented and first models with advection are reviewed. The present status of the solution for a low-luminous optically thin accretion disk model with advection is discussed and the limits for an advection dominated accretion flows (ADAF) imposed by the presence of magnetic field are analysed. Related problems of mass ejection from accretion disks and jet formation are discussed.

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arXiv2002

A Cosmic Battery Reconsidered

We revisit the problem of magnetic field generation in accretion flows onto black holes owing to the excess radiation force on electrons. This excess force may arise from the Poynting-Robertson effect. Instead of a recent claim of the generation of dynamically important magnetic fields, we establish the validity of earlier results from 1977 which show only small magnetic fields are generated. The radiative force causes the magnetic field to initially grow linearly with time. However, this linear growth holds for only a {\it restricted} time interval which is of the order of the accretion time of the matter. The large magnetic fields recently found result from the fact that the linear growth is unrestricted. A model of the Poynting-Robertson magnetic field generation close to the horizon of a Schwarzschild black hole is solved exactly using General Relativity, and the field is also found to be dynamically insignificant. These weak magnetic fields may however be important as seed fields for dynamos.

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arXiv2001

Magnetic Helix Formation Driven by Keplerian Disk Rotation in an External Plasma Pressure --- The Initial Expansion Stage

We study the evolution of a magnetic arcade that is anchored to an accretion disk and is sheared by the differential rotation of a Keplerian disk. By including an extremely low external plasma pressure at large distances, we obtain a sequence of axisymmetric magnetostatic equilibria and show that there is a fundamental difference between field lines that are affected by the plasma pressure and those are not (i.e., force-free). Force-free fields, while being twisted by the differential rotation of the disk, expand outward at an angle of $\sim 60^\circ$ away from the rotation axis, consistent with the previous studies. These force-free field lines, however, are enclosed by the outer field lines which originate from small disk radii and come back to the disk at large radii. These outer fields experience most of the twist, and they are also affected most by the external plasma pressure. At large cylindrical radial distances, magnetic pressure and plasma pressure are comparable so that any further radial expansion of magnetic fields is prevented or slowed down greatly by this pressure. This hindrance to cylindrical radial expansion causes most of the added twist to be distributed on the ascending portion of the field lines, close to the rotation axis. Since these field lines are twisted most, the increasing ratio of the toroidal $B_ϕ$ component to the poloidal component $B_{R,z}$ eventually results in the collimation of magnetic energy and flux around the rotation axis. We discuss the difficulty with adding a large number of twists within the limitations of the magnetostatic approximation.

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arXiv2001

Propagation of Magnetized Neutron Stars Through the Interstellar Medium

This work investigates the propagation of magnetized, isolated old neutron stars through the interstellar medium. We performed axisymmetric non relativistic magnetohydrodynamic (MHD) simulations of the propagation of a non-rotating star with dipole magnetic field aligned with its velocity through the interstellar medium (ISM). Effects of rotation will be discussed in a subsequent work. We consider two cases: (1) where theaccretion radius $R_{acc}$ is comparable to the magnetic standoff distance or Alfvén radius $R_A$ and gravitational focusing is important; and (2) where $R_{acc}<< R_A$ and the magnetized star interacts with the ISM as a ``magnetic plow'', without significant gravitational focusing. For the first case simulations were done at a low Mach number ${\cal M}=3$ for a range of values of the magnetic field $B_*$. For the second case, simulations were done for higher Mach numbers, ${\cal M}=10,$ 30, and 50. In both cases, the magnetosphere of the star represents an obstacle for the flow, and a shock wavestands in front of the star. Magnetic field lines are stretched downwind from the star and form a hollow elongated magnetotail. Reconnection of the magnetic field is observed in the tail which may lead to acceleration of particles. Similar powers are estimated to be released in the bow shock wave and in the magnetotail. The estimated powers are, however, below present detection limits. Results of our simulations maybe applied to other strongly magnetized stars, for example, white dwarfs and magnetic Ap stars. Future more sensitive observations may reveal long magnetotails of magnetized stars moving through the ISM.

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arXiv2001

Funnel Flows from Disks to Magnetized Stars

This work considers flows from an accretion disk corotating with the aligned dipole magnetic field of a rotating star. Ideal magnetohydrodynamics (MHD) is assumed with the pressure and density related as $p \propto ρ^γ$ and with $ρ{\bf v}^2 \ll {\bf B}^2/4π$, where ${\bf v}$ is the flow velocity. This limit corresponds to the Alfvén radius for the disk accretion larger than the corotation radius. Transonic flows, which go from subsonic motion near the disk to supersonic inflow near the star, are shown to be possible only for a narrow range of $R_d \sim r_c \equiv (GM/Ω^2)^{1/3}$, where $R_d$ is the radius at which the dipole field line intersects the disk, $r_c$ the corotation radius, $M$ the mass of the star, and $Ω$ its angular rotation rate. The transonic flows have very different behaviors for $γ> 7/5$ and $< 7/5$. In both cases, the plasma flow velocity ${\bf v}$ (which is parallel to ${\bf B}$) increases with decreasing distance $R$ from the star. However, for $γ>7/5$, the Mach number ${\cal M} \equiv |{\bf v}|/c_s$ (with $c_s$ the sound speed) initially increases with $R$ decreasing from $R_d$, but for $R$ decreasing from $\approx 0.22 R_d$ (for $γ=5/3$)the Mach number surprisingly {\it decreases}. In the other limit, $γ< 7/5$, ${\cal M}$ increases monotonically with decreasing $R$. Application of these results is made to funnel flows to rotating magnetized neutron stars and young stellar objects.

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arXiv2001

Poynting Jets from Accretion Disks

The powerful narrow jets observed to emanate from many compact accreting objects may arise from the twisting of a 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, {\it hydromagnetic outflows}, which have a significant mass flux and have energy and angular momentum carried by both the matter and the electromagnetic field and, Poynting outflows, where the mass flux is negligible and energy and angular momentum are carried predominantly by the electromagnetic field. Here we consider a Keplerian disk initially threaded by a dipole-like magnetic field and we present solutions of the force-free Grad-Shafranov equation for the coronal plasma. We find solutions with Poynting jets where there is a continuous outflow of energy and toroidal magnetic flux from the disk into the external space. This behavior contradicts the commonly accepted ``theorem'' of Solar plasma physics that the motion of the footpoints of a magnetic loop structure leads to a stationary magnetic field configuration with zero power and flux outflows. In addition we discuss recent magnetohydrodynamic (MHD) simulations which establish that quasi-stationary collimated Poynting jets similar to our Grad-Shafranov solutions arise from the inner part of a disk threaded by a dipole-like magnetic field. At the same time we find that there is a steady uncollimated hydromagnetic outflow from the outer part of the disk. The Poynting jets represent a likely model for the jets from active galactic nuclei, microquasars, and gamma ray burst sources.

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arXiv2000

Asymmetry in Isolated, Morphologically Normal Sa Galaxies

We have examined the morphological and dynamical HI symmetry properties of a sample of moderately inclined Sa galaxies which are classified as morphologically normal. The sample galaxies were known a prior to exhibit kinematic peculiarities ranging from warps to independent, wholly decoupled disks, and are possibly the remnants of minor mergers. We compare the asymmetry of the rotation curves to global kinematic asymmetry, and find a relationship between rotation curve asymmetry and the kinematic n=2 mode. We have also examined the kinematics of these galaxies following the discussion of Briggs (1990) and find that the warps observed in the HI disks of these galaxies deviate significantly from the simple rules for warps that commonly apply.

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arXiv2000

Departures From Axisymmetric Morphology and Dynamics in Spiral Galaxies

New HI synthesis data have been obtained for six face-on galaxies with the Very Large Array. These data and reanalyses of three additional data sets make up a sample of nine face-on galaxies analyzed for deviations from axisymmetry in morphology and dynamics. This sample represents a subsample of galaxies already analyzed for morphological symmetry properties in the R-band. Four quantitative measures of dynamical nonaxisymmetry are compared to one another and to the quantitative measures of morphological asymmetry in HI and R-band to investigate the relationships between nonaxisymmetric morphology and dynamics. We find no significant relationship between asymmetric morphology and most of the dynamical measures in our sample. A possible relationship is found, however, between morphology and dynamical position angle differences between approaching and receding sides of the galaxy.

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arXiv1999

Rossby Wave Instability of Thin Accretion Disks -- II. Detailed Linear Theory

In earlier work we identified a global, non-axisymmetric instability associated with the presence of an extreme in the radial profile of the key function ${\cal L}(r) \equiv (ΣΩ/κ^2) S^{2/Γ}$ in a thin, inviscid, nonmagnetized accretion disk. Here, $Σ(r)$ is the surface mass density of the disk, $Ω(r)$ the angular rotation rate, $S(r)$ the specific entropy, $Γ$ the adiabatic index, and $κ(r)$ the radial epicyclic frequency. The dispersion relation of the instability was shown to be similar to that of Rossby waves in planetary atmospheres. In this paper, we present the detailed linear theory of this Rossby wave instability and show that it exists for a wider range of conditions, specifically, for the case where there is a ``jump'' over some range of $r$ in $Σ(r)$ or in the pressure $P(r)$. We elucidate the physical mechanism of this instability and its dependence on various parameters, including the magnitude of the ``bump'' or ``jump,'' the azimuthal mode number, and the sound speed in the disk. We find large parameter range where the disk is stable to axisymmetric perturbations, but unstable to the non-axisymmetric Rossby waves. We find that growth rates of the Rossby wave instability can be high, $\sim 0.2 Ω_{\rm K}$ for relative small ``jumps'' or ``bumps''. We discuss possible conditions which can lead to this instability and the consequences of the instability.

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