arXiv subjects

Maxim Lyutikov

Publications and source records attributed to Maxim Lyutikov.

At least 145 records · Page 8Linked to original sources

arXiv2011

Schwarzschild black holes as unipolar inductors: expected electromagnetic power of a merger

(Abridged) The motion of a Schwarzschild black hole with velocity $v_0 = β_0 c$ through a constant magnetic field $B_0$ in vacuum induces a component of the electric field along the magnetic field, generating a non-zero second Poincare electromagnetic invariant $ ^* F \cdot F \neq 0$. This will produce (e.g., via radiative effects and vacuum breakdown) an electric charge density of the order of $ρ_{\rm ind}= B_0 β_0 /(2 πe R_G)$, where $R_G = 2 G M/c^2$ is the Schwarzschild radius and $M$ is the mass of the black hole; the charge density $ρ_{\rm ind}$ is similar to the Goldreich-Julian density. The magnetospheres of moving black holes resemble in many respects the magnetospheres of rotationally-powered pulsars, with pair formation fronts and outer gaps, where the sign of the induced charge changes. As a result, the black hole will generate bipolar electromagnetic jets each consisting of two counter-aligned current flows (four current flows total), each carrying an electric current of the order $I \approx e B_0 R_G β_0$. The electromagnetic power of the jets is $L \approx (G M)^2 B_0^2 β_0^2/c^3$; for a particular case of merging black holes the resulting Poynting power is $ L \approx {(G M)^3 B_0^2 /(c^5 R)}$, where $R$ is the radius of the orbit. In addition, in limited regions near the horizon the first electromagnetic invariant changes sign, so that the induced electric field becomes larger than the magnetic field, $E>B$. The total energy loss from a system of merging BHs is a sum of two components with similar powers, one due to the rotation of space-time within the orbit, driven by the non-zero angular momentum in the system, and the other due to the linear motion of the BHs through the magnetic field.

astro-ph.HE↗

arXiv2011

Coronal mass ejections as expanding force-free structures

We mode Solar coronal mass ejections (CMEs) as expanding force-fee magnetic structures and find the self-similar dynamics of configurations with spatially constant α, where {\bf J} =α{\bf B}, in spherical and cylindrical geometries, expanding spheromaks and expanding Lundquist fields correspondingly. The field structures remain force-free, under the conventional non-relativistic assumption that the dynamical effects of the inductive electric fields can be neglected. While keeping the internal magnetic field structure of the stationary solutions, expansion leads to complicated internal velocities and rotation, induced by inductive electric field. The structures depends only on overall radius R(t) and rate of expansion \dot{R}(t) measured at a given moment, and thus are applicable to arbitrary expansion laws. In case of cylindrical Lundquist fields, the flux conservation requires that both axial and radial expansion proceed with equal rates. In accordance with observations, the model predicts that the maximum magnetic field is reached before the spacecraft reaches the geometric center of a CME.

astro-ph.SR↗

arXiv2010

Electromagnetic extraction of energy from merging black holes

We calculate the evolution of the prompt intrinsic Poynting power generated by merging black holes. Orbiting black holes induce rotation of the space-time. In a presence of magnetic field supported by an accretion disk outside of the orbit, this results in a generation of an electromagnetic outflow via the Blandford-Znajek-type process with luminosity $L_{EM} \sim G^3 M^3 B^2/(c^5 R_{orb}) $ and reaching a fairly low maximum values of $L_{EM} = 10^{37}-10^{39} m_6 $ erg s$^{-1}$ ($m_6$ is the masses of black holes in millions of Solar mass) at the time of the merger. Dissipation of the wind power may produce two types of observed signatures: a highly variable collimated emission coming from the internal dissipation within the jets and a broad-band near-isotropic emission generated at the termination shocks.

astro-ph.HE↗

arXiv2010

Double explosions and jet formation in GRB-supernova progenitors

Progenitors of long GRBs, and core-collapse supernovae in general, may have two separate mechanisms driving the outflows: quasi-isotropic neutrino-driven supernova explosions followed by a highly collimated relativistic outflow driven by the GRB central engine, a black hole or a magnetar. We consider the dynamics of the second GRB-driven explosion propagating through expanding envelope generated by the passage of the primary supernova shock. Beyond the central core, in the region of steep density gradient created by the SN shock breakout, the accelerating secondary quasi-spherical GRB shock become unstable to corrugation and under certain conditions may form a highly collimated jet, a "chimney", when a flow expands almost exclusively along a nearly cylindrically collimated channel. Thus, weakly non-spherical driving and/or non-spherical initial conditions of the wind cavity may produce highly non-spherical, jetted outflows. For a constant luminosity GRB central engine, this occurs for density gradient in the envelope ρ~ r^{-ω} steeper than ω>4.

astro-ph.HE↗

arXiv2010

Structure of magnetic fields in intracluster cavities

Observations of clusters of galaxies show ubiquitous presence of X-ray cavities, presumably blown by the AGN jets. We consider magnetic field structures of these cavities. Stability requires that they contain both toroidal and poloidal magnetic fields, while realistic configurations should have vanishing magnetic field on the boundary. For axisymmetric configurations embedded in unmagnetized plasma, the continuity of poloidal and toroidal magnetic field components on the surface of the bubble then requires solving the elliptical Grad-Shafranov equation with both Dirichlet and Neumann boundary conditions. This leads to a double eigenvalue problem, relating the pressure gradients and the toroidal magnetic field to the radius of the bubble. We have found fully analytical stable solutions. This result is confirmed by numerical simulation. We present synthetic X-ray images and synchrotron emission profiles and evaluate the rotation measure for radiation traversing the bubble.

astro-ph.HE↗

arXiv2010

Resolving Doppler-factor crisis in AGNs: non-steady magnetized outflows

Magnetically-driven non-stationary acceleration of jets in AGNs results in the leading parts of the flow been accelerated to much higher Lorentz factors than in the case of steady state acceleration with the same parameters. The higher Doppler-boosted parts of the flow may dominate the high energy emission of blazar jets. We suggest that highly variable GeV and TeV emission in blazars is produced by the faster moving leading edges of highly magnetized non-stationary ejection blobs, while the radio data trace the slower-moving bulk flow. Model predictions compare favorably with the latest Fermi gamma-ray and MOJAVE radio VLBI results.

astro-ph.HE↗

arXiv2010

Dynamics of strongly magnetized ejecta in Gamma Ray Bursts

We consider dynamical scales in magnetized GRB outflows, using the solutions to the Riemann problem of expanding arbitrarily magnetized outflows (Lyutikov 2010). For high ejecta magnetization, the behavior of the forward shock closely resembles the so-called thick shell regime of the hydrodynamical expansion. The exception is at small radii, where the motion of the forward shock is determined by the dynamics of subsonic relativistic outflows. The behaviors of the reverse shock is different in fluid and magnetized cases: in the latter case, even for medium magnetization, sigma ~ 1, the reverse shock forms at fairly large distances, and may never form in a wind-type external density profile.

astro-ph.HE↗

arXiv2010

Probing Shock Properties with Non-thermal X-ray Filaments in Cas A

Thin non-thermal X-ray filaments are often seen in young supernova remnants. We used data from the 1 Ms Chandra observation of Cassiopeia A to study spectral properties of some of the filaments in this remnant. For all the cases that we examined, the X-ray spectrum across the filaments hardens, at about 10% level, going outward, while observed filament widths depend only weakly on the photon energy. Using a model that includes radiative cooling, advection and diffusion of accelerated particles behind the shock, we estimated the magnetic field, turbulence level and shock obliquity.

astro-ph.HE↗

arXiv2010

Gamma Ray Bursts: back to the blackboard

Exceptional observational breakthroughs in the field of Gamma Ray Burst research are not paralleled by theoretical advances. In this review, based on the introductory talk given at the "The Shocking Universe" meeting, I argue that any present day model of GRBs, especially of Short type, is grossly incomplete. I will highlight various contradictions with observations that many models face and briefly mention a number of ideas that might or might not work. In particular, I will discuss (i) a possibility that early X-ray afterglows are coming from internal dissipation, and not from the forwards shock; (ii) that prompt radiation is beamed in the outflow frame.

astro-ph.HE↗

arXiv2009

Structure of Cosmic Ray-modified Perpendicular Shocks

Kinetic diffusion of cosmic rays ahead of perpendicular shocks induces charge non-neutrality, which is mostly, yet not completely, screened by the bulk plasma via polarization drift current. Hydrodynamic shear instabilities as well as modified Buneman instability of the polarization current generate the turbulence necessary for a Fermi-type acceleration. Thus, similar to the case of parallel shocks, in perpendicular shocks the diffusing cosmic rays generate unstable plasma currents that in turn excite turbulence. This allows a self-consistent evolution of a shock-cosmic rays system. In the kinetic regime of the modified Buneman instability, electrons may be heated in the cosmic ray precursor.

astro-ph.HE↗

arXiv2009

High sigma model of pulsar wind nebulae

Pulsars and central engines of long gamma ray burst -- collapsars -- may produce highly magnetized (Poynting flux dominated) outflows expanding in a dense surrounding (interstellar medium or stellar material). For certain injection conditions, the magnetic flux of the wind cannot be accommodated within the cavity. In this case, ideal (non-dissipative) MHD models, similar to the Kennel and Coroniti (1984) model of the Crab nebular, break down (the so-called sigma problem). This is typically taken to imply that the wind should become particle-dominated on scales much smaller than the size of the cavity. The wind is then slowed down by a fluid-type (low magnetization) reverse shock. Recent Fermi results, indicating that synchrotron spectrum of the Crab nebula extends well beyond the upper limit of the most efficient radiation reaction-limited acceleration, contradict the presence of a low sigma reverse shock. We propose an alternative possibility, that the excessive magnetic flux is destroyed in a reconnection-like process in two regions: near the rotational axis and near the equator. We construct an example of such highly magnetized wind having two distinct reconnection regions and suggest that these reconnection cites are observed as tori and jets in pulsar wind nebulae. The model reproduces, qualitatively, the observed morphology of the Crab nebula. In parts of the nebular the dissipation occurs in a relativistically moving wind, alleviating the requirements on the acceleration rate.

astro-ph.HE↗

arXiv2009

Structure of magnetic fields in non-convective stars

(Abridged) We develop a theoretical framework to construct axisymmetric magnetic equilibria in stars, consisting of both poloidal and toroidal magnetic field components. In a stationary axisymmetric configuration, the poloidal current is a function of the poloidal magnetic flux only, and thus should vanish on field lines extending outside of the star. Non-zero poloidal current is limited to a set of toroid-shape flux surfaces fully enclosed inside the star. If we demand that there are no current sheets, then on the separatrix delineating the regions of zero and finite toroidal magnetic field both the poloidal flux function and its derivative should match. Thus, for a given magnetic field in the bulk of the star, the elliptical Grad-Shafranov equation that describes magnetic field structure inside the toroid is an ill-posed problem, with both Dirichlet and Newman boundary conditions and {\it a priori} unknown distribution of toroidal and poloidal electric currents. We discuss a procedure which allows to solve this ill-posed problem by adjusting the unknown current functions. We find a poloidal current-carrying solution that leaves the shape of the flux function and, correspondingly, the toroidal component of the electric current the same as in the case of no poloidal current. The equilibria discussed in this paper may have arbitrary large toroidal magnetic field, and may include a set of stable equilibria. The method developed here can also be applied to magnetic structure of differentially rotating stars, as well as to calculate velocity field in incompressible isolated fluid vortex with a swirl.

astro-ph.SR↗

arXiv2009

On reversal of centrifugal acceleration in special relativity

The basic principles of General Theory of Relativity historically have been tested in gedanken experiments in rotating frame of references. One of the key issues, which still evokes a lot of controversy, is the centrifugal acceleration. Machabeli & Rogava (1994) argued that centrifugal acceleration reverse direction for particles moving radially with relativistic velocities within a "bead on a wire" approximation. We show that this result is frame-dependent and reflects a special relativistic dilution of time (as correctly argued by de Felice (1995)) and is analogous to freezing of motion on the black hole horizon as seen by a remote observer. It is a reversal of coordinate acceleration; there is no such effect as measured by a defined set of observers, e.g., proper and/or comoving. Frame-independent velocity of a "bead" with respect to stationary rotating observers increases and formally reaches the speed of light on the light cylinder. In general relativity, centrifugal force does reverse its direction at photon circular orbit, r=3M in Schwarzschild metric, as argued by Abramowicz (1990).

gr-qc↗

arXiv2009

Magnetocentrifugal launching of jets from disks around Kerr black holes

Strong magnetic fields modify particle motion in the curved space-time of spinning black holes and change the stability conditions of circular orbits. We study conditions for magnetocentrifugal jet launching from accretion disks around black holes, whereby large scale black hole lines anchored in the disk may fling tenuous coronal gas outward. For a Schwarzschild black hole, magnetocentrifugal launching requires that the poloidal component of magnetic fields makes an angle less than $60^\circ$ to the outward direction at the disk surface, similar to the Newtonian case. For prograde rotating disks around Kerr black holes, this angle increases and becomes $90^\circ$ for footpoints anchored to the disk near the horizon of a critically spinning $a=M$ black hole. Thus, a disk around a critically spinning black hole may centrifugally launch a jet even along the rotation axis.

astro-ph.HE↗

arXiv2008

Relativistic Spin Precession in the Double Pulsar

The double pulsar PSR J0737-3039A/B consists of two neutron stars in a highly relativistic orbit that displays a roughly 30-second eclipse when pulsar A passes behind pulsar B. Describing this eclipse of pulsar A as due to absorption occurring in the magnetosphere of pulsar B, we successfully use a simple geometric model to characterize the observed changing eclipse morphology and to measure the relativistic precession of pulsar B's spin axis around the total orbital angular momentum. This provides a test of general relativity and alternative theories of gravity in the strong-field regime. Our measured relativistic spin precession rate of 4.77 (+0.66,-0.65) degrees per year (68% confidence level) is consistent with that predicted by general relativity within an uncertainty of 13%.

astro-ph↗

arXiv2008

Resonant cyclotron scattering in magnetars' emission

(Abridged) We present a systematic fit of a model of resonant cyclotron scattering (RCS) to the X-ray data of ten magnetars, including canonical and transient anomalous X-ray pulsars (AXPs), and soft gamma repeaters (SGRs). In this scenario, non-thermal magnetar spectra in the soft X-rays (i.e. below ~10 keV) result from resonant cyclotron scattering of the thermal surface emission by hot magnetospheric plasma. We find that this model can successfully account for the soft X-ray emission of magnetars, while using the same number of free parameters than the commonly used empirical blackbody plus power-law model. However, while the RCS model can alone reproduce the soft X-ray spectra of AXPs, the much harder spectra of SGRs below ~10 keV, requires the addition of a power-law component (the latter being the same component responsible for their hard X-ray emission). Although this model in its present form does not explain the hard X-ray emission of a few of these sources, we took this further component into account in our modeling not to overlook their contribution in the ~4-10 keV band. We find that the entire class of sources is characterized by magnetospheric plasma with a density which, at resonant radius, is about 3 orders of magnitudes higher than n_{GJ}, the Goldreich-Julian electron density. The inferred values of the intervening hydrogen column densities, are also in better agreement with more recent estimates inferred from the fit of single X-ray edges. For the entire sample of observations, we find indications for a correlation between the scattering depth and the electron thermal velocity, and the field strength. Moreover, in most transient anomalous X-ray pulsars the outburst state is characterized by a relatively high surface temperature which cools down during the decay.

astro-ph↗

arXiv2008

Hartmann flow with Braginsky viscosity: a test problem for intercluster plasma

We consider a Hartmann layer, stationary flow of a viscose and resistive fluid between two plates with superimposed transverse magnetic field, in the limit of gyrotropic plasma, when viscosity across the field is strongly suppressed. For zero cross-field viscosity, the problem is not well posed, since viscosity then vanishes on the boundaries and in the middle of the layer, where there is no longitudinal field. An additional arbitrarily small isotropic viscosity allows one to find magnetic field and velocity profiles which are independent of this viscosity floor and different from flows with isotropic viscosity. Velocity sharply rises in a thin boundary layer, which thickness depends both on the Hartmann number and on the Lundquist number of the flow. The implication of the work is that, in simulating ICM dynamics it is imperative to use numerical schemes which take into account anisotropic viscosity. Although magnetic fields are dynamically subdominant in the ICM they do determine its the dissipative properties, stability of embedded structures and the transition to turbulence.

astro-ph↗

arXiv2007

Inductive acceleration of UHECRs in sheared relativistic jets

Relativistic outflows carrying large scale magnetic fields have large inductive potential and may accelerate protons to ultra high energies. We discuss a novel scheme of Ultra-High Energy Cosmic Ray (UHECR) acceleration due to drifts in magnetized, cylindrically collimated, sheared jets of powerful active galaxies (with jet luminosity $\geq 10^{46}$ erg s$^{-1}$). A positively charged particle carried by such a plasma is in an unstable equilibrium if ${\bf B} \cdot \nabla \times {\bf v}< 0$, so that kinetic drift along the velocity shear would lead to fast, regular energy gain. The highest rigidity particles are accelerated most efficiently implying the dominance of light nuclei for energies above the ankle in our model: from a mixed population of pre-accelerated particle the drift mechanism picks up and boosts protons preferably.

astro-ph↗