Magnetic Field In Accretion Disk Research Articles

Induced Magnetic Field in Accretion Disks around Neutron Stars

In pulsating X-ray sources a magnetized neutron star is surrounded by an accretion disk whosestructure requires a study. In particular, the dipole magnetic field of the star can partially penetrate the diskand, freezing into the matter, can give rise to an induced magnetic field in the disk. The field growth can belimited by its turbulent diffusion. In this paper we calculate such an induced field. The problem is reduced tosolving the induction equation in the presence of diffusion. An analytical solution of the equation has beenobtained, with the radial and vertical structures of the induced field having been calculated simultaneously.The radial structure is close to the previously predicted dependence on the difference of the angular velocitiesof the disk and the magnetosphere: b ∝ Ωs − Ωk, while the vertical structure of the field is close to the linearproportionality between the field and the height above the equator: b ∝ z. The possibility of the existence ofnonstationary quasi-periodic components of the induced magnetic field is discussed.

Large-scale dynamo model for accretion disks

Context. Magnetic fields in accretion disks play an important role in the rich dynamics of these systems. A dynamo theory describing the generation of these magnetic field is in general very complex and requires many assumptions in order to be of practical use. In this respect, a theory with as few assumptions as possible is desirable. Aims. To investigate the generation of magnetic fields in accretion disks around magnetized central objects, a large-scale dynamo model is employed that includes feedback effects on the mass motion due to the Lorentz force. The dynamo model was developed from the fundamental magnetohydrodynamics equations with a minimum of hypothesis, and was tested in the case of the Sun and other stars. It is applied to accretion disks for the first time. Methods. The magnetic field in the disk, generated by the mentioned dynamo theory, was matched to that of the central object, considered dipolar, and to that of a magnetosphere described with the Grad–Shafranov equation. The relation between axial current and magnetic flux required in the Grad–Shafranov equation was not imposed, but was self-consistently determined along with the full solution. Results. The model is able to reproduce the patterns of magnetic field lines obtained in several works, such as closed magnetic lines near the central object and open lines for larger radii. The maximum value of the field is located near the internal radius of the accretion disk, where the currents in the disk force the concentration of field lines of the central object in the magnetosphere around this region. By varying the values of stellar mass, stellar magnetic field, mass accretion rate, and internal radius of the disk, it is found that the stellar magnetic field is the most important parameter in the determination of the disk magnetic field. The stellar mass is of secondary importance. It affects the azimuthal component of the disk magnetic field. The internal radius of the disk affects the disk zonal magnetic field and is likewise less important.

Primordial black holes as seeds of magnetic fields in the universe

Although it is assumed that magnetic fields in accretion disks are dragged from the interstellar medium, the idea is likely not applicable to primordial black holes (PBHs) formed in the early universe. Here we show that magnetic fields can be generated in initially unmagnetized accretion disks around PBHs through the Biermann battery mechanism, and therefore provide the small scale seeds of magnetic field in the universe. The radial temperature and vertical density profiles of these disks provide the necessary conditions for the battery to operate naturally. The generated seed fields have a toroidal structure with opposite sign in the upper and lower half of the disk. In the case of a thin accretion disk around a rotating PBH, the field generation rate increases with increasing PBH spin. At a fixed $r/r_{\rm isco}$, where $r$ is the radial distance from the PBH and $r_{\rm isco}$ is the radius of the innermost stable circular orbit, the battery scales as $M^{-9/4}$, where $M$ is the PBH's mass. The very weak dependency of the battery on accretion rate, makes this mechanism a viable candidate to provide seed fields in an initially unmagnetized accretion disk, following which the magnetorotational instability could take over.

Dynamo generated magnetic configurations in accretion discs and the nature of quasi-periodic oscillations in accreting binary systems

Magnetic fields are important for accretion disc structure. Magnetic fields in a disc system may be transported with the accreted matter. They can be associated with either the central body and/or jet, and be fossil or dynamo excited in situ. We consider dynamo excitation of magnetic fields in accretion discs of accreting binary systems in an attempt to clarify possible configurations of dynamo generated magnetic fields. We first model the entire disc with realistic radial extent and thickness using an alpha-quenching non-linearity. We then study the simultaneous effect of feedback from the Lorentz force from the dynamo-generated field. We perform numerical simulations in the framework of a relatively simple mean-field model which allows the generation of global magnetic configurations. We explore a range of possibilities for the dynamo number, and find quadrupolar-type solutions with irregular temporal oscillations that might be compared to observed rapid luminosity fluctuations. The dipolar symmetry models with $R_\alpha<0$ have lobes of strong toroidal field adjacent to the rotation axis that could be relevant to jet launching phenomena. We have explored and extended the solutions known for thin accretion discs.

Spatially resolved magnetic field structure in the disk of a T Tauri star.

Magnetic fields in accretion disks play a dominant part during the star formation process but have hitherto been observationally poorly constrained. Field strengths have been inferred on T Tauri stars and possibly in the innermost part of their accretion disks, but the strength and morphology of the field in the bulk of a disk have not been observed. Spatially unresolved measurements of polarized emission (arising from elongated dust grains aligned perpendicularly to the field) imply average fields aligned with the disks. Theoretically, the fields are expected to be largely toroidal, poloidal or a mixture of the two, which imply different mechanisms for transporting angular momentum in the disks of actively accreting young stars such as HL Tau (ref. 11). Here we report resolved measurements of the polarized 1.25-millimetre continuum emission from the disk of HL Tau. The magnetic field on a scale of 80astronomical units is coincident with the major axis (about 210 astronomical units long) of the disk. From this we conclude that the magnetic field inside the disk at this scale cannot be dominated by a vertical component, though a purely toroidal field also does not fit the data well. The unexpected morphology suggests that the role of the magnetic field in the accretion of a T Tauri star is more complex than our current theoretical understanding.

DISK CORONA INTERACTION: MECHANISM FOR THE DISK TRUNCATION AND SPECTRUM CHANGE IN LOW-LUMINOSITY ACTIVE GALACTIC NUCLEI

The truncation of an optically thick, geometrically thin accretion disk is investigated in the context of low luminosity AGN (LLAGN). We generalize the disk evaporation model used in the interpretative framework of black hole X-ray binaries by including the effect of a magnetic field in accretion disks surrounding supermassive black holes. The critical transition mass accretion rate for which the disk is truncated is found to be insensitive to magnetic effects, but its inclusion leads to a smaller truncation radius in comparison to a model without its consideration. That is, a thin viscous disk is truncated for LLAGN at an Eddington ratio less than 0.03 for a standard viscosity parameter ($\alpha = 0.3$). An increase of the viscosity parameter results in a higher critical transition mass accretion rate and a correspondingly smaller truncation distance, the latter accentuated by greater magnetic energy densities in the disk. Based on these results, the truncation radii inferred from spectral fits of LLAGN published in the literature are consistent with the disk evaporation model. The infrared emission arising from the truncated geometrically thin accretion disks may be responsible for the red bump seen in such LLAGN.

Advection/diffusion of large scale magnetic field in accretion disks

Abstract. 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. We derive a sixth-order differential equation for the radial flow velocity vr(z) which depends mainly on the midplane thermal to magnetic pressure ratio β&gt;1 and the Prandtl number of the turbulence P=viscosity/diffusivity. Boundary conditions at the disk surface take into account a possible magnetic wind or jet and allow for a surface current in the highly conducting surface layer. The stationary solutions we find indicate that a weak (β&gt;1) large-scale field does not diffuse away as suggested by earlier work.

The Influence of Magnetic Field Geometry on the Evolution of Black Hole Accretion Flows: Similar Disks, Drastically Different Jets

Because the magnetorotational instability is capable of exponentially amplifying weak pre-existing magnetic fields, it might be hoped that the character of the magnetic field in accretion disks would be independent of the nature of the seed field. However, the divergence-free nature of magnetic fields in highly conducting fluids ensures that their large-scale topology is preserved, no matter how greatly the field intensity is changed. By performing global two- and three-dimensional general relativistic magnetohydrodynamic disk simulations with several different topologies for the initial magnetic field, we explore the degree to which the character of the flows around black holes depends on the initial topology. We find that while the qualitative properties of the accretion flow are nearly independent of field topology, jet launching is very sensitive to it; a sense of vertical field consistent for at least an inner disk inflow time is essential to the support of strong jets.

Advection of Magnetic Fields in Accretion Disks: Not So Difficult After All

We show that a large-scale, weak magnetic field threading a turbulent accretion disk tends to be advected inward, contrary to previous suggestions that it will be stopped by outward diffusion. The efficient inward transport is a consequence of the diffuse, magnetically-dominated surface layers of the disk, where the turbulence is suppressed and the conductivity is very high. This structure arises naturally in three-dimensional simulations of magnetorotationally unstable disks, and we demonstrate here that it can easily support inward advection and compression of a weak field. The advected field is anchored in the surface layer but penetrates the main body of the disk, where it can generate strong turbulence and produce values of alpha (i.e., the turbulent stress) large enough to match observational constraints; typical values of the vertical magnetic field merely need to reach a few percent of equipartition for this to occur. Overall, these results have important implications for models of jet formation which require strong, large-scale magnetic fields to exist over a region of the inner accretion disk.

Dissipation of Magnetic Flux in Primordial Gas Clouds

We report the strength of seed magnetic flux of accretion disk surrounding the PopIII stars. The magnetic field in accretion disk might play an important role in the transport of angular momentum because of the turbulence induced by Magneto-Rotational Instability (MRI). On the other hand, since the primordial star-forming clouds contain no heavy elements and grains, they experience much different thermal history and different dissipation history of the magnetic field in the course of their gravitational contraction, from those in the present-day star-forming molecular clouds. In order to assess the magnetic field strength in the accretion disk of PopIII stars, we calculate the thermal history of the primordial collapsing clouds, and investigate the coupling of magnetic field with primordial gas. As a result, we find that the magnetic field strongly couple with primordial gas cloud throughout the collapse, i.e. the magnetic field are frozen to the gas as far as initial field strength satisfies $ B\lesssim 10^{-5}(n_{\rm H}/10^3{\rm cm^{-3}})^{0.55}{\rm G}$.

Turbulent dynamo action in a shear flow

We consider the mean electromotive force and a dynamo-generated magnetic field, taking into account the stretching of turbulent magnetic field lines by a shear flow. Calculations are performed by making use of the second-order correlation approximation. In the presence of shear, the mirror symmetry of turbulence can be broken; thus turbulent motions become suitable for the generation of a large-scale magnetic field. Regardless of the shear law, turbulence can lead to a rapid amplification of the mean magnetic field. The growth rate of the mean magnetic field depends on the length-scale: it is faster for the fields with smaller length-scale. The mechanism considered is qualitatively different from the alpha dynamo, and can generate only a magnetic field that is inhomogeneous in the direction of flow. In contrast to the alpha dynamo, this mechanism also allows the generation of two-dimensional fields. The suggested mechanism may play an important role in the generation of magnetic fields in accretion discs, galaxies and jets.

The effects of dynamo-generated magnetic fields on accretion disc structure

The effects of dynamo-generated magnetic fields in accretion discs are considered for the two main types of diffusion mechanisms of turbulence and magnetic buoyancy. Thin disc solutions are possible in both cases, and it is shown that the dynamo equations and vertical equilibrium place limits on the turbulent parameters. A lower limit arises for the Prandtl number in the case of turbulent diffusion.

Magnetic Fields in Accretion Disks

AbstractThis paper summarises our work on the role of magnetic fields in accretion disks presented in two papers elsewhere. In the first part (a summary of part of Kuncic &amp; Bicknell 1999), we present a formal development of the equations governing the structure of an accretion disk containing magnetohydrodynamic turbulence. The importance of the different terms in the energy and momentum equations is discussed, and a parametrisation of the unresolved processes is suggested that could be used to make further progress. We briefly explore whether an MHD accretion disk can transport a significant part of the gravitational power into a corona by buoyancy. In the second part, we present some exploratory calculations of the vertical structure of accretion disks, in which non-local dissipation of energy due to the buoyant transport of magnetic field energy is taken into account. It is argued that the efficiency of buoyant magnetic transport depends very strongly on the size of the coherent magnetic regions. If the size of the buoyant cells is not very close to the disk thickness, magnetic energy generated by dynamo action inside the disk will be dissipated locally, and will not be available to transport a significant part of the accretion luminosity into a corona.

Proceedings of the Workshop on Magnetic Fields and Accretion

"It has been clear for many years now that the process of accretion on to a compact object is largely dominated by magnetic fields. The magnetic field channels the accretion flow onto strongly magnetized stars such as magnetic white dwarfs (cataclysmic variables), and neutron stars (accreting X-ray pulsars). Magnetic fields are likely to govern transport processes in accretion discs, and may give rise to collimated outflows in the form of jets. Additionally, the strong field close to the surface of compact stars influences the microphysical emission processes themselves. Explaining the wealth of observations being collected on accreting objects poses a formidable challenge to theoretical astrophysicists. Since several research groups in Australia are working on problems related to magnetic accretion, the ANUATC organised its 1998 annual Theory Workshop around this subject. The next three papers summarise the three main topics of the meeting: accretion onto magnetic white dwarfs, magnetic fields in accretion discs and a collection of issues relating specifically to the X-ray pulsar GX1+4. The organisers would like to thank all participants for their contribution to what we consider to have been a very focussed and succesful workshop, and special thanks go to Michelle Storey for her help in publishing these proceedings. "

Analysis of Toroidal Magnetic Fields in Accretion Disks Using the Cross-Helicity Effect and Estimate of the Jet Velocity

The magnetic fields in an accretion disk are examined using a magnetohydrodynamic (MHD) turbulent dynamo model consisting of transport equations for the mean fields, turbulent energy, dissipation rate, and cross helicity. The velocity of accreting gases is assumed to obey the Keplerian and rigid rotations in the outer and inner regions of a disk, respectively, except for the central part. Under the condition of axisymmetry around the rotation axis and uniformity in the direction perpendicular to the disk, the turbulent model is examined both numerically and analytically. As a result, it is pointed out that the cross-helicity effect generates a toroidal magnetic field, resulting in the occurrence of a current in a direction perpendicular to the disk in the central part. This toroidal magnetic field enables gas to escape from a central high-mass body as bipolar jets, because the magnetic energy may become comparable to the gravitational one. The velocity of the jets in a protoplanetary system was estimated by a numerical simulation of the preset model.

Enhancement of Turbulent Viscosity by Global Magnetic Fields in Accretion Disks

A model of magnetohydrodynamic (MHD) turbulence in accretion disks with global magnetic fields is constructed using a second-order closure modeling of turbulence. The transport equations of the Reynolds stress tensor, the Maxwell stress tensor, and the cross-helicity tensor (the correlation of velocity fluctuation and magnetic fluctuation) are closed by second-order quantities using atwo-scale direct interaction approximation(TSDIA). The quantities appearing these equations are considered to be those averaged in the vertical direction of the disks. The turbulence is assumed to be stationary. We are interested only in the effects of the global magnetic fields on the turbulence in the disks, i.e., no dynamo processes are considered, and the global magnetic fields are supposed to be embedded in the diska priori.

An implicit FE-time solver for axisymmetric magnetic dynamo equations

Magnetic fields are an essential element in investigations for the behaviour of interstellar matter, molecular clouds, galactic disks, accretion disks for stellar and collapsed objects as well as for the collimation and propagation of galactic and extragalactic jets. The time-evolution of magnetic fields in such turbulent media follows from the mean-field electrodynamics which includes the effects of advection, shear, rotation, helicity and diffusivity of the underlying fluid. Under axisymmetry, the induction equation splits into a coupled system of diffusion-type equations for the magnetic flux function ψ ( t, R, z) and the poloidal current stream-function T ( t, R, z). For this non-linearly coupled system, a fully implicit time-solver has been developed based on the methods of finite elements. This code can easily be implemented on any modern RISC workstation. With this code, magnetic fields in galactic disks evolving on the Hubble time can be followed up. Similarly, the time-evolution of magnetic fields in accretion disks around stellar and compact objects can be simulated for a few hundred Keplerian revolution times.

Dynamo generation of magnetic fields in accretion disks

Dynamo generation of magnetic fields in accretion disks

DYNAMO MAGNETIC-FIELD GENERATION IN TURBULENT ACCRETION DISKS

ABSTRACT Magnetic fields can play important roles in the dynamics and evolution of accretion disks. The presence of strong differential rotation and vertical density gradients in turbulent disks allows the alpha-omega dynamo mechanism to offset the turbulent dissipation and maintain strong magnetic fields. It is found that MHD dynamo magnetic-field normal modes in an accretion disk are highly localized to restricted regions of a disk. Implications for the character of real, dynamically constrained magnetic fields in accretion disks are discussed. The magnetic stress due to the mean magnetic field is found to be of the order of a viscous stress. The dominant stress, however, is likely to come from small-scale fluctuating magnetic fields. These fields may also give rise to energetic flares above the disk surface, providing a possible explanation for the highly variable hard X-ray emission from objects like Cyg X-l.

Dynamo Driven Mean Magnetic Field in Accretion Disks of Compact Astrophysical Objects and its Manifestations

The simplest case of the nonlinear turbulent dynamo mechanism is proposed. It is shown that under certain conditions the generated mean magnetic field can become stronger than the small-scale one. Some manifestations (the model of Cyg X-1 bimodal behaviour, asymmetric accretion onto the magnetized rotating compact star) of this mean field are discussed.