In this paper, we initiate a new study of steady funnel-flow accretion onto strongly magnetized neutron stars, including a full treatment of shock generation. As a first step, we adopt a simplified model considering the flow within Newtonian theory and neglecting radiative pressure and cooling. The flow is taken to start from an accretion disc and then to follow magnetic field lines, forming a transonic funnel flow onto the magnetic poles. A standing shock occurs at a certain point in the flow and beyond this material accretes subsonically onto the star with high pressure and density. We calculate the location of the standing shock and all other features of the flow within the assumptions of our model. Applications to observed X-ray pulsars are discussed.
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 m…
Aims : We re-examine the conditions required to steadily deviate an accretion flow from a circumstellar disc into a magnetospheric funnel flow onto a slow rotating young forming star. Methods : New analytical constraints on the formation of accretion funnels flows due to the presence of a dipolar stellar magnetic field disrupting the disc are derived. The Versatile Advection Code is used to confirm these constraints numerically. Axisymmetric MHD simulations are performed, where a stellar dipole field enters the resistive accretion disc, whose structure is self-consistently computed. Results : …
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 $R_d$ i…
We report on the first global three-dimensional (3D) MHD simulations of disk accretion onto a rotating magnetized star through the Rayleigh-Taylor instability. The star has a dipole field misaligned relative to the rotation axis by a small angle Theta. Simulations show that, depending on the accretion rate, a star may be in the stable or unstable regime of accretion. In the unstable regime, matter penetrates deep into the magnetosphere through several elongated "tongues" which deposit matter at random places on the surface of the star, leading to stochastic light-curves. In the stable regime, …
We have carried out fully relativistic numerical simulations of accretion disks in the Kerr metric. In this paper we focus on the unbound outflows that emerge self-consistently from the accretion flow. These outflows are found in the axial funnel region and consist of two components: a hot, fast, tenuous outflow in the axial funnel proper, and a colder, slower, denser jet along the funnel wall. Although a rotating black hole is not required to produce these unbound outflows, their strength is enhanced by black hole spin. The funnel-wall jet is excluded from the axial funnel due to elevated ang…
Non-stationary column accretion onto a surface of a magnetized neutron star is studied with a numerical code based on modified first-order Godunov method with splitting. Formation and evolution of shocks in the column is modeled for accretion rates ranging from 10^{15} g/s to 10^{16} g/s and surface magnetic fields ranging from 5*10^{11} G to 10^{13} G. Non-stationary solutions with plasma deceleration at collisionless oscillating shocks are found. The kinetic energy of the accreting flow efficiently transforms into a cyclotron radiation field. Collisionless stopping of the flow allows a subst…
The propeller regime of disk accretion to a rapidly rotating magnetized star is investigated here for the first time by axisymmetric 2.5D magnetohydrodynamic simulations. An expanded, closed magnetosphere forms in which the magnetic field is predominantly toroidal. A smaller fraction of the star's poloidal magnetic flux inflates vertically, forming a magnetically dominated tower. Matter accumulates in the equatorial region outside magnetosphere and accretes to the star quasi-periodically through elongated funnel streams which cause the magnetic field to reconnect. The star spins-down owing to …
We present results of 3D simulations of MHD instabilities at the accretion disk-magnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a fairly broad range of accretion rates and stellar rotation rates and magnetic fields. It produces tall, thin tongues of plasma that penetrate the magnetosphere in the equatorial plane. The shape and number of the tongues changes with time on the inner-disk dynamical timescale. In contrast with funnel flows, which deposit matter mainly in the polar region, the tongues deposit matter much closer to the stellar equator. The instability appe…
We present results of 3D simulations of MHD instabilities at the accretion disk-magnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a fairly broad range of accretion rates and stellar rotation rates and magnetic fields. It manifests itself in the form of tall, thin tongues of plasma that penetrate the magnetosphere in the equatorial plane. The shape and number of the tongues changes with time on the inner-disk dynamical timescale. In contrast with funnel flows, which deposit matter mainly in the polar region, the tongues deposit matter much closer to the stellar equato…
It is well known that the accretion disk around a magnetized compact star can penetrate inside the magnetospheric boundary, so the magnetospheric radius $\ro$ does not represent the true inner edge $\rin$ of the disk; but controversies exist in the literature concerning the relation between $\ro$ and $\rin$. In the model of Ghosh & Lamb, the width of the boundary layer is given by $δ=\ro-\rin\ll\ro$, or $\rin\simeq\ro$, while Li & Wickramasinghe recently argued that $\rin$ could be significantly smaller than $\ro$ in the case of a slow rotator. Here we show that if the star is able to absorb t…
The boundary layers of weakly-magnetized white dwarfs (WDs) accreting at rates <=10^16 g/s are radially extended, hot, optically-thin, and they advect some of their internally-dissipated energy (Narayan & Popham 1993). Motivated by this, I construct here idealized spectral models of an Advection-Dominated Accretion Flow (ADAF) around a WD, for application to quiescent Dwarf Novae (DN). The Bremsstrahlung cooling of the gas in the ADAF, with temperatures ranging from a few keV to a few tens of keV, can account for the X-ray emission properties of quiescent DN. If the energy advected by the flow…
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…
The efficiency of thin disk accretion onto black holes depends on the inner boundary condition, specifically the torque applied to the disk at the last stable orbit. This is usually assumed to vanish. I estimate the torque on a magnetized disk using a steady magnetohydrodynamic inflow model originally developed by Takahashi et al., 1990. I find that the efficiency epsilon can depart significantly from the classical thin disk value. In some cases epsilon > 1, i.e. energy is extracted from the black hole.
The conventional picture of disk accretion onto magnetized neutron stars has been challenged by the spin changes observed in a few X-ray pulsars, and by theoretical results from numerical simulations of disk-magnetized star interactions. These indicate possible accretion during the propeller regime and the spin-down torque increasing with the accretion rate. Here we present a model for the accretion torque exerted by the disk on a magnetized neutron star, assuming accretion continues even for rapid rotators. The accretion torque is shown to have some different characteristics from that in the …
We report on the second phase of our study of slightly rotating accretion flows onto black holes. We consider magnetohydrodynamical (MHD) accretion flows with a spherically symmetric density distribution at the outer boundary, but with spherical symmetry broken by the introduction of a small, latitude-dependent angular momentum and a weak radial magnetic field. We study accretion flows by means of numerical 2D, axisymmetric, MHD simulations with and without resistive heating. Our main result is that the properties of the accretion flow depend mostly on an equatorial accretion torus which is ma…
We discuss three specific modes 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 modes requires that there be a maximum in the angular velocity of the accreting material, and that the fluid is in stable, nearly circular motion near this maximum rather than moving rapidly towards the star or out of the disk plane into funnel flows. It is presently not known if these conditions occur, but we are exploring this with 3D magnetohydrodynamic simulat…
AA Tau was observed for about 5h per XMM orbit (2 days) over 8 successive orbits, which covers two optical eclipse periods (8.2 days). The XMM optical/UV monitor simultaneously provided UV photometry with a ~15 min sampling rate. Some V-band photometry was also obtained from the ground during this period in order to determine the dates of the eclipses. Two X-ray and UV measurements were secured close to the center of the eclipse. The UV flux is the highest just before the eclipse starts and the lowest towards the end of it. We model the UV flux variations with a weekly modulation (inner disk e…
Qualitative arguments are presented to demonstrate that the energy density of magnetic fields in matter accreting onto a black hole inside the marginally stable orbit is automatically comparable to the rest-mass energy density of the accretion flow. Several consequences follow: magnetic effects must be dynamically significant, but cannot be so strong as to dominate; outward energy transport in Alfven waves may alter the effective efficiency of energy liberation; and vertical magnetic stresses in this region may contribute to "coronal" activity.
We use global magnetohydrodynamic simulations, in a pseudo-Newtonian potential, to investigate the temporal variability of accretion discs around Schwarzschild black holes. We use the vertically-averaged magnetic stress in the simulated disc as a proxy for the rest-frame dissipation, and compute the observed emission by folding this through the transfer function describing the relativistic beaming, light bending and time delays near a non-rotating black hole. The temporal power spectrum of the predicted emission from individual annuli in the disc is described by a broken power law, with indice…
Current magnetic-accretion models for classical T-Tauri stars rely on a strong, dipolar magnetic field of stellar origin to funnel the disk material onto the star, and assume a steady-state. In this paper, I critically examine the physical basis of these models in light of the observational evidence and our knowledge of magnetic fields in low-mass stars, and find it lacking. I also argue that magnetic accretion onto these stars is inherently a time-dependent problem, and that a steady-state is not warranted. Finally, directions for future work towards fully-consistent models are pointed out.
Flow of matter onto strongly magnetized neutron stars in X-ray binaries proceeds through accretion funnels that roughly follow geometry of the magnetic field. X-rays originate near surface of the neutron star, and it may happen that the accretion flow passes through the line of sight as the star rotates. We consider the effects of such accretion flow eclipses on the X-ray light curves of accretion powered pulsars, and present a set of X-ray light-curves measured by BATSE for A0535+262 for which this phenomenon is very likely to take place.
We present a scenario for non-radiative accretion onto the supermassive black hole at the galactic center. Conducting MHD simulations with 1400^3 grid zones that break the axial and reflection symmetries of earlier investigations and extend inward from the Bondi radius, we find a quasi-hydrostatic radial density profile rho propto r^{-0.72} with superadiabatic gradient corresponding to an n~0.72 polytrope. Buoyancy generated by magnetic dissipation is resisted by the same fields so effectively that energy is advected inward: a state of magnetically-frustrated convection. This scenario is consi…
The accretion process onto a magnetized isolated neutron star, which captures material from the interstellar medium, is discussed. The evolutionary track of such a star can be presented as a sequence of four states: ejector, supersonic propeller, subsonic propeller, and steady accretor. I show that subsonic propeller - accretor transition does not occur as long as the magnetic field of the star is strong enough to control the accretion flow in the stellar vicinity. During the subsonic propeller state the accretion rate onto the stellar surface is limited to the rate of plasma diffusion into it…
We report on the first phase of our study of slightly rotating accretion flows onto black holes. We consider inviscid accretion flows with a spherically symmetric density distribution at the outer boundary, but with spherical symmetry broken by the introduction of a small, latitude-dependent angular momentum. We study accretion flows by means of numerical 2D, axisymmetric, hydrodynamical simulations. Our main result is that the properties of the accretion flow do not depend as much on the outer boundary conditions (i.e., the amount as well as distribution of the angular momentum) as on the geo…
We study the self-similar collapse of an isothermal magnetized rotating cloud in the ideal magnetohydrodynamic (MHD) regime. In the limit of small distance from the accreting protostar we find an analytic solution that corresponds to free-fall onto a central mass point. The density distribution is not spherically symmetric but depends on the mass loading of magnetic field lines, which can be obtained by matching our inner solution to an outer collapse solution previously computed by Allen, Shu & Li. The concentration of magnetic field trapped by the central mass point under field-freezing, ind…
We investigate the photometric variability of magnetized stars, particularly neutron stars, accreting through a magnetic Rayleigh-Taylor-type instability at the disk-magnetosphere interface, and compare it with the variability during stable accretion, with the goal of looking for possible quasi-periodic oscillations. The lightcurves during stable accretion show periodicity at the star's frequency and sometimes twice that, due to the presence of two funnel streams that produce antipodal hotspots near the magnetic poles. On the other hand, lightcurves during unstable accretion through tongues pe…
We analyze three-dimensional magnetohydrodynamic (MHD) simulations of a nonradiative accretion flow around a black hole using a pseudo-Newtonian potential. The flow originates from a torus initially centered at 100 gravitational (Schwarzschild) radii. Accretion is driven by turbulent stresses generated self-consistently by the magnetorotational instability. The resulting flow has three well-defined dynamical components: a hot, thick, rotationally-dominated Keplerian disk; a surrounding magnetized corona with vigorous circulation and outflow; and a magnetically-confined jet along the centrifuga…
We consider the spin evolution of highly magnetized neutron stars in a hypercritical inflow just after their birth in supernovae. Presence of a strong magnetic field could deform the star and if the symmetry axis of the field is misaligned with that of stellar rotation, the star will be an emitter of gravitational wave. Here we investigate the possibility of gravitational radiation from such a star when there is a hypercritical inflow onto it. The stellar configuration is slightly deformed away from spherical by the intense magnetic field; the rotational angular frequency of the star is determ…
Quasi-spherical supersonic (Bondi-type) accretion to a star with a dipole magnetic field is investigated using resistive magnetohydrodynamic simulations. A systematic study is made of accretion to a non-rotating star, while sample results for a rotating star are also presented. A new stationary subsonic accretion flow is found with a steady rate of accretion to the magnetized star smaller than the Bondi accretion rate. Dependences of the accretion rate and the flow pattern on the magnetic momentum of the star and the magnetic diffusivity are presented. For slow star's rotation the accretion fl…
We report on the fourth phase of our study of slightly rotating accretion flows onto black holes. The main new element of this study is that we used fully three dimensional (3-D) numerical simulations. We consider hydrodynamics of inviscid accretion flows. We assume a spherically symmetric density distribution at the outer boundary, but brake the flow symmetry by introducing a small, latitude-dependent angular momentum. We also consider cases where angular momentum at large radii is latitude- and azimuth-dependent. For the latitude-dependent angular momentum, 3-D simulations confirm axisymmetr…
A hypermassive neutron star (HMNS) is a possible transient formed after the merger of a neutron star binary. In the latest magnetohydrodynamic simulations in full general relativity, we find that a magnetized HMNS undergoes `delayed' collapse to a rotating black hole (BH) as a result of angular momentum transport via magnetic braking and the magnetorotational instability. The outcome is a BH surrounded by a massive, hot torus with a collimated magnetic field. The torus accretes onto the BH at a quasi-steady accretion rate ~10 solar mass/s; the lifetime of the torus is ~10 ms. The torus has a t…
We present an idealized, spherical model of the evolution of a magnetized molecular cloud due to ambipolar diffusion. This model allows us to follow the quasi-static evolution of the cloud's core prior to collapse and the subsequent evolution of the remaining envelope. By neglecting the thermal pressure gradients in comparison with magnetic stresses and by assuming that the ion velocity is small compared with the neutral velocity, we are able to find exact analytic solutions to the MHD equations. We show that, in the case of a centrally condensed cloud, a core of finite mass collapses into the…
The radiation spectra of many of the brightest ultraluminous X-ray sources (ULXs) are dominated by a hard power law component, likely powered by a hot, optically thin corona that Comptonizes soft seed photons emitted from a cool, optically thick black hole accretion disk. Before its dissipation and subsequent conversion into coronal photon power, the randomized gravitational binding energy responsible for powering ULX phenomena must separate from the mass of its origin by a means other than, and quicker than, electron scattering-mediated radiative diffusion. Therefore, the release of accretion…
We demonstrate that the formation of collapsing cores in subcritical clouds is accelerated by nonlinear flows, by performing three-dimensional non-ideal MHD simulations. An initial random supersonic (and trans-Alfvenic) turbulent-like flow is input into a self-gravitating gas layer that is threaded by a uniform magnetic field (perpendicular to the layer) such that the initial mass-to-flux ratio is subcritical. Magnetic ambipolar diffusion occurs very rapidly initially due to the sharp gradients introduced by the turbulent flow. It subsequently occurs more slowly in the traditional near-quasist…
The recent emergence of a new class of accretion-powered, transient, millisecond X-ray pulsars presents some difficulties for the conventional picture of accretion onto rapidly rotating magnetized neutron stars and their spin behavior during outbursts. In particular, it is unclear from the standard paradigm how these systems manage to accrete over such a wide range in dM/dt (i.e., >~ a factor of 50), and why the neutron stars exhibit a high rate of spindown in at least a number of cases. Following up on prior suggestions, we propose that `fast' X-ray pulsars can continue to accrete, and that t…
The widely accepted assumption is that metals detected in the spectra of a few cool helium-rich white dwarfs cannot be of primordial origin and therefore must be accreted from the interstellar medium. However, the observed abundances of hydrogen are much too low to be compatible with the high accretion rates inferred from metal accretion. Hydrogen accretion is therefore suppressed compared to metal accretion. The hypothesis most widely discussed as cause for this ``hydrogen screening'' is the propeller mechanism: Metals are accreted in the form of grains onto a slowly rotating, weakly magnetiz…
We describe the results of a sequence of simulations of gravitational collapse in a turbulent magnetized region. The parameters are chosen to be representative of molecular cloud material. We find that several protostellar cores and filamentary structures of higher than average density form. The filaments inter-connect the high density cores. Furthermore, the magnetic field strengths are found to correlate positively with the density, in agreement with recent observations. We make synthetic channel maps of the simulations and show that material accreting onto the cores is channelled along the …
From a long time series of high resolution (R=115,000) HARPS spectra and simultaneous broad-band photometry, we report new evidence for magnetospheric accretion as well as ejection processes in the nearly edge-on classical T Tauri star AA Tau. AA Tau's light curve is modulated with a period of 8.22d. The recurrent luminosity dips are due to the periodic occultation of the central star by the magnetically-warped inner disk edge located at about 9 Rstar. Balmer line profiles exhibit a clear rotational modulation of high-velocity redshifted absorption components with a period of 8.22 days as well…
The accurate modelling of astrophysical scenarios involving compact objects and magnetic fields, such as the collapse of rotating magnetized stars to black holes or the phenomenology of gamma-ray bursts, requires the solution of the Einstein equations together with those of general-relativistic magnetohydrodynamics. We present a new numerical code developed to solve the full set of general-relativistic magnetohydrodynamics equations in a dynamical and arbitrary spacetime with high-resolution shock-capturing techniques on domains with adaptive mesh refinements. After a discussion of the equatio…
