GRMHD simulations of accreting neutron stars I: Nonrotating dipoles (original) (raw)
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GRMHD simulations of accreting neutron stars with non-dipole fields
Monthly Notices of the Royal Astronomical Society
NASA’s NICER telescope has recently provided evidence for non-dipolar magnetic field structures in rotation-powered millisecond pulsars. These stars are assumed to have gone through a prolonged accretion spin-up phase, begging the question of what accretion flows on to stars with complex magnetic fields would look like. We present results from a suite of general relativistic magnetohydrodynamic simulations of accreting neutron stars for dipole, quadrupole, and quadrudipolar stellar field geometries. This is a first step towards simulating realistic hotspot shapes in a general relativistic framework to understand hotspot variability in accreting millisecond pulsars. We find that the location and size of the accretion columns resulting in hotspots changes significantly depending on initial stellar field strength and geometry. We also find that the strongest contributions to the stellar torque are from disc-connected field lines and the pulsar wind, leading to spin-down in almost the e...
Dynamics of the flows accreting onto a magnetized neutron star
Astronomy Letters, 2004
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 −1 to 10 16 g s −1 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 substantial part of accreting CNO nuclei to avoid spallation and reach the surface. The nuclei survival fraction depends on the surface magnetic field, being higher at lower magnetic fields.
The Astrophysical …, 2008
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 V. 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, matter accretes in ordered funnel streams and the light curves are almost periodic. A star may switch between these two regimes depending on the accretion rate and may thus show alternate episodes of ordered pulsations and stochastic light curves. In the intermediate regime, both stochastic and ordered pulsations are observed. For , the instability is suppressed and stable accretion V 1 30Њ through funnel streams dominates.
2013
Inspiraling and merging binary neutron stars are not only important source of gravitational waves, but also promising candidates for coincident electromagnetic counterparts. These systems are thought to be progenitors of short gamma-ray bursts (sGRBs). We have shown previously that binary neutron star mergers that undergo delayed collapse to a black hole surrounded by a weighty magnetized accretion disk can drive magnetically powered jets. We now perform magnetohydrodynamic simulations in full general relativity of binary neutron stars mergers that undergo prompt collapse to explore the possibility of jet formation from black hole-light accretion disk remnants. We find that after t − t BH ∼ 26ðM NS =1.8 M ⊙ Þ ms (M NS is the ADM mass) following prompt black hole formation, there is no evidence of mass outflow or magnetic field collimation. The rapid formation of the black hole following merger prevents magnetic energy from approaching force-free values above the magnetic poles, which is required for the launching of a jet by the usual Blandford-Znajek mechanism. Detection of gravitational waves in coincidence with sGRBs may provide constraints on the nuclear equation of state (EOS): the fate of an NSNS merger-delayed or prompt collapse, and hence the appearance or nonappearance of an sGRBdepends on a critical value of the total mass of the binary, and this value is sensitive to the EOS.
On the Rotational Dynamics of Magnetically Threaded Disks around Neutron Stars
The Astrophysical Journal, 2004
We investigate the rotational dynamics of disk accretion around a strongly magnetized neutron star with an aligned dipole field. The magnetospheric field is assumed to thread the disk plasma both inside and outside the corotation radius. As a result of disk-star interaction, the magnetic torque on the disk affects the structure of accretion flow to yield the observed spin-up or spin-down rates for a source of given fastness, magnetic field strength, and mass accretion rate. Within the model we obtain a prescription for the dynamical viscosity of such magnetically modified solutions for a Keplerian disk. We then use this prescription to find a model solution for the rotation rate profile throughout the entire disk, including the non-Keplerian inner disk. We find that the non-Keplerian angular velocity transition region is not necessarily narrow for a source of given spin state. The boundary layer approximation, as in the standard magnetically threaded disk model, holds only in the case of dynamical viscosity decreasing all the way to the innermost edge of the disk. These results are applied to several observed disk-fed X-ray pulsars that have exhibited quasi-periodic oscillations (QPOs). The QPO frequencies provide a constraint on the fastness parameter and enable one to determine uniquely the width of the angular velocity transition zone for each source within model assumptions. We discuss the implications of these results on the value of the critical fastness parameter for a magnetized star in spin equilibrium. Applications of our model are also made with relevant parameters from recent numerical simulations of quasi-stationary disk-magnetized star interactions.
Accretion onto strongly magnetized neutron stars: Partly Applicable for MECOs
In this project the fundamentals of accretion phenomena onto ultracompact neutron stars (NS) and their connection to the formation and acceleration of ultra-relativistic jets are presented and discussed. From basic physics we know that gravitationally-bound rotating plasmas around compact objects form accretion disks. Depending on their Mass-to-kinetic energy ratio, they may become geometrically thin or thick. As all normal stars in the local universe are observed to rotate sub-Keplerian, a boundary layer between the accretion disk and the slowly rotating central object must form, where the rotational velocity of the inflowing matter deviates significantly from the corresponding Keplerian profile. In the case of a strongly magnetized NS, the disk truncates at a certain radius, where the inflowing matter start collimating along the magnetic field lines and shocks as it hits the solid surface in the polar cap, thereby emitting hard x-rays. Accretion disks around weakly magnetized NS's however are expected to penetrate deeper in the gravitational well of the NS, giving rise to plasma ejection from the boundary layer that subsequently collimate into jets. Magnetic fields both of the NS and of the accreted disk-matter are considered to be efficient converters of rotational into kinetic and magnetic energies that power most of the jets observed in X-ray binaries and in active galactic nuclei. Depending on ratio of mass-to-total energy of the jet, the jet content can either be leptonic or hadronic-dominated, though the latter is preferable.
2D and 3D MHD simulations of disk accretion by rotating magnetized stars: Search for variability
Advances in Space Research, 2006
We performed 2D and full 3D magnetohydrodynamic simulations of disk accretion to a rotating star with an aligned or misaligned dipole magnetic field. We investigated the rotational equilibrium state and derived from simulations the ratio between two main frequencies: the spin frequency of the star and the orbital frequency at the inner radius of the disk. In 3D simulations we observed different features related to the non-axisymmetry of the magnetospheric flow. These features may be responsible for high-frequency quasi-periodic oscillations (QPOs). Variability at much lower frequencies may be connected with restructuring of the magnetic flux threading the inner regions of the disk. Such variability is specifically strong at the propeller stage of evolution.
Magnetorotational instability in relativistic hypermassive neutron stars
Physical Review D, 2013
A differentially rotating hypermassive neutron star (HMNS) is a metastable object which can be formed in the merger of neutron-star binaries. The eventual collapse of the HMNS into a black hole is a key element in generating the physical conditions expected to accompany the launch of a short gamma-ray burst. We investigate the influence of magnetic fields on HMNSs by performing three-dimensional simulations in general-relativistic magnetohydrodynamics. In particular, we provide direct evidence for the occurrence of the magnetorotational instability (MRI) in HMNS interiors. For the first time in simulations of these systems, rapidly-growing and spatially-periodic structures are observed to form with features like those of the channel flows produced by the MRI in other systems. Moreover, the growth time and wavelength of the fastest-growing mode are extracted and compared successfully with analytical predictions. The MRI emerges as an important mechanism to amplify magnetic fields over the lifetime of the HMNS, whose collapse to a black hole is accelerated. The evidence provided here that the MRI can actually develop in HMNSs could have a profound impact on the outcome of the merger of neutron-star binaries and on its connection to short gamma-ray bursts. PACS numbers: 97.60.Jd, 95.30.Qd, 97.60.Lf
Relativistic magnetohydrodynamics winds from rotating neutron stars
Monthly Notices of the Royal Astronomical Society, 2006
We solve for the time-dependent dynamics of axisymmetric, general relativistic magnetohydrodynamic winds from rotating neutron stars. The mass-loss rate as a function of latitude is obtained self-consistently as a solution to the magnetohydrodynamics equations, subject to a finite thermal pressure at the stellar surface. We consider both monopole and dipole magnetic field geometries and we explore the parameter regime extending from low magnetization (low σ 0), almost thermally driven winds to high magnetization (high σ 0), relativistic Poyntingflux-dominated outflows (σ = B 2 /4πργc 2 β 2 ≈ σ 0 /γ ∞ ,β = v/c with σ 0 = ω 2 2 /Ṁ, where ω is the rotation rate, is the open magnetic flux, andṀ is the mass flux). We compute the angular momentum and rotational energy-loss rates as a function of σ 0 and compare with analytic expectations from the classical theory of pulsars and magnetized stellar winds. In the case of the monopole, our high-σ 0 calculations asymptotically approach the analytic force-free limit. If we define the spindown rate in terms of the open magnetic flux, we similarly reproduce the spindown rate from recent force-free calculations of the aligned dipole. However, even for σ 0 as high as ∼20, we find that the location of the Y-type point (r Y), which specifies the radius of the last closed field line in the equatorial plane, is not the radius of the Light Cylinder R L = c/ω (R = cylindrical radius), as has previously been assumed in most estimates and force-free calculations. Instead, although the Alfvén radius at intermediate latitudes quickly approaches R L as σ 0 exceeds unity, r Y remains significantly less than R L. In addition, r Y is a weak function of σ 0 , suggesting that high magnetizations may be required to quantitatively approach the force-free magnetospheric structure, with r Y = R L. Because r Y < R L , our calculated spindown rates thus exceed the classic 'vacuum dipole' rate: equivalently, for a given spindown rate, the corresponding dipole field is smaller than traditionally inferred. In addition, our results suggest a braking index generically less than 3. We discuss the implications of our results for models of rotation-powered pulsars and magnetars, both in their observed states and in their hypothesized rapidly rotating initial states.
Accretion to magnetized stars through the Rayleigh–Taylor instability: global 3D simulations
Monthly Notices of the Royal Astronomical Society, 2008
We present results of 3D simulations of magnetohydrodynamics (MHD) instabilities at the accretion disc-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 disc 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 appears for relatively small misalignment angles, 30 • , between the star's rotation and magnetic axes, and is associated with higher accretion rates. The hotspots and light curves during accretion through instability are generally much more chaotic than during stable accretion. The unstable state of accretion has possible implications for quasi-periodic oscillations and intermittent pulsations from accreting systems, as well as planet migration.