A major uncertainty in the structure and dynamics of magnetized, radiation pressure dominated neutron star accretion columns in X-ray pulsars and pulsating ultraluminous X-ray sources is that they are thought to be subject to the photon bubble instability. We present the results of two dimensional radiation relativistic magnetohydrodynamic simulations of a non-accreting, static atmosphere to study the development of this instability assuming isotropic Thomson scattering in the slow diffusion regime that is relevant to neutron star accretion columns. Photon bubbles generally grow faster toward shorter wavelengths, until a maximum growth rate is achieved at the radiation viscosity length scale, which is generally quite small and requires high numerical resolution to simulate. We confirm the consistency between our simulation results and linear theory in detail, and show that the nonlinear evolution inevitably leads to collapse of the atmosphere with the higher resolution simulation collapsing faster due to the presence of shorter length scale nonlinear structures. At least in static atmospheres with horizontally periodic boundary conditions, this resolution dependence may make simulations of the nonlinear dynamics of photon bubble instability in neutron star accretion columns challenging. It remains to be seen whether these difficulties will persist upon inclusion of an accretion flow through the top and magnetically-confined horizontal boundaries through which photons can escape. Our results here provide a foundation for such future work.
We discuss basic features of steady accretion disk morphology around magnetized compact astrophysical objects. A comparison between the standard model of accretion based on visco-resistive MHD and the plasma instabilities, like ballooning modes, triggered by very low value of resistivity, is proposed.
This work presents an implementation of the resistive MHD equations for a generic algebraic Ohm's law which includes the effects of finite resistivity within full General Relativity. The implementation naturally accounts for magnetic-field-induced anisotropies and, by adopting a phenomenological current, is able to accurately describe electromagnetic fields in the star and in its magnetosphere. We illustrate the application of this approach in interesting systems with astrophysical implications; the aligned rotator solution and the collapse of a magnetized rotating neutron star to a black hole…
Neutron stars are among the most fascinating astrophysical sources, being characterized by strong gravity, densities about the nuclear one or even above, and huge magnetic fields. Their observational signatures can be extremely diverse across the electromagnetic spectrum, ranging from the periodic and low-frequency signals of radio pulsars, up to the abrupt high-energy gamma-ray flares of magnetars, where energies of ~10^46 erg are released in a few seconds. Fast-rotating and highly magnetized neutron stars are expected to launch powerful relativistic winds, whose interaction with the supernov…
The interactions between a relativistic magnetized collisionless shock and dense clumps have been expected to play a crucial role on the magnetic field amplification and cosmic-ray acceleration. We investigate this process by two-dimensional Particle-In-Cell (PIC) simulations for the first time, where the clump size is much larger than the gyroradius of downstream particles. We also perform relativistic magnetohydrodynamic (MHD) simulations for the same condition to see the kinetic effects. We find that particles escape from the shocked clump along magnetic field lines in the PIC simulations, …
We present in this paper both a linear study and numerical relativistic MHD simulations of the non-resonant streaming instability occurring in the precursor of relativistic shocks. In the shock front restframe, we perform a linear analysis of this instability in a likely configuration for ultra-relativistic shock precursors. This considers magneto-acoustic waves having a wave vector perpendicular to the shock front and the large scale magnetic field. Our linear analysis is achieved without any assumption on the shock velocity and is thus valid for all velocity regimes. In order to check our ca…
Shocks waves are a ubiquitous feature of many astrophysical plasma systems, and an important process for energy dissipation and transfer. The physics of these shock waves are frequently treated/modeled as a collisional, fluid MHD discontinuity, despite the fact that many shocks occur in the collisionless regime. In light of this, using fully kinetic, 3D simulations of non-relativistic, parallel propagating collisionless shocks comprised of electron-positron plasma, we detail the deviation of collisionless shocks form MHD predictions for varying magnetization/Alfvénic Mach numbers, with particu…
The properties of a new-born neutron star, produced in a core-collapse supernova, can be strongly affected by the possible late fallback which occurs several hours after the explosion. This accretion occurs in the regime dominated by neutrino cooling, explored initially in this context by Chevalier (1989). Here we revisit this approach in a 1D spherically symmetric model and carry out numerical simulations in 2D in an accretion column onto a neutron star considering detailed microphysics, neutrino cooling and the presence of magnetic fields in ideal MHD. We compare our numerical results to the…
We analyze a 900-ks stacked Chandra/HETG spectrum of NGC 3783 in the context of magnetically-driven accretion-disk wind models in an effort to provide tight constraints on the global conditions of the underlying absorbers. Motivated by the earlier measurements of its absorption measure distribution (AMD) indicating X-ray-absorbing ionic columns that decrease slowly with decreasing ionization parameter, we employ 2D magnetohydrodynamic (MHD) disk-wind models to describe the global outflow. We compute its photoionization structure along with the wind kinematic properties allowing us to further c…
We report work towards a relativistic formulation for modeling strongly magnetized neutron stars, rotating or in a close circular orbit around another neutron star or black hole, under the approximations of helical symmetry and ideal MHD. The quasi-stationary evolution is governed by the first law of thermodynamics for helically symmetric systems, which is generalized to include magnetic fields. The formulation involves an iterative scheme for solving the Einstein-Maxwell and relativistic MHD-Euler equations numerically. The resulting configurations for binary systems could be used as self-con…
Laplace-Fourier linear stability analysis of non-relativistic magnetized rotational jets: mode identification by Hamiltonian analysis and the occurrence of mode transitions
This work provides a concrete implementation of E. Fermi's model of particle acceleration in magnetohydrodynamic (MHD) turbulence, connecting the rate of energization to the gradients of the velocity of magnetic field lines, which it characterizes within a multifractal picture of turbulence intermittency. It then derives a transport equation in momentum space for the distribution function. This description is shown to be substantiated by a large-scale numerical simulation of strong MHD turbulence. The present, general framework can be used to model particle acceleration in a variety of environ…
The surfaces of neutron stars are likely sources of strongly polarized soft X rays due to the presence of strong magnetic fields. Scattering transport in the surface layers is critical to the determination of the emergent anisotropy of light intensity, and is strongly influenced by the complicated interplay between linear and circular polarization information. We have developed a magnetic Thomson scattering simulation to model the outer layers of fully-ionized atmospheres in such compact objects. Here we summarize emergent intensities and polarizations from extended atmospheric simulations, sp…
Relativistic magnetized jets, such as those from AGN, GRBs and XRBs, are susceptible to current- and pressure-driven MHD instabilities that can lead to particle acceleration and non-thermal radiation. Here we investigate the development of these instabilities through 3D kinetic simulations of cylindrically symmetric equilibria involving toroidal magnetic fields with electron-positron pair plasma. Generalizing recent treatments by Alves et al. (2018) and Davelaar et al. (2020), we consider a range of initial structures in which the force due to toroidal magnetic field is balanced by a combinati…
It is widely accepted that quasars and other active galactic nuclei (AGN) are powered by accretion of matter onto a central supermassive black hole. While numerical simulations have demonstrated the importance of magnetic fields in generating the turbulence believed necessary for accretion, so far they have not produced the high mass accretion rates required to explain the most powerful sources. We describe new global 3D simulations we are developing to assess the importance of radiation and non-ideal MHD in generating magnetized outflows that can enhance the overall rates of angular momentum …
We study the gravitational collapse and formation of the Kerr black hole from the rotating progenitor star. We follow the evolution of black hole spin, coupled with its increasing mass. We study the effect of different level of rotation endowed in the progenitor's envelope, and we out some constraints on the final black hole parameters. Our method is based on semi-analytical computations that involve stellar-evolution models of different progenitors. We also follow numerically the black hole evolution and spacetime metric changes during the collapse, via General Relativistic MHD modeling. (abr…
We examine the range of rest-mass densities, temperatures and magnetic fields involved in simulations of binary neutron star mergers (BNSM) and identify the conditions under which the ideal magneto-hydrodynamics (MHD) breaks down using recently computed conductivities of warm, magnetized plasma created in such systems. While previous dissipative MHD studies of BNSMs assumed that dissipation sets in due to low conduction at low rest-mass densities, we show that this paradigm must be shifted: the ideal MHD is applicable up to the regime where the hydrodynamic description of matter breaks down. W…
We conduct global galactic--scale magnetohydrodynamical (MHD) simulations of the cosmic--ray driven dynamo. We assume that exploding stars deposit small--scale, randomly oriented, dipolar magnetic fields into the differentially rotating ISM, together with a portion of cosmic rays, accelerated in supernova shocks. Our simulations are performed with the aid of a new parallel MHD code PIERNIK. We demonstrate that dipolar magnetic fields supplied on small SN--remnant scales, can be amplified exponentially by the CR--driven dynamo to the present equipartition values, and transformed simultaneously …
Collisionless, magnetized turbulence offers a promising framework for the generation of non-thermal high-energy particles in various astrophysical sites. Yet, the detailed mechanism that governs particle acceleration has remained subject to debate. By means of 2D and 3D PIC, as well as 3D (incompressible) magnetohydrodynamic (MHD) simulations, we test here a recent model of non-resonant particle acceleration in strongly magnetized turbulence~\cite{2021PhRvD.104f3020L}, which ascribes the energization of particles to their continuous interaction with the random velocity flow of the turbulence, …
We have written and tested a new general relativistic magnetohydrodynamics (GRMHD) code, capable of evolving MHD fluids in dynamical spacetimes with adaptive-mesh refinement (AMR). Our code solves the Einstein-Maxwell-MHD system of coupled equations in full 3+1 dimensions, evolving the metric via the Baumgarte-Shapiro Shibata-Nakamura (BSSN) formalism and the MHD and magnetic induction equations via a conservative, high-resolution shock-capturing scheme. The induction equations are recast as an evolution equation for the magnetic vector potential, which exists on a grid that is staggered with …
Global simulations of accretion flows onto perturbers embedded in magnetized disks -I. MRI and jet formation in ideal MHD
Charles University（查理大学）；Instituto de Astronomía, Universidad Nacional Autónoma de México（墨西哥国立自治大学天文研究所）；Niels Bohr International Academy, Niels Bohr Institute（尼尔斯·玻尔国际学院，尼尔斯·玻尔研究所）
The results of MHD numerical simulations of the formation and development of magnetized jets are presented. Similarity criteria for comparisons of the results of laboratory laser experiments and numerical simulations of astrophysical jets are discussed. The results of laboratory simulations of jets generated in experiments at the Neodim laser installation are presented.
Black hole-torus systems from compact binary mergers are possible engines for gamma-ray bursts (GRBs). During the early evolution of the post-merger remnant, the state of the torus is determined by a combination of neutrino cooling and magnetically-driven heating processes, so realistic models must include both effects. In this paper, we study the post-merger evolution of a magnetized black hole-neutron star binary system using the Spectral Einstein Code (SpEC) from an initial post-merger state provided by previous numerical relativity simulations. We use a finite-temperature nuclear equation …
In this paper we evolve magnetized and unmagnetized circumbinary accretion disks around supermassive black hole binaries in the relativistic regime. We use a post-Newtonian expansion to construct an analytical spacetime and determine how the order of the post-Newtonian (PN) expansion affects the dynamics of the gas. We find very small differences in the late-time bulk dynamics of non-magnetized hydrodynamic evolutions between the two spacetimes down to separations of approximately $40GM/c^2$ where $M$ is the total mass of the binary. For smaller separations, the differences due to PN-order bec…
Equilibriums of extremely magnetized compact stars with force-free magnetotunnels
University of the Ryukyus（琉球大学）；Tohoku University（东北大学）；Observatoire de Paris, Université PSL（巴黎天文台，巴黎文理研究大学）；University of Cambridge（剑桥大学）；National Center for Supercomputing Applications, University of Illinois at Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校超级计算应用国家中心）；Queen Mary University of London（伦敦大学玛丽女王学院）；University of Tokyo（东京大学）；Tokyo University of Technology（东京工科大学）；University of Illinois at Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校）
Astrophysical plasmas are turbulent and magnetized. The interaction between cosmic rays (CRs) and magnetohydrodynamic (MHD) turbulence is a fundamental astrophysical process. Based on the current understanding of MHD turbulence, we revisit the trapping of CRs by magnetic mirrors in the context of MHD turbulence. In compressible MHD turbulence, isotropic fast modes dominate both trapping and gyroresonant scattering of CRs. The presence of trapping significantly suppresses the pitch-angle scattering and the spatial diffusion of CRs along the magnetic field. The resulting parallel diffusion coeff…
Relativistic magnetized jets are key elements in Active Galactic Nuclei and in other astrophysical environments. Their structure and evolution involves a complex nonlinear physics that can be approached by numerical studies only. Still, owing to a number of challenging computational aspects, only a few numerical investigations have been undertaken so far. In this paper, we present high-resolution three dimensional numerical simulations of relativistic magnetized jets carrying an initially toroidal magnetic field. The presence of a substantial toroidal component of the field is nowadays commonl…
High-energy phenomena in astrophysics involve quite generally a combination of relativistic motions and strong gravity. The simultaneous solution of Einstein equations and General Relativistic MHD equations is thus necessary to model with accuracy such phenomena. The so-called Conformally Flat Condition (CFC) allows a simplified treatment of Einstein equations, that can be particularly efficient in those contexts where gravitational wave emission is negligible, like core-collapse, or the formation/evolution of neutron stars. We have developed a set of codes to model axisymmetric MHD flows, in …
Astrophysical fluids are turbulent, magnetized and frequently partially ionized. As an example of astrophysical turbulence, the interstellar turbulence extends over a remarkably large range of spatial scales and participates in key astrophysical processes happening in different ranges of scales. A significant progress has been achieved in the understanding of the magnetohydrodynamic (MHD) turbulence since the turn of the century, and this enables us to better describe turbulence in magnetized and partially ionized plasmas. In fact, the modern revolutionized picture of the MHD turbulence physic…
Coalescence of binary neutron stars (BNSs) is one of the sources of gravitational waves (GWs) able to be detected by ground-based interferometric detectors. The event GW170817 was the first observed in the gravitational and electromagnetic spectra, showing through this joint analysis a certain compatibility with the models of short gamma-ray bursts (sGRBs) to explain the signature of this system. Due to the intense magnetic fields of the neutron stars, the plasma magnetosphere stays strongly magnetized and the propagation of the GW through plasma can excite magnetohydrodynamic (MHD) modes such…
We present an extension to the general relativistic radiation magnetohydrodynamic code HARMRAD to account for emission and absorption by thermal cyclo-synchrotron, double Compton, bremsstrahlung, low-temperature OPAL opacities as well as Thomson and Compton scattering. We approximate the radiation field as a Bose-Einstein distribution and evolve it using the radiation number-energy-momentum conservation equations in order to track photon hardening. We perform various simulations to study how these extensions affect the radiative properties of magnetically-arrested disks accreting at Eddington …
We investigated the impacts of magnetorotational instability (MRI) on the dynamics of weakly magnetized, rapidly rotating core-collapse by conducting high resolution MHD simulations in axisymmetry with simplified neutrino transfer. We found that an initially sub-magnetar class magnetic field is drastically amplified by MRI and substantially affects the dynamics thereafter. Although the magnetic pressure is not strong enough to eject matter, the amplified magnetic field efficiently transfers angular momentum from higher to lower latitudes, which causes the expansion of the heating region at low…
A wide variety of astrophysical phenomena involve the flow of turbulent magnetized gas with relativistic velocity or energy density. Examples include gamma-ray bursts, active galactic nuclei, pulsars, magnetars, micro-quasars, merging neutron stars, X-ray binaries, some supernovae, and the early universe. In order to elucidate the basic properties of the relativistic magnetohydrodynamical (RMHD) turbulence present in these systems, we present results from numerical simulations of fully developed driven turbulence in a relativistically warm, weakly magnetized and mildly compressible ideal fluid…
