Torus Systems Research Articles

Secular outflows from 3D MHD hypermassive neutron star accretion disc systems

ABSTRACT Magnetized hypermassive neutron stars (HMNSs) have been proposed as a way for neutron star mergers to produce high electron fraction, high-velocity ejecta, as required by kilonova models to explain the observed light curve of GW170817. The HMNS drives outflows through neutrino energy deposition and mechanical oscillations, and raises the electron fraction of outflows through neutrino interactions before collapsing to a black hole (BH). Here, we perform 3D numerical simulations of HMNS–torus systems in ideal magnetohydrodynamics, using a leakage/absorption scheme for neutrino transport, the nuclear APR equation of state, and Newtonian self-gravity, with a pseudo-Newtonian potential added after BH formation. Due to the uncertainty in the HMNS collapse time, we choose two different parametrized times to induce collapse. We also explore two initial magnetic field geometries in the torus, and evolve the systems until the outflows diminish significantly ($\sim\!\! 1\!\! - \!\!2\ \mathrm{s}$). We find bluer, faster outflows as compared to equivalent BH–torus systems, producing M ∼ 10−3 M⊙ of ejecta with Ye ≥ 0.25 and v ≥ 0.25c by the simulation end. Approximately half the outflows are launched in disc winds at times $t\lesssim 500 \ \mathrm{ms}$, with a broad distribution of electron fractions and velocities, depending on the initial condition. The remaining outflows are thermally driven, characterized by lower velocities and electron fractions. Nucleosynthesis with tracer particles shows patterns resembling solar abundances in all models. Although outflows from our simulations do not match those inferred from two-component modelling of the GW170817 kilonova, self-consistent multidimensional detailed kilonova models are required to determine whether our outflows can power the blue kilonova.

Impact of systematic nuclear uncertainties on composition and decay heat of dynamical and disc ejecta in compact binary mergers

ABSTRACT Theoretically predicted yields of elements created by the rapid neutron capture (r-)process carry potentially large uncertainties associated with incomplete knowledge of nuclear properties and approximative hydrodynamical modelling of the matter ejection processes. We present an in-depth study of the nuclear uncertainties by varying theoretical nuclear input models that describe the experimentally unknown neutron-rich nuclei. This includes two frameworks for calculating the radiative neutron capture rates and 14 different models for nuclear masses, β-decay rates, and fission properties. Our r-process nuclear network calculations are based on detailed hydrodynamical simulations of dynamically ejected material from NS–NS or NS–BH binary mergers plus the secular ejecta from BH–torus systems. The impact of nuclear uncertainties on the r-process abundance distribution and the early radioactive heating rate is found to be modest (within a factor of ∼20 for individual A > 90 abundances and a factor of 2 for the heating rate). However, the impact on the late-time heating rate is more significant and depends strongly on the contribution from fission. We witness significantly higher sensitivity to the nuclear physics input if only a single trajectory is used compared to considering ensembles with a much larger number of trajectories (ranging between 150 and 300), and the quantitative effects of the nuclear uncertainties strongly depend on the adopted conditions for the individual trajectory. We use the predicted Th/U ratio to estimate the cosmochronometric age of six metal-poor stars and find the impact of the nuclear uncertainties to be up to 2 Gyr.

Prospects for multi-messenger extended emission from core-collapse supernovae in the Local Universe

Multi-messenger emissions from SN1987A and GW170817/GRB170817A suggest a Universe rife with multi-messenger transients associated with black holes and neutron stars. For LIGO-Virgo, soon to be joined by KAGRA, these observations promise unprecedented opportunities to probe the central engines of core-collapse supernovae (CC-SNe) and gamma-ray bursts. Compared to neutron stars, central engines powered by black hole-disk or torus systems may be of particular interest to multi-messenger observations by the relatively large energy reservoir $E_J$ of angular momentum, up to 29\% of total mass in the Kerr metric. These central engines are expected from relatively massive stellar progenitors and compact binary coalescence involving a neutron star. We review prospects of multi-messenger emission by catalytic conversion of $E_J$ by a non-axisymmetric disk or torus. Observational support for this radiation process is found in a recent identification of ${\cal E}\simeq (3.5\pm1)\%M_\odot c^2$ in Extended Emission to GW170817 at a significance of 4.2\,$\sigma$ concurrent with GRB170817A. A prospect on similar emissions from nearby CC-SNe justifies the need for all-sky blind searches of long duration bursts by heterogeneous computing.

On the dynamics of tilted black hole–torus systems

We present results from three-dimensional, numerical relativity simulations of a {\it tilted} black hole-thick accretion disc system. The simulations are analysed using tracer particles in the disc which are advected with the flow. Such tracers, which we employ in these new simulations for the first time, provide a powerful means to analyse in detail the complex dynamics of tilted black hole-torus systems. We show how its use helps to gain insight in the overall dynamics of the system, discussing the origin of the observed black hole precession and the development of a global non-axisymmetric $m=1$ mode in the disc. Our three-dimensional simulations show the presence of quasi-periodic oscillations (QPOs) in the instantaneous accretion rate, with frequencies in a range compatible with those observed in low mass X-ray binaries with either a black hole or a neutron star component. The frequency ratio of the dominant low frequency peak and the first overtone is $o_1/f \sim 1.9$, a frequency ratio not attainable when modelling the QPOs as $p$-mode oscillations in axisymmetric tori.

Comprehensive nucleosynthesis analysis for ejecta of compact binary mergers

We present the first comprehensive study of r-process element nucleosynthesis in the ejecta of compact binary mergers (CBMs) and their relic black-hole (BH)-torus systems. The evolution of the BH-accretion tori is simulated for seconds with a Newtonian hydrodynamics code including viscosity effects, pseudo-Newtonian gravity for rotating BHs, and an energy-dependent two-moment closure scheme for the transport of electron neutrinos and antineutrinos. The investigated cases are guided by relativistic double neutron star (NS-NS) and NS-BH merger models, producing ~3-6 Msun BHs with rotation parameters of A~0.8 and tori of 0.03-0.3 Msun. Our nucleosynthesis analysis includes the dynamical (prompt) ejecta expelled during the CBM phase and the neutrino and viscously driven outflows of the relic BH-torus systems. While typically ~20-25% of the initial accretion-torus mass are lost by viscously driven outflows, neutrino-powered winds contribute at most another ~1%, but neutrino heating enhances the viscous ejecta significantly. Since BH-torus ejecta possess a wide distribution of electron fractions (0.1-0.6) and entropies, they produce heavy elements from A~80 up to the actinides, with relative contributions of A>130 nuclei being subdominant and sensitively dependent on BH and torus masses and the exact treatment of shear viscosity. The combined ejecta of CBM and BH-torus phases can reproduce the solar abundances amazingly well for A>90. Varying contributions of the torus ejecta might account for observed variations of lighter elements with 40<Z<56 relative to heavier ones, and a considerable reduction of the prompt ejecta compared to the torus ejecta, e.g. in highly asymmetric NS-BH mergers, might explain the composition of heavy-element deficient stars.

On the development of the Papaloizou–Pringle instability of the black hole–torus systems and quasi-periodic oscillations

We present the numerical study of dynamical instability of a pressure-supported relativistic torus, rotating around the black hole with a constant specific angular momentum on a fixed space-time background, in case of perturbation by a matter coming from the outer boundary. Two dimensional hydrodynamical equations are solved at equatorial plane using the HRSCS to study the effect of perturbation on the stable systems. We have found that the perturbed torus creates an instability which causes the gas falling into the black hole in a certain dynamical time. All the models indicate an oscillating torus with certain frequency around their instant equilibrium. The dynamic of the accreted torus varies with the size of initial stable torus, black hole spin and other variables, such as Mach number, sound speed, cusp location of the torus, etc. The mass accretion rate is slightly proportional to the torus-to-hole mass ratio in the black hole-torus system, but it strongly depends on the cusp location of the torus. The cusp located in the equipotential surfaces of the effective potential moves outwards into the torus. The dynamical change of the torus increases the mass accretion rate and triggers the Papaloizou-Pringle instability. It is also observed that the growth of the $m=1$ mode of the Papaloizou-Pringle instability occurs for a wide range of fluid and hydrodynamical parameters and a black hole spin. We have also computed the QPOs from the oscillating relativistic torus.

Adaptive system-level diagnosis for hypercube multiprocessors using a comparison model

Fault detection and localization are important in multiprocessor computing systems. Several methods of system self-diagnosis have been developed, including the model proposed by Preparata, Metze, and Chien (known as the PMC model) and a comparison model proposed by Maeng and Malek (the MM model). Adaptive diagnosis is a practical system-level diagnostic scheme whose main design objectives are to reduce the number of test rounds and the total number of tests. A number of interesting research results have been presented for adaptive diagnosis using the classical PMC model. However, adaptive diagnosis using the MM model has been discussed only in relation to completely connected systems and torus systems. In this paper we consider the problem of adaptive fault diagnosis in an n-dimensional hypercube using the MM model. The hypercube is an important interconnection network and an n-dimensional hypercube has been shown to be n-diagnosable using the MM model. The goal is to correctly identify the status of all processors, assuming that the number of faulty vertices does not exceed the hypercube dimension. For a cycle of N vertices, we show that at least four test rounds are necessary for complete diagnosis if N is not a multiple of three. We propose a scheme that completely diagnoses a 4-dimensional hypercube in at most seven test rounds and an n-dimensional hypercube, for n⩾5 in at most six test rounds and at most N+2n3+8n2 tests, where N=2n.

Adaptive diagnosis for torus systems under the comparison model

System level diagnosis is an important technique for fault detection and location in multiprocessor computing systems. Adaptive diagnosis, proposed by Nakajima, is one of many practical approach system level diagnostic schemes. As far as we know, the adaptive approach under the MM model has only been discussed in relation to a completely connected system. In this paper, we consider the problem of adaptive fault diagnosis for systems modelled by a cycle and a torus under the MM model. For cycles, we give some useful properties for identifying faulty vertices, show the minimum number of test rounds and provide some efficient test assignments. We also present two adaptive diagnosis algorithms for tori and show the minimum number of tests for these algorithms. Moreover, the two algorithms take linear time both for overall testing and syndrome decoding.

Gravitational Waves from the Papaloizou-Pringle Instability in Black-Hole-Torus Systems

Black hole (BH)-torus systems are promising candidates for the central engine of γ-ray bursts (GRBs), and also possible outcomes of the collapse of supermassive stars to supermassive black holes (SMBHs). By three-dimensional general relativistic numerical simulations, we show that an m = 1 nonaxisymmetric instability grows for a wide range of self-gravitating tori orbiting BHs. The resulting nonaxisymmetric structure persists for a time scale much longer than the dynamical one, becoming a strong emitter of large amplitude, quasiperiodic gravitational waves. Our results indicate that both, the central engine of GRBs and newly formed SMBHs, can be strong gravitational wave sources observable by forthcoming ground-based and spacecraft detectors.

Research plan for divertor simulation making use of a large tandem mirror device

We are planning to start a study of divertor simulation under the closely resemble to actual fusion plasma environment making use of the advantage of open magnetic field configuration and to contribute the solution for realizing the divertor in ITER as a future research plan of Plasma Research Center of the University of Tsukuba. In the research plan, the concepts of two divertor devices are introduced. One has an axi-symmetric divertor configuration with the separatrix which is similar to toroidal divertor of torus systems and the other is a high heat flux divertor simulator by using an end-mirror exit of the existing tandem mirror device. Development of magnetic field configuration for ensuring the MHD stability is under way and a designed example is investigated under the optimal condition for plasma production. Consideration of plasma heating scheme using Fokker–Planck simulation code was successfully performed at both axi-symmetric divertor and end-mirror regions. Preparative experiments using calorimeter, Mach probe and high-speed camera have been started at the end-mirror region and the heat flux density of the level in 1–10 MW m −2 was achieved in standard hot-ion mode plasma-confining experiments, which gives a clear prospect of generating the required heat flux density for divertor studies.

Influence of Self-Gravity on the Runaway Instability of Black-Hole–Torus Systems

Results from the first fully general relativistic numerical simulations in axisymmetry of a system formed by a black hole surrounded by a self-gravitating torus in equilibrium are presented, aiming to assess the influence of the torus self-gravity on the onset of the runaway instability. We consider several models with varying torus-to-black-hole mass ratio and angular momentum distribution orbiting in equilibrium around a nonrotating black hole. The tori are perturbed to induce the mass transfer towards the black hole. Our numerical simulations show that all models exhibit a persistent phase of axisymmetric oscillations around their equilibria for several dynamical time scales without the appearance of the runaway instability, indicating that the self-gravity of the torus does not play a critical role favoring the onset of the instability, at least during the first few dynamical time scales.

Relativistic outflows from remnants of compact object mergers and their viability for short gamma-ray bursts

We present the first general relativistic hydrodynamic models of the launch and evolution of relativistic jets and winds, driven by thermal energy deposition, possibly due to neutrino-antineutrino annihilation, in the close vicinity of black hole-accretion torus systems. The latter are considered to be the remnants of compact object mergers. Our two-dimensional simulations establish the link between models of such mergers and future observations of short gamma-ray bursts by the SWIFT satellite. They show that ultrarelativistic outflow with maximum terminal Lorentz factors around 1000 develops for polar energy deposition rates above some erg s-1 per steradian, provided the merger environment has a sufficiently low baryon density. By the interaction with the dense accretion torus the ultrarelativistic outflow with Lorentz factors Γ above 100 is collimated into a sharp-edged cone that is embedded laterally by a wind with steeply declining Lorentz factor. The typical semi-opening angles of the cone are , corresponding to about of the hemisphere and apparent isotropized energies (kinetic plus internal) up to ≈erg although at most of the deposited energy is transferred to the outflow with . The viability of post-merger black hole-torus systems as engines of short, hard gamma-ray bursts is therefore confirmed. The annihilation of neutrino-antineutrino pairs radiated from the hot accretion torus appears as a suitable energy source for powerful axial outflow even if only ≈erg are deposited within a cone of half-opening angle around the system axis. Although the torus lifetimes are expected to be only between some 0.01 s and several 0.1 s, our models can explain the durations of all observed short gamma-ray bursts, because different propagation velocities of the front and rear ends will lead to a radial stretching of the ultrarelativistic fireball before transparency is reached. The ultrarelativistic flow reveals a highly non-uniform structure caused by the action of Kelvin-Helmholtz instabilities that originate at the fireball-torus interface. Large radial variations of the baryon density (up to several orders of magnitude) are uncorrelated with moderate variations of the Lorentz factor (factors of a few) and fluctuations of the gently declining radiation-dominated pressure. In the angular direction the Lorentz factor reveals a nearly flat plateau-like maximum with values of several hundreds, that can be located up to off the symmetry axis, and a steep decrease to less than 10 for polar angles larger than . Lateral expansion of the ultrarelativistic core of the flow is prevented by a subsonic velocity component of about towards the symmetry axis, whereas the moderately relativistic wings show a subsonic sideways inflation with less than (measured in the frame comoving with the radial flow).

Multicast performance modeling and evaluation for high-speed unidirectional torus networks

This paper evaluates the performance of various unicast-based and path-based multicast protocols for high-speed torus networks. The results of an experimental case study on a Scalable Coherent Interface (SCI) torus network are presented. Small-message latency models of these software-based multicast algorithms as well as analytical projections for larger unidirectional torus systems are also introduced. The strengths and weaknesses of selected multicast protocols are experimentally and analytically illustrated in terms of various metrics, such as startup and completion latency, CPU utilization, and link concentration and concurrency for SCI networks under various networking and multicasting scenarios.

Proposed Gravitational Wave Background from Black Hole–Torus Systems

Cosmological gamma-ray bursts may be powered by rotating black holes with contemporaneous emission of gravitational radiation from a surrounding torus. We calculate the resulting stochastic background radiation assuming strong cosmological evolution and a uniform black hole mass distribution of M = 4-14 M☉. The predicted spectral flux density corresponds to a peak spectral closure density of (1-2) × 10-7 and has comparable contributions at 450 Hz × κ and over 300-450 Hz × κ from nearby and distant sources, respectively, where κ refers to an uncertainty factor of order unity in the radius of the torus. For two optimized advanced Laser Interferometer Gravitational-Wave Observatory-type detectors the proposed gravitational wave background could be detectable within a year of integration.

On the Stability of Thick Accretion Disks around Black Holes

Discerning the likelihood of the so-called runaway instability of thick accretion disks orbiting black holes is an important issue for most models of cosmic gamma-ray bursts. To this aim, we investigate this phenomenon by means of time-dependent, hydrodynamical simulations of black hole plus torus systems in general relativity. The evolution of the central black hole is assumed to be that of a sequence of Kerr black holes of increasing mass and spin, whose growth rate is controlled by the transfer of mass and angular momentum from the material of the disk spiraling in through the event horizon of the black hole. The self-gravity of the disk is neglected. We find that when the black hole mass and spin are allowed to increase, constant angular momentum disks undergo a runaway instability on a dynamical timescale (a few orbital periods). However, our simulations show that a slight increase of the specific angular momentum of the disk outward has a dramatic stabilizing effect. Our results, obtained in the framework of general relativity, are in broad agreement with earlier studies based both on stationary models and on time-dependent simulations with Newtonian and pseudo-Newtonian gravitational potentials.

Gamma-ray bursts: calorimetry on echoes in gravitational waves

Black hole–torus systems may represent astrophysical transient sources powered by the spin energy of a Kerr black hole. Equivalence in poloidal topology to pulsar magnetospheres suggests a long-lived state in suspended accretion when the black hole spins rapidly. The rotational energy is released in several channels: baryon-poor jets along open flux tubes extending from the horizon to infinity, gravitational radiation from the torus, winds and, conceivably, neutrino emissions. Gravitational radiation is expected to be representative of the rotational energy of the black hole, exceeding the output along the open flux tube by some two orders of magnitude. LIGO/VIRGO detections can hereby provide us with a calorimetric test of Kerr black holes.

Proposed source of gravitational radiation from a torus around a black hole.

Long gamma-ray bursts (GRBs) could be emitted from rapidly spinning black-hole-torus systems, resulting from either hypernovae or black-hole-neutron-star coalescence. We show that a nonaxisymmetric torus may also radiate gravitational radiation, powered by the spin energy of the black hole. The coupling to the spin energy of the black hole operates by equivalence in poloidal topology to pulsar magnetospheres. Results calculated in the suspended-accretion state indicate that GRBs are potentially the most powerful LIGO/VIRGO burst sources in the Universe, with an expected duration of 10-15 s on a horizontal branch of 1-2 kHz in the f(f) diagram.

The safety case for JET D-T operation

Formal approval was required to permit tritium operation of the JET machine to proceed and a safety case was prepared in order to justify this. The standards and the methodology used are summarised both for the Active Gas Handling System and for the torus systems. The main design safety principles are described and accident sequences are reviewed. Both deterministic and probabilistic analysis was carried out and the main results in terms of source terms, on-site and off-site doses are presented. It is shown that operation of JET with deuterium-tritium plasmas complies with the ALARP principle.

Runaway Instability of Black Hole--Neutron Torus Systems

view Abstract Citations (22) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Runaway Instability of Black Hole--Neutron Torus Systems Nishida, Shogo ; Eriguchi, Yoshiharu Abstract We have obtained fully general relativistic equilibrium configurations of black hole-neutron torus systems and have analyzed their stability. Although several authors have discussed whether such systems can be sources of gamma-ray bursts, their arguments have been based on the qualitative nature of black hole-neutron torus systems because such configurations have not yet been solved. Moreover, since black hole-neutron torus systems are supposed to form after coalescence of binary neutron stars or after collapse of a massive star, a newly formed torus may be in a rather hot state. Therefore we have developed a numerical code that can handle configurations of cold and hot (0.1, 1, and 10 MeV) neutron tori. Recently, polytropic tori around black holes have been shown to suffer a global runaway instability. As for neutron tori, the self-gravitating tori closely orbiting around the black holes with the mass range of 1-3 Msun are destroyed in a timescale ΔtT&tau; ≪ 1 s as a result of this instability. This is a much shorter timescale than the evolutionary timescale that is relevant for the current model of gamma-ray bursts based on the merger of binary neutron stars. This result probably excludes the possibility of neutron tori models for gamma-ray bursts as sources at cosmological distances. Publication: The Astrophysical Journal Pub Date: April 1996 DOI: 10.1086/177058 Bibcode: 1996ApJ...461..320N Keywords: ACCRETION; ACCRETION DISKS; STARS: BINARIES: CLOSE; BLACK HOLE PHYSICS; GAMMA RAYS: BURSTS; METHODS: NUMERICAL; STARS: NEUTRON full text sources ADS |

The JET preliminary tritium experiment

The first tokamak discharges with deuterium-tritium mixtures have been carried out in the Joint European Torus (JET). The main objectives were to produce more than IMW of fusion power in a controlled way, to determine tritium retention in torus systems and to establish effective means of tritium removal. The experiments were undertaken within limits imposed by restrictions on vessel activation and tritium usage. Deuterium plasmas were heated by high power deuterium neutral beams from fourteen sources and fuelled by two neural beam sources injecting tritium. In the best (D-T discharge), the tritium concentration was about 11% at peak performance, when total neutron emission rate was 6.0*1017 s-1, with 1.7 MW of fusion power. The fusion amplification factor QDT was 0.15. With optimum tritium concentration, this pulse would have produced a fusion power approximately=5 MW and nominal QDT=0.46. The same extrapolation for the best pure deuterium discharge gives about 11 MW and a nominal QDT=1.14. Techniques for introducing, tracking, monitoring and recovering tritium were highly effective.