Shinji Koide

Relativistic Jet Formation from Black Hole Magnetized Accretion Disks: Method, Tests, and Applications of a General RelativisticMagnetohydrodynamic Numerical Code

Relativistic jets are observed in both active galactic nuclei (AGNs) and iimicroquasars ˇˇ in our Galaxy. It is believed that these relativistic jets are ejected from the vicinity of black holes. To investigate the formation mechanism of these jets, we have developed a new general relativistic magnetohydrodynamic (GRMHD) code. We report on the basic methods and test calculations to check whether the code repro- duces some analytical solutions, such as a standing shock and a Keplerian disk with a steady state infall- ing corona or with a corona in hydrostatic equilibrium. We then apply the code to the formation of relativistic MHD jets, investigating the dynamics of an accretion disk initially threaded by a uniform poloidal magnetic —eld in a nonrotating corona (either in a steady state infall or in hydrostatic equilibrium) around a nonrotating black hole. The numerical results show the following: as time goes on, the disk loses angular momentum as a result of magnetic braking and falls into the black hole. The infalling motion of the disk, which is faster than in the nonrelativistic case because of general relativistic eUects below is the Schwarzschild radius), is strongly decelerated around by centrifugal 3r S (r S r \ 2r S force to form a shock inside the disk. The magnetic —eld is tightly twisted by the diUerential rotation, and plasma in the shocked region of the disk is accelerated by the JB force to form bipolar rela- tivistic jets. In addition, and interior to, this magnetically driven jet, we also found a gas-pressuredriven jet ejected from the shocked region by the gas-pressure force. This two-layered jet structure is formed not only in the hydrostatic corona case but also in the steady state falling corona case. Subject headings: accretion, accretion disksblack hole physicsgalaxies: jetsMHDrelativity

General Relativistic Simulations of Early Jet Formation in a Rapidly Rotating Black Hole Magnetosphere

To investigate the formation mechanism of relativistic jets in active galactic nuclei and microquasars, we have developed a new general relativistic magnetohydrodynamic code in Kerr geometry. Here we report on the —rst numerical simulations of jet formation in a rapidly rotating (a \ 0.95) Kerr black hole magnetosphere. We study cases in which the Keplerian accretion disk is both corotating and counter- rotating with respect to the black hole rotation, and investigate the —rst D50 light-crossing times. In the corotating disk case, our results are almost the same as those in Schwarzschild black hole cases: a gas pressuredriven jet is formed by a shock in the disk, and a weaker magnetically driven jet is also gener- ated outside the gas pressuredriven jet. On the other hand, in the counter-rotating disk case, a new powerful magnetically driven jet is formed inside the gas pressuredriven jet. The newly found magneti- cally driven jet in the latter case is accelerated by a strong magnetic —eld created by frame dragging in the ergosphere. Through this process, the magnetic —eld extracts the energy of the black hole rotation. Subject headings: accretion, accretion disksblack hole physicsgalaxies: jetsmagnetic —elds ¨ methods: numericalMHDrelativity

GENERAL RELATIVISTIC MAGNETOHYDRODYNAMIC SIMULATIONS OF JETS FROM BLACK HOLE ACCRETION DISKS: TWO-COMPONENT JETS DRIVEN BY NONSTEADY ACCRETION OF MAGNETIZED DISKS

The radio observations have revealed the compelling evidence of the existence of relativistic jets not only from active galactic nuclei but also from “microquasars” in our Galaxy. In the cores of these objects, it is believed that a black hole exists and that violent phenomena occur in the black hole magnetosphere, forming the relativistic jets. To simulate the jet formation in the magnetosphere, we have newly developed the general relativistic magnetohydrodynamic code. Using the code, we present a model of these relativistic jets, in which magnetic fields penetrating the accretion disk around a black hole play a fundamental role of inducing nonsteady accretion and ejection of plasmas. According to our simulations, a jet is ejected from a close vicinity to a black hole (inside , where is the Schwarzschild radius) at a maximum speed of »90% of the light velocity (i.e., a Lorentz 3 rr SS factor of »2). The jet has a two-layered shell structure consisting of a fast gas pressure‐driven jetin the inner part and a slow magnetically driven jet in the outer part, both of which are collimated by the global poloidal magnetic field penetrating the disk. The former jet is a result of a strong pressure increase due to shock formation in the disk through fast accretion flow (“advection-dominated disk”) inside 3 rS, which has never been seen in the nonrelativistic calculations. Subject headings: accretion, accretion disks — black hole physics — galaxies: jets — magnetic field — methods: numerical — MHD — relativity

A General Relativistic Magnetohydrodynamic Simulation of Jet Formation

We have performed a fully three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulation of jet formation from a thin accretion disk around a Schwarzschild black hole with a free-falling corona. The initial simulation results show that a bipolar jet (velocity ~0.3c) is created, as shown by previous two-dimensional axisymmetric simulations with mirror symmetry at the equator. The three-dimensional simulation ran over 100 light crossing time units (τS = rS/c, where rS ≡ 2GM/c2), which is considerably longer than the previous simulations. We show that the jet is initially formed as predicted owing in part to magnetic pressure from the twisting of the initially uniform magnetic field and from gas pressure associated with shock formation in the region around r = 3rS. At later times, the accretion disk becomes thick and the jet fades resulting in a wind that is ejected from the surface of the thickened (torus-like) disk. It should be noted that no streaming matter from a donor is included at the outer boundary in the simulation (an isolated black hole not binary black hole). The wind flows outward with a wider angle than the initial jet. The widening of the jet is consistent with the outward-moving torsional Alfven waves. This evolution of disk-jet coupling suggests that the jet fades with a thickened accretion disk because of the lack of streaming material from an accompanying star.

A Two-dimensional Simulation of a Relativistic Magnetized Jet

We have performed a magnetohydrodynamic simulation of a relativistic jet in slab geometry. The simulation code employs a simplified total variation diminishing method. We compare the weakly and strongly magnetized relativistic jets. The result shows that both relativistic effects and the parallel magnetic field collimate the jets. Stronger reversed magnetic fields are formed in the weakly magnetized jet than in the strongly magnetized jet.

General Relativistic Magnetohydrodynamic Simulations of Collapsars

We have performed 2.5-dimensional general relativistic magnetohydrodynamic (MHD) simulations of the gravitational collapse of a magnetized rotating massive star as a model of gamma-ray bursts (GRBs). The current calculation focuses on general relativistic MHD with simplified microphysics (we ignore neutrino cooling, physical equation of state, and photodisintegration). Initially, we assume that the core collapse has failed in this star. A few M☉ black hole is inserted by hand into the calculation. The simulations presented in the paper follow the accretion of gas into a black hole that is assumed to have formed before the calculation begins. The simulation results show the formation of a disklike structure and the generation of a jetlike outflow inside the shock wave launched at the core bounce. We have found that the jet is accelerated by the magnetic pressure and the centrifugal force and is collimated by the pinching force of the toroidal magnetic field amplified by the rotation and the effect of geometry of the poloidal magnetic field. The maximum velocity of the jet is mildly relativistic (~0.3c). The velocity of the jet becomes larger as the initial rotational velocity of stellar matter gets faster. On the other hand, the dependence on the initial magnetic field strength is a bit more complicated: the velocity of the jet increases with the initial field strength in the weak field regime, then is saturated at some intermediate field strength, and decreases beyond the critical field strength. These results are related to the stored magnetic energy determined by the balance between the propagation time of the Alfven wave and the rotation time of the disk (or twisting time).

General Relativistic Magnetohydrodynamic Simulations of Collapsars: Rotating Black Hole Cases

We have performed 2.5-dimensional general relativistic magnetohydrodynamic (MHD) simulations of collapsars including a rotating black hole. This paper is an extension of our previous paper. The current calculation focuses on the effect of black hole rotation using general relativistic MHD with simplified microphysics; i.e., we ignore neutrino cooling, physical equation of state, and photodisintegration. Initially, we assume that the core collapse has failed in this star. A rotating black hole of a few solar masses is inserted by hand into the calculation. We consider two cases, a corotating case and a counterrotating case with respect to the black hole rotation. Although the counterrotating case may be unrealistic for collapsars, we perform it as the maximally dragging case of a magnetic field. The simulation results show the formation of a disklike structure and the generation of a jetlike outflow near the central black hole. The jetlike outflow propagates outwardly with the twisted magnetic field and becomes collimated. We have found that the jets are generated and accelerated mainly by the magnetic field. The total jet velocity in the rotating black hole case is comparable to that of the nonrotating black hole case, ~0.3c. When the rotation of the black hole is faster, the magnetic field is twisted strongly owing to the frame-dragging effect. The magnetic energy stored by the twisting magnetic field is directly converted to kinetic energy of the jet rather than propagating as an Alfven wave. Thus, as the rotation of the black hole becomes faster, the poloidal velocity of the jet becomes faster. In the rapidly rotating black hole case the jetlike outflow can be produced by the frame-dragging effect only through twisting of the magnetic field, even if there is no stellar rotation.

Three-dimensional Magnetohydrodynamic Simulations of Relativistic Jets Injected along a Magnetic Field

We present the first numerical simulations of moderately hot, supersonic jets propagating initially along the field lines of a denser magnetized background medium with Lorentz factor W = 4.56 and evolving in a four-dimensional spacetime. Compared with previous simulations in two spatial dimensions, the resulting structure and kinematics differ noticeably: the density of the Mach disk is lower, and the head speed is smaller. This is because the impacted ambient fluid and its embedded magnetic field make efficient use of the third spatial dimension as they are deflected circularly off of the head of the jet. As a result, a significant magnetic field component normal to the jet is created near the head. If the field is strong, backflow and field reversals are strongly suppressed; upstream, the field closes back on the surface of the beam and assists the collimation of the jet. If the field is weak, backflow and field reversals are more pronounced, although still not as extended as in the corresponding plane-parallel case. In all studied cases, the high-pressure region is localized near the jet head irrespective of the presence/strength of the magnetic field, and the head decelerates efficiently by transferring momentum to the background fluid that recedes along a thin bow shock in all directions. Furthermore, two oppositely directed currents circle near the surface of the cylindrical beam, and a third current circles on the bow shock. These preliminary results underline the importance of performing fully three-dimensional simulations to investigate the morphology and propagation of relativistic extragalactic jets.

Relativistic outflow magnetically driven by black hole rotation

In the universe, several kinds of relativistic jets have been discovered, and it is believed that they are formed by violent phenomena near black holes. Despite the advancement of observations and black hole physics, their acceleration mechanisms are still not understood. Here, using numerical simulations, we show that the relativistic outflow is formed spontaneously by the magnetic field very near the rapidly rotating black hole. Previous simulations showed electromagnetic energy emission and nonrelativistic outflow formation in the black hole magnetosphere, but actual relativistic outflows have not yet been obtained. The present simulation shows that magnetic flux tubes, oblique to the rotation axis across the black hole horizon, are twisted into the shape of screws and propel plasma around the black hole to the relativistic regime.

A Two-dimensional Simulation of a Relativistic Jet Bent by an Oblique Magnetic Field

We have investigated a relativistic jet injected into an oblique magnetic field in a slab geometry using a newly developed relativistic magnetohydrodynamic simulation code. The simulation code employs a simplified total variation diminishing method. We compare the nonrelativistic and relativistic jets to find the relativistic effects when the jet propagates across the oblique magnetic field. Numerical results show that both the nonrelativistic and relativistic jets are bent by the oblique magnetic field. However, the relativistic jet is weakly bent compared to the nonrelativistic one. We try to express the bending scale detected in the simulation with a simple model and find good agreement between them. We apply the results to the explanation of the bending of the extragalactic jets.