The Acceleration Mechanism of Resistive MHD Jets Launched from Accretion Disks

We analyzed the results of non-linear resistive magnetohydrodynamical (MHD) simulations of jet formation to study the acceleration mechanism of axisymmetric, resistive MHD jets. The initial state is a constant angular momentum, polytropic torus threaded by weak uniform vertical magnetic fields. The time evolution of the torus is simulated by applying the CIP-MOCCT scheme extended for resistive MHD equations. We carried out simulations up to 50 rotation period at the innermost radius of the disk created by accretion from the torus. The acceleration forces and the characteristics of resistive jets were studied by computing forces acting on Lagrangian test particles. Since the angle between the rotation axis of the disk and magnetic field lines is smaller in resistive models than in ideal MHD models, magnetocentrifugal acceleration is smaller. The effective potential along a magnetic field line has maximum around $z \sim 0.5r_0$ in resistive models, where $r_0$ is the radius where the density of the initial torus is maximum. Jets are launched after the disk material is lifted to this height by pressure gradient force. Even in this case, the main acceleration force around the slow magnetosonic point is the magnetocentrifugal force. The power of the resistive MHD jet is comparable to the mechanical energy liberated in the disk by mass accretion. Joule heating is not essential for the formation of jets.Comment: 15 pages, 15 figures, 1 table, accepted for publication in Ap

Nonlinear Hydromagnetic Wave Support of a Stratified Molecular Cloud

We perform numerical simulations of nonlinear MHD waves in a gravitationally stratified molecular cloud that is bounded by a hot and tenuous external medium. We study the relation between the strength of the turbulence and various global properties of a molecular cloud, within a 1.5-dimensional approximation. Under the influence of a driving source of Alfvenic disturbances, the cloud is lifted up by the pressure of MHD waves and reaches a steady-state characterized by oscillations about a new time-averaged equilibrium state. The nonlinear effect results in the generation of longitudinal motions and many shock waves; however, the wave kinetic energy remains predominantly in transverse, rather than longitudinal, motions. There is an approximate equipartition of energy between the transverse velocity and fluctuating magnetic field (aspredicted by small-amplitude theory) in the region of the stratified cloud which contains most of the mass; however, this relation breaks down in the outer regions, particularly near the cloud surface, where the motions have a standing-wave character. This means that the Chandrasekhar-Fermi formula applied to molecular clouds must be significantly modified in such regions. Models of an ensemble of clouds show that, for various strengths of the input energy, the velocity dispersion in the cloud $\sigma \propto Z^{0.5}$, where $Z$ is a characteristic size of the cloud.Furthermore, $\sigma$ is always comparable to the mean Alfven velocity of the cloud, consistent with observational results.Comment: 16 pages, 15 figures, emulateapj, to appear in ApJ, 2003 Oct 1, higher resolution figures at http://www.astro.uwo.ca/~basu/pub.html or http://www.astro.uwo.ca/~kudoh/pub.htm

Analytical Studies on the Structure and Emission of the SS433 Jets

We study the structure and emission of the SS 433 jets in the X-ray emitting region and in the inner and hotter portion inside the X-ray emitting region. In order to consider the jet structure from the inner to outer regions we develop the hybrid model combining the conical beam and the model beam whose cross section grows with the distance more slowly. We find that the jet beams in the inner and hotter portion are of two-temperature and emit a large amount of high energy gamma photons. Our analyses suggest the thick absorbing envelope to exist in the SS 433 system. Based on our results, we discuss the possible acceleration mechanism for the SS 433 jets.Comment: 15 pages. Accepted for publication by Publ. Astron. Soc. Japa

Gravitational Wave Background from Neutrino-Driven Gamma-Ray Bursts

We discuss the gravitational wave background (GWB) from a cosmological population of gamma-ray bursts (GRBs). Among various emission mechanisms for the gravitational waves (GWs), we pay a particular attention to the vast anisotropic neutrino emissions from the accretion disk around the black hole formed after the so-called failed supernova explosions. The produced GWs by such mechanism are known as burst with memory, which could dominate over the low-frequency regime below \sim 10Hz. To estimate their amplitudes, we derive general analytic formulae for gravitational waveform from the axisymmetric jets. Based on the formulae, we first quantify the spectrum of GWs from a single GRB. Then, summing up its cosmological population, we find that the resultant value of the density parameter becomes roughly \Omega_{GW} \approx 10^{-20} over the wide-band of the low-frequency region, f\sim 10^{-4}-10^1Hz. The amplitude of GWB is sufficiently smaller than the primordial GWBs originated from an inflationary epoch and far below the detection limit.Comment: 6 pages, 4 figures, accepted for publication in MNRA

Six-dimensional localized black holes: numerical solutions

To test the strong-gravity regime in Randall-Sundrum braneworlds, we consider black holes bound to a brane. In a previous paper, we studied numerical solutions of localized black holes whose horizon radii are smaller than the AdS curvature radius. In this paper, we improve the numerical method and discuss properties of the six dimensional (6D) localized black holes whose horizon radii are larger than the AdS curvature radius. At a horizon temperature $\mathcal{T} \approx 1/2\pi \ell$, the thermodynamics of the localized black hole undergo a transition with its character changing from a 6D Schwarzschild black hole type to a 6D black string type. The specific heat of the localized black holes is negative, and the entropy is greater than or nearly equal to that of the 6D black strings with the same thermodynamic mass. The large localized black holes show flattened horizon geometries, and the intrinsic curvature of the horizon four-geometry becomes negative near the brane. Our results indicate that the recovery mechanism of lower-dimensional Einstein gravity on the brane works even in the presence of the black holes.Comment: 17 pages, 9 figures, RevTeX4, typos correcte

Second order perturbations in the radius stabilized Randall-Sundrum two branes model

The nonlinear gravitational interaction is investigated in the Randall-Sundrum two branes model with the radius stabilization mechanism. As the stabilization model, we assume a single scalar field which has the potential in the bulk and the potential on each brane. We develop the formulation of the second order gravitational perturbations under the assumption of the static and axial-symmetric 5-dimensional metric that is spherically symmetric in the 4-dimensional sense. After deriving the formal solutions for the perturbations, we discuss the gravity on each brane induced by the matter on its own side, taking the limit of the large coupling of the scalar field interaction term on the branes. We show using the Goldberger-Wise stabilization model that the 4-dimensional Einstein gravity is approximately recovered in the second order perturbations

Three-dimensional MHD Simulations of Jets from Accretion Disks

We report the results of 3-dimensional magnetohydrodynamic (MHD) simulations of a jet formation by the interaction between an accretion disk and a large scale magnetic field. The disk is not treated as a boundary condition but is solved self-consistently. To investigate the stability of MHD jet, the accretion disk is perturbed with a non-axisymmetric sinusoidal or random fluctuation of the rotational velocity. The dependences of the jet velocity $(v_z)$, mass outflow rate $(\dot{M}_w)$, and mass accretion rate $(\dot{M}_a)$ on the initial magnetic field strength in both non-axisymmetric cases are similar to those in the axisymmetric case. That is, $v_z \propto B_0^{1/3}$, $\dot{M}_w \propto B_0$ and $\dot{M}_a \propto B_0^{1.4}$ where $B_0$ is the initial magnetic field strength. The former two relations are consistent with the Michel's steady solution, $v_z \propto (B_0^2/\dot{M}_w)^{1/3}$, although the jet and accretion do not reach the steady state. In both perturbation cases, a non-axisymmetric structure with $m=2$ appears in the jet, where $m$ means the azimuthal wave number. This structure can not be explained by Kelvin-Helmholtz instability and seems to originate in the accretion disk. Non-axisymmetric modes in the jet reach almost constant levels after about 1.5 orbital periods of the accretion disk, while all modes in the accretion disk grow with oscillation. As for the angular momentum transport by Maxwell stress, the vertical component, $$, is comparable to the radial component,$$, in the wide range of initial magnetic field strength.Comment: Accepted for publication in ApJ. The pdf file with high resolution figures can be downloaded at http://www.kusastro.kyoto-u.ac.jp/~hiromitu/3j050806.pd

3-D General Relativistic MHD Simulations of Generating Jets

We have performed a first fully 3-D GRMHD simulation with Schwarzschild black hole with a free falling corona. The initial simulation results show that a jet is created as in previous axisymmetric simulations. However, the time to generate the jet is longer than in the 2-D simulations. We expect that due to the additional azimuthal dimension the dynamics of jet formation can be modified.Comment: 4 pages Proc. Oxford Radio Galaxy Workshop ed. R. Laing & K. Blundell (San Francisco: PASP) in press (revised

Thermodynamics of four-dimensional black objects in the warped compactification

We reinvestigate the thermodynamics of black objects (holes and strings) in four-dimensional braneworld models that are originally constructed by Emparan, Horowitz and Myers based on the anti-de Sitter (AdS) C-metric. After proving the uniqueness of slicing the AdS C-metric, we derive thermodynamic quantities of the black objects by means of the Euclidean formulation and find that we have no necessity of requiring any regularization to calculate their classical action. We show that there exist the Bekenstein-Hawking law and the thermodynamic first law. The thermodynamic mass of the localized black hole on a flat brane is negative, and it differs from the one previously derived. We discuss the thermodynamic stabilities and show that the BTZ black string is more stable than the localized black holes in a canonical ensemble, except for an extreme case. We also find a braneworld analogue of the Hawking-Page transition between the BTZ black string and thermal AdS branes. The localized black holes on a de Sitter brane is discussed by considering Nariai instanton, comparing the study of "black cigar" in the five-dimensional braneworld model.Comment: 15 pages, 4 figures, RevTex4, typos fixed, minor correction