arXiv subjects

H. Li

Publications and source records attributed to H. Li.

245 records · Page 14Linked to original sources

arXiv2000

The Origin of the Magnetic Fields of the Universe: The Plasma Astrophysics of the Free Energy of the Universe

(abridged) The interpretation of Faraday rotation measure maps of AGNs within galaxy clusters has revealed regions, $\sim 50-100$ kpc, that are populated with large, $\sim 30 μ$ G magnetic fields. The magnetic energy of these coherent regions is $\sim 10^{59-60}$ ergs, and the total magnetic energy over the whole cluster ($\sim 1$ Mpc across) is expected to be even larger. A sequence of physical processes that are responsible for the production, redistribution and dissipation of these magnetic fields is proposed. These fields are associated with single AGNs within the cluster and therefore with all galaxies during their AGN phase, simply because only the central supermassive black holes ($\sim 10^8 M_{\odot}$) have an accessible energy, $\sim 10^{61}$ ergs. We propose an $α-Ω$ dynamo process in an accretion disk. The disk rotation naturally provides a large winding number, $\sim 10^{11}$ turns, sufficient to make both large gain and large flux. The helicity of the dynamo can be generated by the differential plume rotation derived from star-disk collisions. This helicity generation process has been demonstrated in the laboratory and the dynamo gain was simulated numerically. A liquid sodium analog of the dynamo is being built. Speculations are that the back reaction of the saturated dynamo will lead to the formation of a force-free magnetic helix, which will carry the energy and flux of the dynamo away from the accretion disk and redistribute the field within the clusters and galaxy walls. The magnetic reconnection of a small fraction of this energy logically is the source of the AGN luminosity, and the remainder of the field energy should then dominate the free energy of the present-day universe.

astro-ph↗

arXiv2000

Rossby Wave Instability of Thin Accretion Disks - III. Nonlinear Simulations

(abridged) We study the nonlinear evolution of the Rossby wave instability in thin disks using global 2D hydrodynamic simulations. The key questions we are addressing in this paper are: (1) What happens when the instability becomes nonlinear? Specifically, does it lead to vortex formation? (2) What is the detailed behavior of a vortex? (3) Can the instability sustain itself and can the vortex last a long time? Among various initial equilibria that we have examined, we generally find that there are three stages of the disk evolution: (1) The exponential growth of the initial small amplitude perturbations. This is in excellent agreement with the linear theory; (2) The production of large scale vortices and their interactions with the background flow, including shocks. Significant accretion is observed due to these vortices. (3) The coupling of Rossby waves/vortices with global spiral waves, which facilitates further accretion throughout the whole disk. Even after more than 20 revolutions at the radius of vortices, we find that the disk maintains a state that is populated with vortices, shocks, spiral waves/shocks, all of which transport angular momentum outwards. We elucidate the physics at each stage and show that there is an efficient outward angular momentum transport in stages (2) and (3) over most parts of the disk, with an $α$ in the range from $10^{-4}$ to $10^{-2}$. We show why such vortices prove to be almost ideal ``units'' in transporting angular momentum outwards, namely by positively correlating the radial and azimuthal velocity components. We find some special cases in which entropy can remain the same while angular momentum is transported. We conclude that Rossby wave/vortex instability is an efficient, purely hydrodynamic mechanism for angular momentum transport in thin disks.

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arXiv2000

Electron Acceleration and Time Variability of High Energy Emission from Blazars

Blazars are known to emit a broad band emission from radio to gamma-rays with rapid time variations, particularly, in X- and gamma-rays. Synchrotron radiation and inverse Compton scattering are thought to play an important role in emission and the time variations are likely related to the acceleration of nonthermal electrons. As simultaneous multiwavelength observations with continuous time spans are recently available, some characteristics of electron acceleration are possibly inferred from the spectral changes of high energy emission. In order to make such inferences, we solve the time-dependent kinetic equations of electrons and photons simultaneously using a simple model for electron acceleration. We then show how the time variations of emission are dependent on electron acceleration. We also present a simple model for a flare in X-rays and TeV gamma-rays by temporarily changing the acceleration timescale. Our model will be used, in future, to analyze observed data in detail to obtain information on electron acceleration in blazars.

astro-ph↗

arXiv2000

Temporal and Spectral Variabilities of High Energy Emission from Blazars Using Synchrotron Self-Compton Models

Multiwavelength observations of blazars such as Mrk 421 and Mrk 501 show that they exhibit strong short time variabilities in flare-like phenomena. Based on the homogeneous synchrotron self-Compton (SSC) model and assuming that time variability of the emission is initiated by changes in the injection of nonthermal electrons, we perform detailed temporal and spectral studies of a purely cooling plasma system. One important parameter is the total injected energy E and we show how the synchrotron and Compton components respond as E varies. We discuss in detail how one could infer important physical parameters using the observed spectra. In particular, we could infer the size of the emission region by looking for exponential decay in the light curves. We could also test the basic assumption of SSC by measuring the difference in the rate of peak energy changes of synchrotron and SSC peaks. We also show that the trajectory in the photon-index and flux plane evolves clockwise or counter-clockwise depending on the value of E and observed energy bands.

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arXiv1999

Rossby Wave Instability of Thin Accretion Disks -- II. Detailed Linear Theory

In earlier work we identified a global, non-axisymmetric instability associated with the presence of an extreme in the radial profile of the key function ${\cal L}(r) \equiv (ΣΩ/κ^2) S^{2/Γ}$ in a thin, inviscid, nonmagnetized accretion disk. Here, $Σ(r)$ is the surface mass density of the disk, $Ω(r)$ the angular rotation rate, $S(r)$ the specific entropy, $Γ$ the adiabatic index, and $κ(r)$ the radial epicyclic frequency. The dispersion relation of the instability was shown to be similar to that of Rossby waves in planetary atmospheres. In this paper, we present the detailed linear theory of this Rossby wave instability and show that it exists for a wider range of conditions, specifically, for the case where there is a ``jump'' over some range of $r$ in $Σ(r)$ or in the pressure $P(r)$. We elucidate the physical mechanism of this instability and its dependence on various parameters, including the magnitude of the ``bump'' or ``jump,'' the azimuthal mode number, and the sound speed in the disk. We find large parameter range where the disk is stable to axisymmetric perturbations, but unstable to the non-axisymmetric Rossby waves. We find that growth rates of the Rossby wave instability can be high, $\sim 0.2 Ω_{\rm K}$ for relative small ``jumps'' or ``bumps''. We discuss possible conditions which can lead to this instability and the consequences of the instability.

astro-ph↗

arXiv1999

Correlated Spectral and Temporal Variability in the High-Energy Emission from Blazars

Blazar flare data show energy-dependent lags and correlated variability between optical/X-ray and GeV-TeV energies, and follow characteristic trajectories when plotted in the spectral-index/flux plane. This behavior is qualitatively explained if nonthermal electrons are injected over a finite time interval in the comoving plasma frame and cool by radiative processes. Numerical results are presented which show the importance of the effects of synchrotron self-Compton cooling and plasmoid deceleration. The use of INTEGRAL to advance our understanding of these systems is discussed.

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arXiv1998

On the Particle Heating and Acceleration in Black Hole Accretion Systems

The lack of our knowledge on how angular momentum is transported in accretion disks around black holes has prevented us from fully understanding their high energy emissions. We briefly highlight some theoretical models, emphasizing the energy flow and electron energization processes. More questions and uncertainties are raised from a plasma physics point of view.

astro-ph↗

arXiv1998

Rossby Wave Instability of Keplerian Accretion Disks

We find a linear instability of non-axisymmetric Rossby waves in a thin non-magnetized Keplerian disk when there is a local maximum in the radial profile of a key function ${\cal L}(r) \equiv {\cal F}(r) S^{2/Γ}(r)$, where ${\cal F}^{-1} = \hat {\bf z}\cdot ({\bf \nabla}\times {\bf v}) /Σ$ is the potential vorticity, $S = P/Σ^Γ$ is the entropy, $Σ$ is the surface mass density, $P$ is the vertically integrated pressure, and $Γ$ is the adiabatic index. We consider in detail the special case where there is a local maximum in the disk entropy profile $S(r)$. This maximum acts to trap the waves in its vicinity if its height to width ratio ${\rm max}(S)/Δr$ is larger than a threshold value. The pressure gradient derived from this entropy variation provides the restoring force for the wave growth. We show that the trapped waves act to transport angular momentum outward. A plausible way to produce an entropy variation is when an accretion disk is starting from negligible mass and temperature, therefore negligible entropy. As mass accumulates by either tidal torquing, magnetic torquing, or Roche-lobe overflow, confinement of heat will lead to an entropy maximum at the outer boundary of the disk. Possible nonlinear developments from this instability include the formation of Rossby vortices and the formation of spiral shocks. What remains to be determined from hydrodynamic simulations is whether or not Rossby wave packets (or vortices) ``hold together'' as they propagate radially inward.

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arXiv1997

Twisted representations of vertex operator algebras and associative algebras

Let V be a vertex operator algebra and g an automorphism of order T. We construct a sequence of associative algebras A_{g,n}(V) with n\in\frac{1}{T}\Z nonnegative such that A_{g,n}(V) is a quotient of A_{g,n+1/T}(V) and a pair of functors between the category of A_{g,n}(V)-modules which are not A_{g,n-1/T}(V)-modules and the category of admissible V-modules. These functors exhibit a bijection between the simple modules in each category. We also show that V is g-rational if and only if all A_{g,n}(V) are finite-dimensional semisimple algebras.

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arXiv1996

The radical of a vertex operator algebra

The radical $J(V)$ of a vertex operator algebra $V$ is defined to be the subspace of $V$ consisting of vectors $v$ such that the zero mode $o(v)=0$ on $V$ where $o(v)=v_{wt v-1}$ if $v$ is homogeneous. We establish various facts about $o(v),$ including the determination of $J(V)$ which is shown to be essentially equal to $(L(0)+L(-1))V.$

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arXiv1996

Associative subalgebras of the Griess algebra and related topics

It is shown how certain idempotents in the Griess algebra generate the discrete series representations for the Virasoro algebra inside the Frenkel-Lepowsky-Meurman's moonshine module vertex operator algebra. It is also shown that each Niemeier lattice determines (in many ways) certain maximal associative subalgebras of the Griess algebra.

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