Accretion disks around stellar-mass black holes are now thought to be the engines which power classical gamma-ray bursts (GRBs). These disks are formed almost exclusively in binaries, and to study the characteristics of the progenitors of these black-hole accretion disk (BHAD) GRBs, we must understand the uncertainties in binary population synthesis calculations. Kicks imparted onto nascent neutron stars and black holes are among the most misunderstood concepts of binary population synthesis. In this paper, we outline the current understanding (or lack of understanding) of these kicks and discuss their effect on BHAD GRBs and binary population synthesis as a whole.

We discuss the properties of the very energetic Type Ic supernovae (SNe Ic) 1998bw and 1997ef, and of Type IIn supernova (SN IIn) 1997cy. SNe Ic 1998bw and 1997ef are characterized by their large luminosity and very broad spectral features. Their observed properties can be explained if they are very energetic SN explosions (EK ≳ 1 × 1052 erg), originating probably from the core collapse of the bare C+O cores of massive stars (~ 30–40M⊙). At late times, both the light curve and the spectra suggest that the explosion may have been asymmetric; this may help us understand the claimed connection with GRBs. Type IIn SN 1997cy is even more luminous than SN 998bw, and the light curve declines more slowly than the 56Co decay. We model such a light curve with circumstellar interaction, which requires the explosion energy of ~ 5 × 1052 erg. Because these kinetic energies of explosion are much larger than in normal core-collapse SNe, we call objects like these SNe “hypernovae”.

The square of the four-momentum of a photon in vacuum is zero. However, in an unmagnetized plasma, it is equal to the square of the plasma frequency. Further, the electron-photon coupling vertex is modified in a plasma to include the effect of the plasma medium. I calculate the cross sections of three processes in a plasma—Compton scattering and electron-positron pair annihilation and production. At high plasma densities, the cross sections are found to change significantly. Such high plasma densities exist in several astrophysical sources.

A model for the spatial distribution of relativistic electrons in the Crab Nebula is proposed. Particles injected in the vicinity of the pulsar propagate in a magnetic field of time-dependent but spatially constant intensity. A good description of the nebular synchrotron emission, from radio to X-ray frequencies, is obtained if particle diffusion with respect to the azimuthal field lines is taken into account.

We have performed a 2.5D, nonsteady, general-relativistic MHD simulation. Initially, we assumed a uniform magnetic field, a geometrically thin accretion disk rotating at Keplerian velocity, and a hydrostatic corona around a Schwarzschild black hole. We have investigated the formation mechanism of gas-pressure driven jets expected by Koide et al. and found the strong dependence of jet velocities Lorentz factor of jets) on the ratio of the density of the accretion disk to that of the corona (ρd/ρc), where γ2j - γj ∝ (ρd/ρc)0.75.

Acceleration, propagation, and energy loss of particles energized in solar flares cannot be studied separately because their radiative signatures observed in the form of hard X-ray bremsstrahlung or radio gyrosynchrotron emission represent a convolution of all these processes. We analyze hard X-ray emission from solar flares using a kinematic model that includes free-streaming electrons (having an energy-dependent time-of-flight delay) as well as temporarily trapped electrons (which are pitch-angle scattered by Coulomb collisional scattering) to determine various physical parameters (trapping times, flux asymmetry, loss-cone angles, magnetic mirror ratios) in flare loops with asymmetric magnetic fields.

The Extreme Ultraviolet Explorer data archive has been used to study flare energy distributions and infer the role of flares in coronal heating. We have constructed flare number distributions as a function of observed EUV and X-ray emitted thermal energy. We find cumulative flare distributions of the form N(> E) ∝ E−α+1 with α ≍ 1.5–2.6. We present results in the context of spectral type classification.

We study the possibility that the recently detected hard X-ray excess from some clusters of galaxies is due to bremsstrahlung emission of a nonthermal tail in the electron distribution produced by stochastic acceleration in a background of MHD waves. We find that this scenario requires an overall level of fluctuations which is only δB2/B2 ~ 3% for a magnetic field B on the order of μG. Despite the reasonable values for these parameters, a large rate of injection of waves is needed to compensate the damping of the waves.

Accretion flows onto compact astronomical sources are likely to be supersonic, and shock waves may therefore be common in such flows. Plasma passing through a shock front will be compressed and heated according to the jump conditions across the shock discontinuity. Shocks in accretion flows may therefore have important consequences for the flow structure and emission characteristics. The equations governing adiabatic (nonradiative) shocks in relativistic plasmas are presented including the effects of radiation pressure and energy density, and pair equilibria in the postshock flow. We find that postshock states for accretion flows within cool, optically thick, accretion-driven sources such as AGN become radiation- or pair-dominated, and the postshock plasma will likely become optically thin before returning to steady-state conditions.

Plasma motion in pulsar magnetospheres is quasi-classical due to curvature radiation of highly energetic gamma-ray photons, implying an extension to the kinetic theory of plasmas. But with high energies involved, other quantum radiative processes become important in the context of vacuum (quantum) electrodynamics.

We discuss the detection of hard, power-law emission components in the X-ray spectra of six nearby, giant elliptical galaxies observed with the ASCA satellite and its implication for low-radiative efficiency accretion models around the central, supermassive black holes.

Blackman & Field have shown that a working α-Ω dynamo requires finite but opposite flows of small- and large-scale magnetic helicity through a body's surface. The helicity is accompanied by magnetic energy available for dissipation. The observed solar coronal nonthermal power is consistent with the derived lower limit required. This link between dynamo field generation and nonthermal emission should generally apply to stars, spiral galaxies, and accretion disks.

The Crab and other pulsars suffer sudden and permanent increases in their spin-down rates in association with glitches, suggesting that the external torque on these objects grows in steps. Here, we describe how torque changes may arise from starquakes, occurring as the star spins down and its rigid crust becomes less oblate. We study the evolution of strain in the crust, the initiation of starquakes, the effects on the magnetic field geometry, and possible observable consequences for neutron star spin down. We find that the stellar crust begins breaking at the rotational equator, forming a fault inclined at an angle to the equator and directed toward the magnetic poles. The resulting asymmetric matter redistribution produces a misalignment of the angular momentum and spin axes. Subsequently, damped precession to a new rotational state increases the angle between rotation and magnetic axes. The change in this angle could increase the external torque, producing a permanent increase in the spin-down rate.

We analyze quiescent radio spectra of main-sequence cool stars in terms of a simple gyrosynchrotron model. We find that the underlying (assumed) power-law electron distribution is often very hard, with power-law indices (in dN/dE ∝ E−δ) mostly around δ ≍2–3.5.

We investigate the reaction of a coronal loop in the case of repetitive flares, with a power-law distribution in energy, injected into a rigid magnetic loop. Emission measure distributions and temperature-dependent modulations of the radiation are briefly discussed.

Given the prevailing physical conditions in the cosmic environment, the similarity of solar transients and cosmic gamma-ray bursts suggests that the most promising energy source for the latter may be primordial flares, which derive energy from magnetic reconnection.

Using Chandrasekhar's MHD equations, we solve for the steady part of the internal rotation of AB Doradus. We estimate the size of the convective envelope to be ~ 40% of the radius and the rotation velocity at the base to be not less than 1.42 × 10−4 rad/sec. This study also yields the steady part of the toroidal magnetic field which is distributed throughout the convective envelope. We present rotational and toroidal magnetic field profiles in the interior and conjecture on the time-dependent part of the magnetic field.

We performed 2.5-dimensional, nonsteady MHD simulations of jets from geometrically thin accretion disks and investigated the acceleration forces of jets in detail.

Particle dynamics and nonthermal emission therefrom in the magnetospheres of collapsing stars with initial dipole magnetic fields and a certain initial energy distribution of charged particles (power-law, relativistic Maxwell, and Boltzmann distributions) are considered. The radiation fluxes are calculated for various collapsing stars with initial dipole magnetic fields and an initial power-law particle energy distribution in the magnetosphere. The effects can be observed by means of modern instruments.

We performed 2.5-dimensional, nonsteady MHD numerical simulations to investigate the acceleration and collimation of magnetically driven outflows from accretion disks, including the accretion process itself, consistently. As an initial condition, we used a paraboloidal magnetic field line that is produced by electric current on the equatorial plane. We found that the outflow ejected from the accretion disk is collimated by the pinch effect of the toroidal component of the magnetic field that is produced by the rotation of the disk.