The Hubble Space Telescope for the first time enables optical and UV images of jets to be obtained with spatial resolution comparable to radio interferometric techniques. Because synchrotron emission at these wavelengths is a diagnostic of particles several orders of magnitude higher in energy and correspondingly shorter in lifetime than those probed by radio, the sites of particle acceleration may be more readily identified in the optical and UV. Short-lifetime optical synchrotron emission in the SE inner radio lobe of M87 argues strongly for the presence of an invisible counterjet. A detailed comparison of HST and VLA images of the M87 jet shows there are very strong similarities in overall morphology, but that there are also significant differences. The optical and UV radiation is more localized within the knots than the radio emission and appears more confined toward the jet center. These differences may arise from localized shocks, sited at the optical knots with diffusion of relativistic particles away from those knots, or else they may be due to nonuniform magnetic fields and distributed acceleration processes. The UV fluxes of the knots are consistent with a single power law from optical to X-ray. Optical jets in other radio galaxies show diverse properties—the jet of PKS 0521–36 is smoothly resolved by HST while that of 3C 66B shows previously unsuspected filmentary structure. Likewise, the jet of 3C 273 shows newly observed optical filaments and an intensity distribution quite unlike that of the radio emission. Like M87, the optical jet of 3C 273 is narrower than the radio jet. The serendipitous discovery of a new optical synchrotron jet in 3C 264 suggests that optical jets may be relatively common.

Subject heading: galaxies: jets

During the period from 1992 May to early 1992 November, the Energetic Gamma-Ray Experiment Telescope (EGRET) on board the Compton Gamma Ray Observatory obtained high-energy gamma-ray data for most of the sky. A total of 18 active galaxies have been seen with high certainty, and it is expected that more will be found in the data when a more thorough analysis is complete. All of those that have been seen are radio-loud quasars or BL Lacertae objects; most have already been identified as blazars. No Seyfert galaxies have been found thus far. If the spectra are represented as a power law in energy, spectral slopes ranging from approximately −1.7 to −2.4 are found. A wide range of z-values exists in the observed sample, eight having values in excess of 1.0. Time variations have been seen, with the timescale for a significant change being as short as days in at least one case. These results imply the existence of very large numbers of relativistic particles, probably close to the central object. Although a large extrapolation is required, their existence also suggests that these active galactic nuclei may be the source of the extragalactic cosmic rays.

Subject headings: acceleration of particles — galaxies: active — gamma rays: observations — quasars: general

Two recent findings concerning high-energy radiation properties of active galactic nuclei—discovery of breaks in hard X-ray spectra of Seyfert galaxies, and discovery of huge fluxes of hard gamma rays from blazars—seem to press us to change our standard views about radiation production in these objects. I review briefly the existing radiation models, confront them with the newest observations, and discuss newly emerging theoretical pictures which attempt to account for the discoveries.

Subject headings: galaxies: active — galaxies: nuclei — radiation mechanisms: nonthermal

We review the transport and acceleration of cosmic rays concentrating on the origin of galactic cosmic rays. Quasi-linear theory for the acceleration rates and propagation parameters of charged test particles combined with the plasma wave viewpoint of modeling weak cosmic electromagnetic turbulence provides a qualitatively and quantitatively correct description of key observations. Incorporating finite frequency effects, dispersion, and damping of the plasma waves are essential in overcoming classical discrepancies with observations as the Kfit - Kql discrepancy of solar particle events. We show that the diffusion-convection transport equation in its general form contains spatial convection and diffusion terms as well as momentum convection and diffusion terms. In particular, the latter momentum diffusion term plays a decisive role in the acceleration of cosmic rays at super-Alfvénic supernova shock fronts, and in the acceleration of ultra-high-energy cosmic rays by distributed acceleration in our own galaxy.

Subject headings: acceleration of particles — convection — cosmic rays — diffusion — shock waves

Our current understanding of acceleration processes for Galactic and extragalactic cosmic rays is briefly reviewed. Shock acceleration in supernova remnants remains the most favored process for cosmic rays up to the “knee” of the all-particle total energy spectrum at 1014 - 1015 eV. The highest energy particles are almost certainly extragalactic, and the most favored sources are associated with active galactic nuclei in one way or another. The intermediate region between rigidities of 1014 and 1018 V is more difficult to understand, although a galactic origin is preferred at present. The problem of making a smooth join in the spectrum at the knee suggests that these particles should not be considered to be independent of those at lower energies.

Subject headings: acceleration of particles — cosmic rays — shock waves

Compton scattering of external radiation by nonthermal particles in outflowing blazar jets is dominated by accretion-disk photons rather than scattered radiation to distances ~ 0.01-0.1 pc from the central engine for standard parameters, thus clarifying the limits of validity of the model by the present authors and the model of Sikora, Begelman, & Rees. On the basis of contemporaneous Ginga X-ray and EGRET gamma-ray observations, we estimate the radius of 3C 279’s gamma-ray photosphere to be smaller than estimated by Blandford. There is thus no need to require that the acceleration and emission sites of gamma-ray blazars to be located farther than ~ 102 - 103 gravitational radii from the central engine. We argue that lineless BL Lac objects, rather than quasars, are more likely to be detected in the TeV energy range.

Subject headings: acceleration of particles — BL Lacertae objects: general — galaxies: jets — gamma rays: theory — radiation mechanisms: nonthermal

Accretion disks around rapidly rotating black holes provide one of the few plausible models for the production of intense radiation in AGNs above energies of several hundred MeV. The rapid rotation of the hole increases the binding energy per nucleon in the last stable orbit relative to the Schwarzschild case, and naturally leads to ion temperatures in the range 1012 -1013 K for sub-Eddington accretion rates. The protons in the hot inner region of a steady, two-temperature disk form a reservoir of energy that is sufficient to power the observed EGRET outbursts if the black hole mass is 1010M⊙ . Moreover, the accretion timescale for the inner region is comparable to the observed transient timescale of ~1 week. Hence EGRET outbursts may be driven by instabilities in hot, two-temperature disks around supermassive black holes. In this paper we discuss turbulent (stochastic) acceleration in hot disks as a possible source of GeV particles and radiation. We constrain the model by assuming the turbulence is powered by a collective instability that drains energy from the hot protons. We also provide some ideas concerning new, high-energy Penrose processes that produce GeV emission be directly tapping the rotational energy of Kerr black holes.

Subject headings: acceleration of particles — accretion, accretion disks — galaxies: nuclei — quasars: individual (3C 279) — radiation mechanisms: nonthermal

In the generally accepted models of high-luminosity extragalactic radio sources, the relativistic electron distribution evolves significantly outside of its source in the galactic nucleus; synchrotron losses occur throughout the source, and the harder (flatter) spectra seen in the hot spots are one indicator of relativistic particle acceleration. Our reanalysis and interpretation of the Carilli et al. (1991) data on Cygnus A, on the other hand, showed no evidence for evolution of the electron distribution function, and no evidence of the universally assumed injection power-law distribution. Having found a spectrum that describes an entire source, we have also introduced the technique of correcting synchrotron maps for frequency-dependent effects. In this way, a partial deconvolution of the underlying physical parameters is possible. In Cygnus A, this correction procedure led to the dominance of a hitherto unrecognized structure, an edge-brightened channel girdled by rings. We briefly discuss some implications of this work and our plans to continue this line of investigation.

Subject headings: acceleration of particles — galaxies: individual (Cygnus A) — galaxies: nuclei — radiation mechanisms: nonthermal — radio continuum: galaxies

Observations of GRO and UHEGR show that a number of Galactic and extragalactic accreting systems release most of their energy in the ultrarelativistic energy range (more than GeV). This result contradicts one of the principal conclusions of the standard models of accretion about a predominantly thermal character of energy release. This contradiction is caused by ignoring in the standard approach the processes of generation and amplification of magnetic field in the case of accretion onto a magnetized gravitating center. A new approach taking into account the processes mentioned above is applied to disk accretion onto a nonmagnetized gravitating center, as well as onto a magnetosphere. It is shown that in both cases the accretion is strictly controlled by the magnetic field, which leads to new conditions of equilibrium and stability and turns on nonthermal processes of energy release. The resulting configuration of the magnetic field, in which the main energy release takes place in both cases, is called “Z-pinch,” and is formed in the polar region of an accreting object. Effective particle acceleration occurs in it owing to the chain of MHD and resistive plasma instabilities, resulting in current discontinuity with the formation of “double layers” and generation of electric fields close to the Dreicer limit in them. The maximum energies of the accelerated particles are limited by the value 10 EeV, that coincides with the results of UHEGR observations.

Subject headings: acceleration of particles — accretion, accretion disks — gamma rays: theory — MHD

We present calculations of favored dynamo modes when advection of ambient magnetic fields onto accretion disks is important. These models are relevant for compact binary systems and young stellar objects and can be extended to active galactic nuclei (AGNs). The dynamo equation, including the standard α-effect, is modified to take into account advected magnetic fields. Vacuum boundary conditions are assumed outside the disk and the dynamo number switches sign across the equatorial plane. For the local steady state problem, critical dynamo numbers for various modes are obtained analytically. Our motivation is to investigate whether the dominant dynamo generation of quadrupolar magnetic fields and accretion of dipolar magnetic fields is likely to lead to particle acceleration in the form of jets. The results shown here are for a particular choice of boundary conditions and geometry of the advected field. Besides examining other choices, we shall calculate growth rates for different modes, and the influence of the initial seed field configuration on the evolution of the magnetic fields in subsequent work.

Subject headings: acceleration of particles — accretion, accretion disks — binaries: close — galaxies: nuclei — MHD — stars: pre-main-sequence

Time evolution of plane, cosmic-ray modified shocks has been simulated numerically for the case with parallel magnetic fields. Computations were done in a “three-fluid” dynamical model incorporating cosmic-ray and Alfvén-wave energy transport equations. Nonlinear feedback from the cosmic rays and Alfvén waves is included in the equation of motion for the underlying plasma, as is the finite propagation speed and energy dissipation of the Alfvén waves. Exploratory results confirm earlier, steady state analyses that found these Alfvén transport effects to be potentially important when the upstream Alfvén speed and gas sound speeds are comparable. As noted earlier, Alfvén transport effects tend to reduce the transfer of energy through a shock from gas to energetic particles. These studies show as well that the timescale for modification of the shock is altered in nonlinear ways. It is clear, however, that the consequences of Alfvén transport are strongly model dependent and that both advection of cosmic rays by the waves and dissipation of wave energy in the plasma will be important to model correctly when quantitative results are needed. Comparison is made between simulations based on a constant diffusion coefficient and more realistic diffusion models allowing the diffusion coefficient to vary in response to changes in Alfvén wave intensity. No really substantive differences were found between them.

Subject headings: cosmic rays — MHD — shock waves

We present the first results of time-dependent two-fluid cosmic-ray (CR) modified MHD shock simulations. The calculations were carried out with a new numerical code for one-dimensional ideal MHD. By coupling this code with the CR energy transport equation we can simulate the time-dependent evolution of MHD shocks, including the acceleration of the CR and their feedback on the shock structures. We report tests of the combined numerical method including comparisons with analytical steady state results published earlier by Webb, as well as internal consistency checks for more general MHD CR shock structures after they appear to have converged to dynamical steady states. We also present results from an initial time-dependent simulation which extends the parameter space domain of previous analytical models. These new results support Webb’s suggestion that equilibrium oblique shocks are less effective than parallel shocks in the acceleration of CR. However, for realistic models of anisotropic CR diffusion, oblique shocks may achieve dynamical equilibrium on shorter timescales than parallel shocks.

Subject headings: cosmic rays — methods: numerical — MHD — shock waves

A numerical solution to the problem of self-consistent diffusive shock acceleration is presented. The cosmic rays are scattered, accelerated and exert a back-reaction on the gas through their interaction with turbulence frozen into the local fluid frame. Using a grid with a hierarchical spacetime structure the physically interesting limit of Bohm diffusion (к ∝ pv), which introduces a wide range of diffusion lengthscales and acceleration timescales, can be studied. Some implications for modified shocks and particle acceleration are presented.

Subject headings: acceleration of particles — cosmic rays — diffusion — shock waves