The paucity of old, isolated accreting neutron stars in ROSAT observations is used to derive a lower limit on the mean velocity of neutron stars at birth. The secular evolution of the population is simulated following the paths of a statistical sample of stars for different values of the initial kick velocity, drawn from an isotropic, Gaussian distribution with mean velocity 0 ≤ 〈V〉 ≤ 550 km s−1. The spin-down, induced by dipole losses and the interaction with the ambient medium, is tracked together with the dynamical evolution in the Galactic potential, allowing for the determination of the fraction of stars which are, at present, in each of the four possible stages: Ejector, Propeller, Accretor, and Georotator. Taking from the ROSAT All-Sky Survey an upper limit of ~ 10 accreting neutron stars within ~ 140 pc from the Sun, we infer a lower bound for the mean kick velocity, 〈V〉 ≳ 200–300 km s−1. The same conclusion is reached for both a constant (B ~ 1012 G) and an exponentially decaying magnetic field with a timescale ~ 109 yr. Present results, moreover, constrain the fraction of low-velocity stars which could have escaped pulsar statistics to ≲ 1%.

The synchrotron emission observed in plerions depends on the characteristics of the magnetic fields and relativistic electrons in these supernova remnants. Therefore, an analysis of the spectral and spatial properties of this emission, combined with models for the evolution and structure of plerions, would allow one to investigate the evolution of the synchrotron nebula, the structure of the magnetic field, and the distribution of the relativistic electrons, as well as to put constraints on the history of the energy input from the associated neutron star.

The identification of a new class of plerionic remnants, with spectral properties different from the Crab Nebula, has been proposed. The spectra of these objects typically show a sharp spectral break at very low frequencies (below 50 GHz), with a steep spectrum right beyond the break. In order to model the properties of their emission, a nonstandard evolution of the pulsar output seems to be required.

X-ray observations of the synchrotron emission from the Crab Nebula are shown. They are compared with previous data, and their implications on the structure of the Crab are discussed. Recent millimetric data of this object are also presented. A spatially resolved analysis, based on radio, millimetric, and X-ray data, will be carried out also for other plerions.

I discuss the impact of cooling flows on the observable X-ray properties of clusters of galaxies. I show that accounting for the effects of cooling flows and subcluster merger events leads to consistent determinations of the distribution of gravitating matter in clusters from X-ray and gravitational lensing studies. Accounting for the effects of cooling flows significantly reduces the scatter in the LBol/TX relation determined for the hottest, most luminous systems and changes the best-fitting slope of the relation to a value close to LBol ∝ T2X, in agreement with theoretical models. A clear segregation between the mean metallicities of cooling-flow and non-cooling-flow clusters is observed, which can be explained by the presence of metallicity gradients in the cooling-flow systems.

X-ray studies of ultrasoft narrow-line Seyfert 1 galaxies are revealing surprising new phenomena that are expanding our understanding of Seyfert activity, and they may well allow us to observe the effects of high mass accretion rates onto supermassive black holes. I briefly review the basic properties of these galaxies, their importance in a general context, and the prospects for studying them further in the future.

In the present paper, we stress the importance of the magnetic field in the problem of acceleration and collimation of astrophysical jets, and discuss our proposed generic picture for such “central gravitator + jets + lobes” systems and inherent interpretations of the various observational characteristics of such systems: Mechanisms are proposed for (1) the enhanced liberation of gravitational energy at the central object, (2) the transfer of a part of the liberated energy along the large-scale magnetic field by large-amplitude, torsional Alfvén wave trains that form collimated jets (we call this a sweeping pinch process), (3) the dumping of the transferred energy at the end of the jets when they impinge on the denser region outside the border of the “cavity” from which the mass contracted to the central condensation (central gravitator + accretion disk, as well as the larger-scale condensation surrounding them), and (4) the formation of wiggled jets and lobes as helical kinks and the tucked-up magnetic field produced in the sweeping pinch process, respectively.

We use a three-dimensional pulsar magnetosphere model to study the geometry of outer-magnetospheric gaps. The vertical size of the “outer gap” is first determined by a self-consistent model in which the outer gap size is limited by pair production from collisions between (1) thermal photons produced from polar cap heating by backflow “outer gap” current, and (2) the curvature photons emitted by gap-accelerated charged particles. The transverse size of the outer gap is also determined by local pair production limits. In principle, there are two topologically disconnected outer gaps in the magnetosphere of a pulsar. Both incoming and outgoing particle flows are allowed. However, the emission morphologies produced by incoming particle flow is severely restricted by local pair production in the gap and the absorption of magnetic pair production near the star. Double-peaked light curves with strong bridges are most common. From the three-dimensional structure of the outer gap and its local properties, we calculate the emission morphologies and phase-resolved spectra of gamma-ray pulsars. Applications to the Crab pulsar illustrate the model.

We present a fully relativistic study on the standing shock formation for magnetohydrodynamical plasmas in a stationary and axisymmetric black hole magnetosphere. We express all the postshock physical quantities in terms of the relativistic compression ratio. Then, the downstream state of a shocked plasma is determined by the upstream state of the accretion. We also discuss the dragging-effects of the rotating black hole on the shock conditions.

We present results from numerical simulations of magneto-hydrodynamic turbulence in accretion discs. Our simulations show that the turbulent stresses that drive the accretion are less stratified than the matter; thus, the surface layers are more strongly heated than the interior of the disc.

Relativistic plasma masers operating on the anomalous cyclotron-Cherenkov resonance (ω - k||v|| + ωB/γres = 0) and on the Cherenkovdrift resonance (ω - k||v|| - kxud = 0) are capable of explaining the main observational characteristics of pulsar radio emission. Both electromagnetic instabilities are due to the interaction of the fast particles of the primary beam and from the tail of the distribution with the normal modes of a strongly magnetized, one-dimensional electron-positron plasma. In a typical pulsar, both resonances occur in the outer parts of the magnetosphere at rres ≍ 109 cm.

I describe the evidence that Soft Gamma Repeaters are magnetars—neutron stars in which a decaying magnetic field (rather than rotation) is the dominant source of free energy. The focus here is on the bursting emission of these sources and on direct physical diagnostics of very strong magnetic fields (B ≳ 10 BQED = 4.4 × 1014 G). I also summarize the trapped fireball model of SGR outbursts, the influence of QED processes on their spectra and lightcurves, and the genetic connection between neutron star magnetism and the violent fluid motions in a collapsing supernova core.

Recent rotation-measure observations of a dozen or so galaxyclusters have revealed a surprisingly large number of magnetic fields whose estimated energy and flux are, on average, ~ 1058 ergs and ~ 1041 G cm2, respectively. These quantities are so much larger than any coherent sums of individual galaxies within the cluster that an efficient galactic dynamo is required. We associate these fields with single AGNs within the cluster and, therefore, with all galaxies during their AGN phase. Only the central, massive black hole (BH) has the necessary binding energy, ~ 1061 ergs. Only the accretion disk during the BH formation has the winding number, ~ 1011 turns, necessary to make the gain and magnetic flux. We present a model of a BH accretion-disk dynamo that might create these magnetic fields, where the helicity of the α-Ω dynamo is driven by star-disk collisions. The back reaction of the saturated dynamo forms a force-free field helix that carries the energy and flux of the dynamo and redistributes them within the clusters.

Recent general-relativistic MHD simulations of jets ejected from black-hole magnetospheres (for both Schwarzschild and Kerr holes) have revealed that (1) strong shock waves are formed in the accretion flow inside 3rs, (2) jets show two-layered shell structure consisting of a gas-pressure driven jet and a magnetically driven jet, the former being accelerated from a high-pressure region heated by strong shocks, and (3) in the case of a Kerr hole, magnetically driven jets are produced from the ergosphere by the effect of frame dragging.

We review the basic, observationally driven requirements for a successful model of particle acceleration in impulsive solar flares and then evaluate the viability of three classes of acceleration mechanisms. We argue that stochastic resonant acceleration is by far the most promising of the mechanisms.

The acceleration of particles in the vicinity of astrophysical shocks has become the main paradigm in astrophysics for the production of superthermal and relativistic particles. In this review, I consider the basic theory of shock acceleration and the open questions.

The surprising lack of a high energy cutoff in the cosmic ray spectrum at the highest energies, together with an apparently isotropic distribution of arrival directions, have strongly challenged most models proposed for the acceleration of ultrahigh energy cosmic rays. Young neutron star winds may be able to explain the mystery. We discuss this recent proposal after summarizing the observational challenge and plausible acceleration sites. Young neutrons star winds differ from alternative models in the predictions for composition, spectrum, and angular distribution, which will be tested in future experiments.

Fast magnetic reconnection in extragalactic jets leads to electron acceleration by the DC electric field in the reconnecting current sheet. The maximum electron energy (γ > 106) and the acceleration time (< 106 s) are determined by the magnetic field dynamics in the sheet.

The Very Large Array has been used to detect nonthermal radio emission from nearby stars of late spectral type F, G, K, and M, and has provided unique high-resolution investigations of the Sun's radio emission.

The generation of superthermal electron populations due to resonant energy transfer from Alfvén wave packets can be studied sufficiently well within the Fokker-Planck diffusive formalism, where the particle energization is induced through Landau interaction. From the calculation of the magnetic-field-dependent diffusion operator of an Alfvén wave spectrum, it is demonstrated how resonant energization can be forced in nonuniform magnetic field configurations. On this basis, a mechanism for the acceleration of electrons from thermal to relativistic energies is presented where the specific magnetic field geometry acts as catalyst, providing a highly effective source for synergetic electron energization.

When a black hole accretes slowly, the radiative cooling of the infalling gas is weak and the accretion disk does not form. A hot, collisionless, quasi-spherical halo is formed instead. The properties of such halos are discussed. The rate of accretion, the radiative efficiency, and the temperature and density near the hole are evaluated.

The origin of GRBs has been a mystery for almost 30 years. Their sources emit a huge amount of energy on short time scales, and the process involves extreme relativistic motion with a bulk Lorentz factor of at least a few hundred. In the last two years, “afterglow” emission in X-ray, optical, IR, and radio was detected. The afterglow can be measured up to months and even years after the few-seconds GRB. We review the theories for the γ-ray emission and its afterglow, and show that these are strongly supported by observations. A recent detection of optical emission simultaneous with the GRB agrees well with theoretical predictions and further constrains the free parameters of the models. We discuss the evidence that some of the bursts are jets and discuss the prospects of polarization measurements.