I review recent observations and accretions disks and jets powered by young stellar objects.

Astrotomography resolves accretion flows with micro-arcsecond angular resolution. Eclipses by a binary companion star slice up a disk surface, giving monochromatic maps of the disk, or spectra from any region of its surface. Doppler tomography maps emission-line regions from the changing velocity profile as the binary rotates, revealing radial and azimuthal structure, gas streams, irradiated companion stars, magnetic flows, and slingshot prominences. Echo mapping exploits time delays between the hard radiation from near the compact object at the focus of the flow, and softer emission generated by irradiation of regions farther out. The maximum entropy techniques for fitting intensity maps to data are currently being extended by incorporating local physics and mapping physical parameters such as temperature, density, surface density, and velocity dispersion.

One of the most exciting results from ASCA has been the discovery of relativistic line profiles for iron Kα emission from Seyfert 1 galaxies. Recent results concerning the properties of Fe Kα lines from the inner regions of black-hole accretion disks are reviewed.

A variety of high energy (>1 keV) spectra have been observed in recent years from Black Hole (BH) and Neutron Star (NS) X-ray Binaries (XB). Some common physical components exist between BHXBs and NSXBs, resulting in some high energy spectral features. A common component between a BHXB and a weakly magnetized NSXB is the inner accretion disk region extending very close to the surface (for a NS) or the horizon (for a BH). The inner disk radiation can be described by a multi-color blackbody (MCB) spectral model. The surface radiation of the NS can be approximated by a Single Color Blackbody (SCB) spectrum. For a strongly magnetized NSXB, the high energy emission is from its magnetosphere, characterised by a thermal bremsstrahlung (TB) spectrum. In both BHXBs and weakly magnetized NSXBs, a hot electron cloud may exist, producing the hard X-ray power law (photon index −1.5 to −2.0) with thermal cutoff (50–200 keV). It has been recently proposed that a converging flow may be formed near the horizon of a BH, producing a softer power law (photon index about −2.5) without cutoff up to several hundred keV. Based on these concepts we also discuss possible ways to distinguish between BH and NS XBs. Finally we discuss briefly spectral state transitions in both BH and NS XBs.

The superluminal radio jets produced by GRO J1655–40 during a series of X-ray outbursts in 1994 were the first indications of a delayed form of relativistic outflows from black hole accretion disk environments. In this paper we review the relation between the radio jets and changes in the X-ray environment for the 1994 events in GRO J1655–40 and compare this behavior with similar X-ray-radio correlations that have since been found in GRS 1716–249, GRS 1739–278, and the 1996 recurrence of another correlated event in GRO 1655–40. We also discuss newly found radio-X-ray correlations in a galactic X-ray binary, Cyg X–3.

MOST 843 MHz flux densities are presented for the May 1996 outburst from GRO J1655—40. A deep radio image of the field reveals extended emission regions which may be associated with the radio jets. The optical spectrum during the 1994 outburst shows remarkable similarities to that of a Wolf-Rayet WN star.

A model for GRO J1655–40 is described in which the hard X/γ-ray behavior, and long delay between the X/γ and radio outbursts, are explained by processes which occur when the accretion rate approaches and exceeds the Eddington limit. The principal feature of the model is a dense, optically thick, super-Eddington wind ejected from the center of the accretion disk. The wind is responsible for determining the luminosity and spectral evolution of the object and for suppressing the formation of a fast, relativistic jet while the accretion rate is above the Eddington limit.

Our model makes use of the “magnetic switch” mechanism we recently discovered with MHD simulations of jet production in magnetized accretion disk coronae. A fast jet can be turned on (or off) by increasing (or decreasing) the Alfvén velocity in the corona relative to a critical value. Examination of models of sub- and super-Eddington disks shows that VA remains below the critical value while the wind is present, but could exceed it when the wind disappears and a hot, optically thin corona forms.

Recent work on advection-dominated accretion flows (ADAFs) is reviewed. The article concentrates on an optically thin branch of ADAFs which is present at mass accretion rates below a critical value ~ (10−2 – 10−1) the Eddington rate. Models based on this branch have been quite successful at explaining a number of low-luminosity X-ray binaries and galactic nuclei, and some brighter systems. Some progress has also been made toward understanding the various spectral states of accreting black holes. It is argued that ADAFs may provide one of the best techniques for demonstrating the reality of event horizons in black holes.

The nature of MHD and hydrodynamical turbulence in accretion disks is discussed. Comparison is made with planar Couette flow, a classical system prone to nonlinear shear instability resulting in enhanced turbulent transport. Both Keplerian and non-Keplerian hydrodynamical disks are studied, and it is found that only constant angular momentum disks are unstable to nonlinear disturbances and develop enhanced turbulent transport. Convective instabilities do not lead to enhanced turbulent transport. Hydrodynamical Keplerian disks are quite stable to nonlinear disturbances. Several lines of argument are presented which all lead to this conclusion, but the key to disk turbulence is the interaction between the stress tensor and the mean flow gradients. The nature of this coupling is found to determine completely the stability properties of disks (hydrodynamics and magnetic), and the nature of turbulent transport. The weak field MHD instability, which is of great astrophysical importance, displays the same type of stress tensor – mean flow coupling that all classical local shear instabilities exhibit. Hydrodynamical Keplerian disks, on the other hand, do not. Accretion disk turbulence is MHD turbulence.

The current status of understanding of the X-ray emission from Seyfert galaxies involves Comptonization of soft photons by hot subrelativistic electrons. After briefly reviewing the early theoretical basis for the presence of hot optically thin plasma in or around accretion disks and the key observations that led to develop the presently popular model of an accretion disk with a hot corona, we summarize recent progress in accretion models that take into account energy dissipation and/or angular momentum transport in the corona. Finally, adopting the simple scheme of a homogeneous plane parallel corona, we discuss in detail the dependence of the X-ray spectrum on the coronal parameters. Despite the strong coupling between optical depth and temperature which determines a spectral shape insensitive to their precise values, moderate spectral changes are possible. The spectral variability patterns can be used as diagnostics for coronal physics and should allow to determine whether the optical depth of the corona is dominated by e+e− pairs.

Extremely luminous extragalactic water masers – the so-called “megamasers”, with isotropic luminosities of tens to hundreds of solar luminosities – appear to be uniquely associated with active galactic nuclei. The recent survey of Braatz et al. indicates that 20% of Seyfert 2 galaxies have detectable water maser emission. Although originally suggested to arise in shocks, it now seems likely that the masers arise from the irradiation of high-pressure molecular gas by X-rays from the AGN. Quantitative modelling shows that the observed megamaser luminosities can plausibly be produced in this fashion. Both observational limits on the size scales and the high gas pressures required indicate that the water maser emission arises on very small scales, either in a circumnuclear “torus” or the accretion disk itself. In the best-studied case, NCG 4258, the masers are produced in a geometrically thin, warped accretion disk. The maser models can be used to derive quantitative information about the physical conditions in the disk, namely, the mass accretion rate, and therefore the radiative efficiency. I discuss the implications of water maser observations and models for the study of accretion disks and circumnuclear tori in AGN.

The range of microphysical and dynamical timescales at the centres of Active Galactic Nuclei (AGN) is sufficiently wide to permit multiphase structure. In particular, very dense, cool clouds can coexist with a hot, magnetically-dominated plasma. The strong dynamical forces in this central magnetosphere can give rise to clouds so small that microphysical processes determine whether they can survive for long enough to produce potentially observable spectral signatures due to thermal reprocessing. Specific physical constraints on the scale sizes of such reprocessing clouds are examined. It is found that there exists a parameter subspace in the extreme high density regime for which gas can sustain cool temperatures whilst maintaining pressure equilibrium with the ambient magnetosphere.

An overview is given of the spatial and time-dependent properties of the accretion discs in cataclysmic variable stars.

The properties of accretion disks in low mass X-ray binaries (LMXRB) are reviewed in the context of recent results from the ASCA observatory.

Disk-like structures have been inferred to exist in the nuclei of galaxies over the entire range in nuclear activity. These form the essence of the Unified Scheme, which has had great success in accounting for AGN of a wide variety of perceived types. Recent progress along this front is summarized, including new polarimetric results, high-angular resolution optical imaging, and interferometry at radio wavelengths.

The structure of accretion discs around (super-)massive black holes is discussed in this contribution with special emphasis on the radiation fields. These are of crucial importance for the understanding of these objects since photons control in most cases not only the temperature distributions but also the pressures and shapes. Recent progress in the modelling of photon fields now provides the means for a much improved understanding of the consequences of the multidimensional structure of the discs as well as of the effects of the strong space time curvature and of the high velocities involved. However, the simultaneous inclusion of the NLTE level populations and of many spectral lines is still a major problem. It is also demonstrated that special and general relativity effects strongly distort the apparent brightness distributions and spectra such accretion discs so that a solution of the inverse problem will be very difficult.

Models of vertical structure are calculated for a range of parameters applicable to dwarf novae with two types of viscosity being included: the standard α-disk viscosity and the additional turbulent viscosity in the convective regions. The resulting surface density (∑) υs. effective temperature (Te) relations, compared to those without convective viscosity, show larger separation between the two critical points of the ∑ – Te relation, ∑min and ∑max. This could suggest that with such a modification the dwarf nova outbursts could be reproduced with a single value of α. (Note that in the standard α approach two different α’s are needed on the hot and cool branch of the ∑ – Te relation, with αcool ≈ αhot/4). It turns out, however, that this is not the case. This is due to the fact that additional convective viscosity makes also ∑max smaller than in the pure α case.