We investigate the global stability of a differentially rotating, ideal MHD fluid shell to linear, nonaxisymmetric perturbations. This system, which approximates an accretion disk near its midplane, is known to be unstable to both axisymmetric and nonaxisymmetric local perturbations. Keplerian disks, which are entirely stable in the hydrodynamic case, show the most rapid growth for radially extended shells.

I review the current status of the investigations of the kilohertz quasi-periodic oscillations that have been discovered in low-mass Xray binaries (LMXBs) with NASA’s Rossi X-ray Timing Explorer (RXTE) since February 1996. Seven sources have now shown this new phenomenon, one Z source (Sco X–l) and 6 atoll sources (all X-ray bursters), and some patterns are beginning to emerge. The frequencies of the oscillations are between 500 and 1200 Hz. They can be coherent for more than 102 cycles, but a more typical coherence is several 101. Amplitudes are between 0.5 and 16% (rms) of the total flux. Most sources show double kHz peaks separated by a few 102 Hz, which move up and down in frequency together. In Sco X–l the peak frequencies systematically increase, and the peak separation systematically decreases, with Ṁ. In 4U 1728–34 a third oscillation is seen at a frequency equal to the difference frequency of the two kHz peaks, but only during X-ray bursts. A simple interpretation in terms of a beat-frequency model with the difference frequency being the neutron star’s spin, as suggested by this result, is inconsistent with the fact that in Sco X–l the peak separation varies.

I review several problems which arise when one tries to apply the standard dwarf nova instability model to outbursts of soft X-ray transients

We present a series of time-dependent 2D and 3D numerical simulations illustrating the evolutionary sequence between high mass X-ray binaries fed by wind accretion (where the primary star sits well within its critical tidal lobe) and those fed by Roche lobe overflow (where the primary star extends out to its tidal lobe). When the primary lies well within its critical surface we find negligible tidal mass loss enhancement, and a system that is characterized by wind accretion with the development of a photoionization zone around the compact object. As the surface of the primary nears the critical surface, we observe tidally enhanced mass loss via a thin tidal stream, resulting in higher accretion wake densities. For full RLOF we observe the development of a steady accretion disk characterized by a total shadowing of the X-rays in the orbital plane.

With the advent of RXTE, which is capable of broad spectral coverage and fast timing, as well as other instruments which are increasingly being used in multi-wavelength campaigns (via both space-based and ground-based observations), we must demand more of our theoretical models. No current model mimics all facets of a system as complex as an x-ray binary. However, a modern theory should qualitatively reproduce — or at the very least not fundamentally disagree with – all of Cygnus X–1’s most basic average properties: energy spectrum (viewed within a broader framework of black hole candidate spectral behavior), power spectrum (PSD), and time delays and coherence between variability in different energy bands. Below we discuss each of these basic properties in turn, and we assess the health of one of the currently popular theories: Comptonization of photons from a cold disk. We find that the data pose substantial challenges for this theory, as well as all other currently discussed models.

SS 433 has been observed with ASCA for three years since the launch in 1993. The excellent energy resolution of ASCA revealed the Doppler-shifted emission lines from the both jets, though it had been generally accepted that the X-ray emitting region of the receding jet is hidden behind the accretion disk. Thus the estimation on the properties of the X-ray jet, such as length, temperature, and mass outflow rate, should be revised in accordance with the ASCA data. Modeling the jet as a radiating ballistic plasma, we determined these quantities. It is also tried to determine the mass of the system from a data covering an eclipse.

The lightcurves of non-magnetic X-ray binaries are usually modelled using standard techniques developed for ordinary double stars plus some assumptions about the geometry and temperature distribution of the accretion disk. The analyses of the observed optical lightcurves of systems suggest that the disks must have extremely large rims (H/R ~ 0.3) in order to produce the variable obscuration needed to reproduce the often asymmetric lightcurves. However, even models with large rims have problems explaining the X-ray absorption seen in systems with smaller inclinations. We discuss two phenomena which show that non-standard effects in X-ray binaries can play major roles in the determination of the lightcurves: the “spray” caused by the passage of the accretion stream through the “bow-shock” at the outer rim of the disk; and the effects of soft X-ray illumination of the secondary star.

Based on the detailed analysis of UBV–photometry of HZ Her obtained in 1972–1994, a model of 35–day cycle is proposed in which both forced precession of a tilted accretion disk and free precession of the neutron star are present.

We have performed three-dimensional hydrodynamical simulations of the coalescence of binary neutron stars taking into account the emission and backreaction of gravitational waves in the Newtonian code based on the “Piecewise Parabolic Method”. The use of the physical equation of state (EOS) of Lattimer & Swesty (1991) allowed us to calculate the production of neutrinos. We evaluated our models for the efficiency of v⊽ annihilation in the surroundings of the coalescing neutron stars. The corresponding energy deposition prior to and during merging turned out to be 2–3 orders of magnitude too small to power a typical γ-ray burst (GRB) with an energy output of ~ (1051/4π) erg/sterad at cosmological distances. Analytical estimates of the subsequent evolution of the disk which possibly surrounds the central black hole showed that even under the most favorable conditions the energy provided by v⊽ → e−e+ → γγ falls short by at least an order of magnitude. We discuss the implications of our results and speculate about possibilities how v⊽ annihilation might still be a viable energy source for GRBs.

AM Herculis Binaries (or Polars) are a subclass of the Cataclysmic Variables in which the accreting white dwarf has a strong magnetic field giving rise to highly polarized cyclotron radiation from the shock heated accretion region. A number of AM Herculis binaries are now known in which the white dwarf is eclipsed by the companion star. High time resolution observations of these eclipses allow a particularly detailed study of the process of accretion onto the magnetic white dwarf. Results on a number of systems will be presented and used to derive information on the accretion structure as well as on the fundamental properties of the binaries.

Time-resolved photometry and spectroscopy of cataclysmic variables (CV) are powerful tools for clarifying the physical size, placement, and conditions of emission regions in these compact systems. We present HST time resolved UV spectroscopic data for one self-eclipsing mCV system, ST LMi, and two secondary eclipsing systems, DP Leo and UZ For. These observations clearly show the relative contributions of three emission components: the white dwarf, a hotter region surrounding the accretion shock, and the photoionized accretion stream. UZ For was observed in an unusually active state by both HST and EUVE. These data provide strong evidence that the accretion stream is “clumpy”, with overdensities 100–1000 times the average density. Using photoionization codes such as XSTAR and CLOUDY, we find that steady, supersonic, and homogenous flow in the UZ For accretion stream is unstable to compression heating. As a consequence, we propose a two-phase model for the stream and obtain density enhancements comparable to those deduced from the strengths of the UV emission lines and EUVE absorption features. Similar behavior may be expected from active, asynchronous magnetic CVs with coupling radii greater than several white dwarf radii.

In this paper, I present a review on the properties of accretion curtains in magnetic Cataclysmic Variables and T Tauri stars. In particular, I will show that magnetically confined accretion curtains that are heated by hard, soft x-rays or UV radiation originating from the accretion shocks at the surface of the accreting star can give rise to the observed complex emission line profiles.

We present results from IUE and optical spectra, optical photometry and circular polarimetry during high and low states of the highly luminous soft X-ray cataclysmic variable AR UMa that identifies the primary in this system as a white dwarf with a magnetic field of 230 MG. The high magnetic field likely threads accretion blobs all the way from the secondary to below the surface of the white dwarf, resulting in a lack of polarised cyclotron emission and an extreme soft-X-ray luminosity during the high state.

We have used the hard X-ray spectrum obtained from Ginga to determine the mass of the white dwarf primary star in 13 magnetic CV systems. Our model for the spectral fit includes the temperature and density structure in the post-shock accretion flow, taking into account the additional cooling of the cyclotron radiation. We obtain good fits to the data, and we compare our derived masses to those determined by other means.

We present preliminary results of spectropolarimetric observations of several cataclysmic variables using the ISIS spectropolarimeter on the 4.2m William Hershell Telescope at La Palma. Our aim is to look for Zeeman signatures of magnetic fields in cataclysmic variables. This would provide a direct measurement of magnetic fields in the accretion flows of CVs. Our current dataset allows us to set upper limits to the presence of magnetic fields and constrain the field geometry in different types of cataclysmic variables.

I review the evidence for disc and discless accretion in the asynchronous magnetic CVs, the intermediate polars (IPs). The presence and relative amplitudes of the spin and synodic (beat) X-ray periods, and related orbital sidebands, are very model dependent, and support disc accretion dominating in all the well-studied IPs. A component of non-disc accretion is sometimes observed, a result of stream overflow penetrating to the magnetosphere, and is responsible for the synodic period. The recently discovered polarized IP, RX J1712.6–2414, is a prime candidate (the first) for purely discless, stream-fed accretion in such a system. While the polarisation varies at the spin period, the X-ray variations occur at the synodic period, a result of the modulation in the mass transfer rate for a discless accretor.

We review the current understanding of accretion flows around compact objects with a special emphasis on advective disks. We discuss the influence of the centrifugal pressure supported high density region around compact objects (where shocks may also form) on the emitted spectra. We show that the stationary and non-stationary spectral properties (such as, low and high states, transition of states, quasi-periodic oscillations, quiescent and rising phases of X-ray novae, etc.) of both low mass and supermassive black hole candidates could be satisfactorily explained within the framework of the analytical solution of the advective disks without invoking any ad hoc components such as Compton clouds or magnetic corona.

We describe the dynamics of a class of MHD winds from Keplerian-rotating disks. In this model, all flow velocities are assumed to vary self-similarly with spherical radius r as υ(r, θ) ∝ r−1/2, with density varying as ρ(r, θ) ∝ r−q for arbitrary q. At large distances from the disk, the wind is explicitly required to become cylindrically collimated. We find that the asymptotic wind solution has power-law scalings of all flow variables in the cylindrical radius R = r sin θ, and q < 1 is necessary. We describe how the Alfvén criticality condition limits the space of energy and angular momentum parameters defining these wind solutions. We present an example of the run of density, velocity, and magnetic field for a full solution of the wind equations, and compare the properties of these cylindrically-collimated wind solutions to previous work.

We carried out three-dimensional global MHD simulations of jet formation from an accretion disk threaded by large-scale magnetic fields. Numerical results show that bipolar jets with maximum speed υjet ~ υKepler are created. The surface layer of the disk accretes faster than the equatorial part because magnetic braking most effectively affects that layer. Accretion proceeds along spiral channels which correspond to the surface avalanche flow appearing in previous axisymmetric simulations. Spirally shaped low β (= Pgas/Pmag < 1) regions appear in the innermost part of accretion disks where toroidal magnetic fields become dominant.

We have obtained ultraviolet spectra of the Herbig Ae star HD104237 using the Goddard High Resolution Spectrograph (GHRS) on HST. The high temperature emission and absorption lines show remarkable outflow absorption features, which have very similar profiles that are essentially independent of formation temperature. The profiles are not those expected from a spherically-symmetric stellar wind but have optically-thick absorption to −280 km s−1 and a high velocity plateau extending to −375 km s−1. The profile shape is a manifestation of the nonspherical geometry of the flow. The observed UV emission is too strong to be associated with the hot X–ray emitting plasma seen by ASCA and probably is generated by the interaction of the innermost part of an accretion disk with the corotating outermost magnetospheric field. The outflow is almost certainly the inner part of a biconical disk wind.