A hydrodynamic formulation for accretion flow channeled by a dipolar magnetic field is constructed using a curvi-linear coordinate system natural to the field structure. We solve the hydrodynamic equations and determine the velocity, density and temperature profiles of the post-shock accretion flow. The results are applied to accretion flows in intermediate polars. We have found that for systems with massive white dwarfs (~ 1 M⊙) the temperature profiles in the flow can differ significantly to those obtained from models in which the accretion column is assumed to be cylindrical.

We have used a model of magnetic accretion to investigate the rotational equilibria of magnetic cataclysmic variables (MCVs). This has enabled us to derive a set of equilibrium spin periods as a function of orbital period and magnetic moment which we use to estimate the magnetic moments of all known intermediate polars. We further show how these equilibrium spin periods relate to the polar synchronisation condition and use these results to calculate the theoretical histogram describing the distribution of magnetic CVs as a function of Pspin/Porb. We demonstrate that this is in remarkable agreement with the observed distribution assuming that the number of systems as a function of white dwarf magnetic moment is distributed according to .

Extensive optical photometry of the canditate magnetic CV RX J0524+42 has uncovered three persistent periods at 157, 146 and 136 minutes, which are the manifestation of the orbital motion of the system, the white dwarf spin and a possible side-band period of the latter. Depending on the interpretation of the 146 or the 136 minute period as the spin period, the system is an asynchronous by 7% or 14% (1 – PSpin/Porb), and therefore intermediate between the near-synchronous polars (< 2%) and the two DQ Her stars with long spin periods (EX Hya and V1025 Cen). Sampling the data over the beat period of 1.4 or 0.7 days reveals a prominent double-humped modulation on the spin period for certain fractions of the beat cycle. The phasing of this modulation strongly suggests that either pole switching onto two diametrically opposed accretion sites, or pole migration is operating in this system. With a period right within the period gap, RX J0524+42 is a key object for MCV evolution: it might be the first transition object between the DQ and AM Her subclasses where the white dwarf is currently in the process of synchronisation.

The origin of radio emission in magnetic CVs is a long standing problem without a satisfactory solution. We review the radio observations of both non-magnetic and magnetic CVs and discuss possible emission mechanisms. We pose the following questions: Is the presence of a magnetic white dwarf a necessary condition for radio emission? Is there a single cause for the radio emission or are there several radio emission mechanisms? We revisit the suggestion that asynchronism or at least the lack of strict magnetic locking might be a necessary condition for radio emission.

We conducted coordinated time-resolved observations of the long-period Intermediate Polar V1062 Tauri at the WIYN Observatory in Kitt Peak, Arizona and at Wise Observatory, Israel, and follow-up observations at the second site. We confirm the presence of two previously reported periodicities of the system: a long period (P≈10 h) which was interpreted as the orbital period of the underlying binary system, and a shorter one (P≈ 1 h), which was interpreted as the spin of the white dwarf. Our data also revealed a third photometric periodicity, corresponding to the orbital sideband of the spin period (OSB). The amplitude of the OSB was found to be strongly correlated with the varying brightness of the system at the orbital cycle.

Our observations suggest bi-modality in the photometric characteristics of the star. In one mode, the light of V1062 Tau varies with the spin period of the white dwarf, whereas the OSB is undetected. In the other mode, the OSB is the main modulation at short time-scales, and the spin period is absent from the light curve. Switching between the two modes occurred three times during the 10 weeks that spanned our observations. Also, we detected an outburst of 1.1 mag, which lasted between ~1 and ~5 days. In addition to the outburst, secular variations in the brightness of the star (0.3 mag on time scale of a few of tens of days) suggest that the system was in a brief low state during 2002 January.

We present optical high-resolution spectroscopy of AM Her and QQ Vul. Making use of indirect imaging techniques (Doppler and Roche tomography), we are able to make details visible, which a model of these systems has to be able to account for. Especially the emission line data cannot be explained by current models of polars.

We apply a new method of eclipse mapping to the light curves of eclipsing polars. The technique aims to locate the bright emission associated with the accretion stream, using a technique that makes the fewest prior assumptions about the location of the accretion stream material. We have obtained data of EP Dra and HU Aqr with the S-Cam 2 superconducting tunnel junction camera using the William Herschel Telescope. The location of emission regions in both systems show that previously assumed trajectories are consistent with those found using our technique. Most of the emission is located in a region where we expect material to be confined to magnetic field lines, particularly for HU Aqr, while there appears to be a lack of emission from where we conventionally expect material to follow a ballistic trajectory from the L1 point.

The processes of accretion of the gaseous blobs with different masses and densities onto strongly magnetized white dwarfs in the systems of polars have been modelled. We prove that shot noise in blue wavelengths represents accretion of the smaller and denser blobs than in redder wavelengths. Using combined “smooth particle hydrodynamics - drag force” model, we have predicted a shape of the accretion stream and active regions on the white dwarf surface.

I present the current results of my computational modeling of the accretion stream in polars. These numerical stream models, generated using an SPH code, yield a 3-dimensional map of the gas distribution throughout the stream, including the distribution of material as it impacts the white dwarf surface. The major features of the accretion streams generated by the model code are presented as well as results of a model representing EF Eri. The prospective future development of the model is also discussed.

We present Doppler maps of Halpha emission lines of AM Her from four differents epochs. The tomography shows a strong velocity “spot” emission at (−100,−100) km s−1 and a weaker “spot” emission at (−100,−400) km s−1 which is suggestive of bi-polar accretion onto the white dwarf with stronger emission from the main pole at phase φ=0.0.

We present the first full-orbit X-ray light curve of the remarkable eclipsing magnetic Cataclysmic Variable V1309 Ori obtained with XMM-Newton. The photometric behaviour and the extreme soft-to-hard flux ratio of the bright phase emission suggest that accretion occurs entirely via dense blobs.

We review our understanding of the prototype “Propeller” system AE Aqr and we examine its flaring behaviour in detail. The flares are thought to arise from collisions between high density regions in the material expelled from the system after interaction with the rapidly rotating magnetosphere of the white dwarf. We show calculations of the time-dependent emergent optical spectra from the resulting hot, expanding ball of gas and derive values for the mass, lengthscale and temperature of the material involved. We see that the fits suggest that the secondary star in this system has reduced metal abundances and that, counter-intuitively, the evolution of the fireballs is best modelled as isothermal.

We calculate the Hα luminosity of an outflow nebula resulting from propeller action in AE Aqr. While the outflow is bright enough to be detected as an extended source, 1 arcsec across, it is dominated by the point-spread function (PSF) wings of the central object. By subtracting a PSF model from HST PC data we rule out the extreme models of high mass-loss rate combined with narrow-angle outflow, but more precise measurements are required to explore the propeller parameter space further.

It is shown here that the peculiar properties of AE Aqr can can be accounted for if the mass transfer from an evolved 0.7M⊙ secondary K4-5 star (qi ≈ 0.8, i.e. < 1) initiated when the orbital period of the binary was Porb,i ≈ 8.5 hours and the white dwarf period P*,i ≈ 1 hour. This resulted in a significant amount of orbital angular momentum being accreted by the white dwarf in an initial discless spin-up phase towards P* ≈ 0.1 Porb,i. This destabilized the mass transfer, resulting in a run-away mass transfer from the secondary that lasted for approximately 104 years, with the orbital period evolving to Porb ≈ 11 hours until a critical mass ratio of qcrit = 0.73 had been reached. In this phase the mass transfer from the secondary occurred at a rapid rate of approximately Ṁ2 ≈ 1020 g s-1, resulting in an accretion disc which spun-up the white dwarf to a period of approximately P* ≈ 33 s. For all q ≤ qcrit = 0.73 the mass transfer proceeded on the thermal time scale of the secondary star, i.e. at a much slower rate, resulting in the binary converging and forcing AE Aqr into the propeller phase. Applying stellar wind theory, this allow an estimate of the polar magnetic field of the secondary star, which is of the order of B° ≈ (1600 – 2000) G. It has been shown here that the duration of mass transfer phase q = qcrit → 0.67 (now) lasted for approximately tṀ2 ~ 107 years, similar to the spin-down time scale of the white dwarf, tsd = P*/P* ≈ 107 years. The propeller ejection of matter in the current phase results in the dissipation of mhd power of Lmhd ≈ 1034 erg s-1, probably channeled into mass ejection and non-thermal activity. This explains the non-thermal outbursts that are observed in radio wavelengths, and occasionally also in TeV energies, from AE Aqr.

In this paper we model the non-thermal radio to infra-red flares from AE Aqr. In our model the non-thermal flares originate in highly magnetized (Bblob ≥ 2000 G) blobs that may be among the propeller ejected outflow. It was shown that the condition ß ≤ 1 constrains the frozen-in magnetic field in these blobs to Bblob ≥ 2000 G, which is of the same order of magnitude as the inferred polar field of the secondary. As these magnetized blobs encounter the violent mhd-propeller, processes such as reconnection, magnetic pumping, and shocks will result in continuous acceleration of electrons from (γ = 2 → 30; δ = 2.8 → 2.6) with resultant synchrotron emission. The total radio to infra-red flare spectrum was modelled in terms of such expanding magnetized synchrotron emitting blobs in various stages of their evolution from ρ = (r/r°) = 1 → 400. In terms of our model, the total integrated flux during outbursts, over the wide frequency range from 1 GHz is the result of several (~ 20) synchrotron emitting blobs observed in different stages of their evolution, resulting in a spectrum showing a peak flux of Sv ~ 148 mJy at v ~ 1805 GHz (~ 166 microns), where the spectrum changes from a typical self-absorbed Sv ∝ vα spectrum to Sv ∝ v-(δ-1)/2 spectrum, i.e. where the blobs are combined optically thin.

AE Aquarii radio flares are attributed to synchrotron optically thick emission from expanding plasma clouds, and the turnover frequency and the optically thin part of the synchrotron spectrum seem to be between the sub-millimetre and infra-red wavelengths. We present the various processes which may affect the synchrotron radiation at both very low (radio) frequencies and at higher energies, and what they infer on the properties of the radio flare source and its surroundings. We also envisage that both thermal and non-thermal emission processes may explain the puzzling spectrum of AE Aqr.

I summarize some aspects which have been raised for discussion during Session 6, devoted to magnetic propeller systems.

We review the current observational status of the ROSAT sources RX J1914.4+2456 and RX J0806.3+1527, and the evidence that these are ultra-short period (< 10 min) binary systems. We argue that an Intermediate Polar interpretation can be ruled out, that they are indeed compact binaries with a degenerate secondary, and that the period seen in the X-ray and optical is the orbital period. A white dwarf primary is preferred, but a neutron star cannot be excluded. We examine the capability of the three current double-degenerate models (Polar, Direct Accretor and Electric Star) to account for the observational characteristics of these systems. All models have difficulties with some aspects of the observations, but none can be excluded with confidence at present. The Electric Star model provides the best description, but the lifetime of this phase requires further investigation. These ultra-short period binaries will be strong gravitational wave emitters in the LISA bandpass, and because of their known source properties will be important early targets for gravitational wave studies.

A nearly simultaneous X-ray/optical (Chandra and VLT) observational campaign of RX J0806.3+1527 has been carried out during 2001. These observations allowed us to phase the X-ray and optical light curves for the first time. We measured a phase–shift of ~0.5, in good agreement with the presence of two distinct emission regions and with the X-ray irradiation process predictions. The Chandra data allowed us also to study in details the X-ray spectrum of RX J0806.3+1527, which is consistent with a soft (kT<70eV) and small (RBB <20km) black-body component. We discuss the present findings in the light of the models proposed so far to account for the X-ray emission detected from RX J0806.3+1527, and its twin source RX J1914.4+2456.

An attempt has been made here to seek the origin of the magnetic white dwarfs in Cataclysmic Variables. Following the suggestion made in the case of isolated white dwarfs, we look at the possibility of binaries with magnetic A type stars known as chemically peculiar stars as progenitors of MCVs. The evolution of the binary is discussed and compared with the available statistics.