AM Herculis Type Systems Research Articles

First detection of Zeeman absorption lines in the polar VV Puppis

Aims. We investigated the low state of the polar VV Pup by collecting high S/N time series spectra. Methods. We monitored VV Pup with VLT+FORS1 and analyzed the evolution of its spectroscopic features across two orbits. Results. We report the first detection of photospheric Zeeman lines in VV Puppis. We argue that the photospheric field structure is inconsistent with the assumption that the accretion shocks are located close to the foot points of a closed field line in a dipolar field distribution. A more complex field structure and coupling process is implied making VV Puppis similar to other well studied AM Herculis type systems.

The Three‐dimensional Structure of Extreme‐Ultraviolet Accretion Regions in AM Herculis Stars: Modeling of Extreme‐Ultraviolet Photometric and Spectroscopic Observations

We have developed a model of the high-energy accretion region for magnetic cataclysmic variables and applied it to Extreme Ultraviolet Explorer observations of 10 AM Herculis type systems. The major features of the EUV light curves are well described by the model. The light curves exhibit a large variety of features such as eclipses of the accretion region by the secondary star and the accretion stream and broad dips caused by material very close to the accretion region. While all the observed features of the light curves are highly dependent on viewing geometry, none of the light curves are consistent with a flat, circular accretion spot whose light curve would vary solely from projection effects. The accretion region immediately above the white dwarf (WD) surface is a source of EUV radiation caused by either a vertical extent to the accretion spot or Compton scattering off of electrons in the accretion column or, very likely, both. Our model yields spot sizes averaging 0.06RWD, or f ~ 1 × 10-3 the WD surface area, and average spot heights of 0.023RWD. Spectra extracted during broad-dip phases are softer than spectra extracted during the out-of-dip phases. This spectral ratio measurement leads to the conclusion that Compton scattering, some absorption by a warm absorber, geometric effects, an asymmetric temperature structure in the accretion region, and an asymmetric density structure of the accretion column are all important components needed to fully explain the data. Spectra extracted at phases where the accretion spot is hidden behind the limb of the WD, but with the accretion column immediately above the spot still visible, show no evidence of emission features characteristic of a hot plasma.

The period gap and the AM Herculis type systems

We argue that the period distribution of AM Herculis binaries in the enlarged sample incorporating results from the recent ROSAT X-ray survey differs significantly from that of other cataclysmic variables. In particular, there is no evidence for a pronounced period gap at 2—3 hr and the significance of the period spike at about 114 min is brought into question. We present an alternative evolutionary scenario for the AM Herculis binaries based on the hypothesis that magnetic braking by the stellar wind of the M star secondary either ceases or is severely curtailed when the rotation of the magnetic white dwarf becomes synchronised with the orbital motion, The orbital evolution of the AM Herculis binaries is thereafter driven mainly by angular momentum loss due to gravitational radiation. This scenario not only explains the higher proportion of AM Herculis binaries in the period gap when compared with other cataclysmic variables but also provides a natural explanation for the low mass transfer rates in these binaries and for the existence of an apparent upper limit for the surface magnetic fields of their white dwarfs.

A Study on the Period Distribution of Magnetic Cataclysmic Variables

Abstractwe use Monte Carlo simulations of the period distribution of magnetic cataclysmic variables to show that the synchronised AM Herculis-type systems and the asynchronous Intermediate Polars are essentially unrelated systems following different evolutionary scenarios.

Three-dimensional structured shocks in AM Herculis-type systems – II. Cyclotron emission from ridge-shaped emission regions

The properties of cyclotron emission from accretion shocks in AM Herculis binaries are studied which result from nonuniform accretion onto linearly extended regions on the surface of the magnetic white dwarf. The resulting ridge-shaped postshock structures yield cyclotron spectra with prominent harmonic features over a wide range of viewing aspects, even for high total accretion rates, in contrast to what has been found previously for axisymmetric accretion shocks.

An interpretation of the field alignment in AM Herculis type systems

The magnetic white dwarfs in AM Herculis type systems are known to have field structures which differ significantly from that of a centred dipole. We use the implied quadrupole asymmetry to explain the observed orientations of the accretion regions relative to the red star. In particular, we present an interpretation of the result that in four systems where two accretion shocks are observed, the region with the weaker magnetic pole is the stronger accretor.

Two-pole emission in AM Herculis-type systems

Des methodes d'analyse de donnees de polarisation lineaire sont discutees et les directions du champ dans les regions d'emission et l'inclinaison orbitale sont estimees a partir des angles de la polarisation lineaire. Il est egalement montre que les hypotheses additionnelles selon lesquelles les regions d'emission cyclotron sont localisees aux points inferieurs des lignes de champ fermees d'un champ dipolaire fournissent un modele geometrique qui est en accord avec les donnees de polarisation de AM Herculis et VV Puppis, mais en accord plus faible avec EF Eridanis. Des arguments sont presentes suggerant que ce dernier systeme presente une distribution de champ quadrupolaire

Cyclotron emission from inhomogeneous shocks in AM Herculis-type systems

The properties of cyclotron radiation from accretion shocks located above the atmospheres of magnetic white dwarfs are discussed, taking into account the effects of field spread, density and temperature structure, and geometrical extension. The models are an improvement on previous point-source models and have characteristics that are in better overall agreement with the properties of AM Herculis-type systems. The flat energy distributions seen in some AM Her-type systems are readily explained in terms of a model in which the optical depth parameter varies across the emission region in response to differences in the accretion rate. The lack of harmonic structure in the intensity spectra of most AM Her-type systems is probably a consequence of field spread in an extended emission region. It is also shown that the asymmetries seen in phase-dependent polarization curves can be explained when allowance is made for the displacement of emission regions from the poles.

Electron scattering in cyclotron emission regions in AM Herculis-type systems

The intensity and polarisation properties of magnetic accretion columns are discussed in the regime where electron scattering is important. It is shown that the circular polarisation standstills observed in some AM Herculis type systems are inconsistent with cyclotron emission from scattering dominated regions.

Zeeman features from accretion columns in AM Herculis-type systems

Zeeman features from accretion columns in AM Herculis-type systems

Electron Scattering in Cyclotron Emission Regions

AbstractThe polarization properties of cylindrical magnetic accretion columns are calculated allowing for electron scattering within the emission region. The results are used to estimate the effects of electron scattering and geometry on the observed properties of AM Herculis type systems.

The Effect of Magnetic Field Spread and Temperature and Density Variations in Cyclotron Emission Regions

AbstractWe discuss the properties of cyclotron radiation from accretion shocks located above the atmospheres of magnetic white dwarfs taking into account the effects of field spread and density and temparture structure. The models which are an improvement on previous point source models are shown to have characteristics that are in better overall agreement with the properties of AM Herculis type systems. Our calculations of the polarization properties of large polar cap emission regions are used to discuss the recent null measurements of circular polarization in DQ Herculis type systems.

Electron Scattering in Cyclotron Emission Regions in AM Herculis-Type Systems

AbstractThe intensity and polarisation properties of magnetic accretion columns are discussed in the regime where electron scattering is important. It is shown that the circular polarisation standstills observed in some AM Herculis type systems are inconsistent with cyclotron emission from scattering dominated regions.

The electron temperature of AM Herculis type systems

The electron temperature of AM Herculis type systems

The polarization properties of magnetic accretion columns – III. A grid of uniform temperature and shock front models

We present calculations of cyclotron and free–free emission for a wide range of values of the electron temperature (Te), frequency (ω/ωc), viewing angle with respect to the magnetic field (θ) and size parameter (Λ) both for constant temperature models and models incorporating a temperature structure (shock front models). It is argued that the observed optical properties of AM Herculis-type systems are inconsistent with low-temperature (Te<5 keV) emission regions but are in general agreement with predictions from high-temperature models. The low values of Λ deduced from high-temperature models are the same order of magnitude as the values expected for the size of the shock front – that is, the region over which the electron temperature rises from pre-shock values to its maximum value in the post-shock region. The conditions under which the temperature structure within the shock front can play an important role in determining the properties of the radiation emerging from AM Herculis-type systems are discussed. It is suggested that the narrow range of field strengths observed in AM Herculis-type systems is most probably a selection effect due to the emission properties of cyclotron radiation.

An interpretation of the polarization of AM Herculis type systems

On montre que les valeurs maximum de polarisation observees dans des systemes de type AM Herculis sont en accord avec les valeurs attendues pour un rayonnement cyclotron optiquement epais a partir d'un front d'onde