Herculis Binaries Research Articles

Magnetospherically induced torques in asynchronous AM Herculis binaries

The AM Herculis binaries are an important class of cataclysmic variables in which the accreting white dwarf primary is observed to rotate synchronously with the orbit. The torque needed to spin the primary towards synchronism is believed to be generated by the interaction of the primary's magnetic field with the tidally synchronized secondary star. It is shown here that such a torque is generated by the interaction of the magnetosphere of an asynchronous primary with the turbulent secondary. The effects of magnetic diffusion, shearing and field advection are accounted for and the resulting magnetic torques are calculated for a range of degrees of asynchronism. Non-central magnetic forces enable the primary to exchange angular momentum with the orbit, while a torque acts on the star which spins it towards synchronism. This torque has the form of a resonance curve, with a maximum value at moderate asynchronism.

Soft X-ray spectral variability of AM Herculis

Context. Polars (AM Herculis binaries) are a prominent class of bright soft X-ray sources, many of which were discovered with ROSAT. Aims. We present a homogenous analysis of all the pointed ROSAT PSPC observations of polars subdivided into two papers that discuss the prototype polar AM Her in detail and summarize the class properties of all other polars. Methods. We derive the high-state soft X-ray flux and short-term spectral variability of AM Her using a new detector response matrix and a confirmed flux calibration of the ROSAT PSPC below 0.28 keV. Results. The best-fit mean single-blackbody temperature and integrated bright-phase energy flux of AM Her in its April 1991 high state are 27.2 ± 1. 0e V and (2.6 ± 0.6) × 10 −9 erg cm −2 s −1 , respectively. The total blackbody flux of a multi-temperature model that fits both the soft X-ray and the fluctuating far-ultraviolet components is Fbb = (4.5 ± 1.5) × 10 −9 erg cm −2 s −1 . The total accretion luminosity at a distance of 80 pc, Lbb = (2.1 ± 0.7) × 10 33 erg s −1 , implies an accretion rate of u

The effect of a magnetosphere on asynchronous magnetic torques in AM Herculis binaries

The interaction is investigated between a white dwarf magnetosphere and a red dwarf secondary star in an asynchronous AM Herculis binary. The poloidal magnetic field diffuses into the secondary and shearing motions, due to differential rotation between the star and the magnetosphere, generate azimuthal field. The effects of diffusion and field advection caused by a poloidal flow induced by magnetospheric motions are calculated. Outward radial motions reduce the amount of field penetration and limit the stellar magnetic torque at higher degrees of asynchronism. A resonance torque curve results, similar to the case of a precessing magnetic field with vacuum surroundings. Inward motions in the secondary increase field penetration, allowing the torque to reach higher values for larger degrees of asynchronism.

Superhumps in Cataclysmic Binaries. XXIII. V442 Ophiuchi and RX J1643.7+3402

ABSTRACTWe report the results of long observing campaigns on two novalike variables: V442 Ophiuchi and RX J1643.7+3402. These stars have high‐excitation spectra, complex line profiles signifying mass loss at particular orbital phases, and similar orbital periods (respectively, 0.12433 and 0.12056 days). They are well‐credentialed members of the SW Sex class of cataclysmic variables. Their light curves are also quite complex. V442 Oph shows periodic signals with periods of 0.12090(8) and 4.37(15) days, and RX J1643.7+3402 shows similar signals at 0.11696(8) and 4.05(12) days. We interpret these short and long periods, respectively, as a “negative superhump” and the wobble period of the accretion disk. The superhump could then possibly arise from the heating of the secondary (and structures fixed in the orbital frame) by inner‐disk radiation, which reaches the secondary relatively unimpeded since the disk is not coplanar. At higher frequencies, both stars show another type of variability: quasi‐periodic oscillations with a period near 1000 s. Underlying these strong signals of low stability may be weak signals of higher stability. Similar quasi‐periodic oscillations, and negative superhumps, are quite common features in SW Sex stars. Both can in principle be explained by ascribing strong magnetism to the white dwarf member of the binary; and we suggest that SW Sex stars are borderline AM Herculis binaries, usually drowned by a high accretion rate. This would provide an ancestor channel for AM Hers, whose origin is still mysterious.

Accretion physics of AM Herculis binaries

Accretion physics of AM Herculis binaries

Eclipsing AM Herculis Binaries

AbstractAM 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.

Magnetic Torque Induced High/Low Accretion States in AM Herculis Binaries

AbstractThe AM Herculis binaries (AM Hers) have high and low accretion states. A model is proposed in which the white dwarf’s rotation acts like an angular momentum storage battery for the binary orbit. The accreted material from the Roche lobe-overflowing secondary charges up this battery, whereas the magnetic torque of the white dwarf on the secondary discharges it back into the binary’s orbit. The Roche lobe’s volume becomes alternately larger and smaller than the secondary’s volume. This alternation causes the mass flow to vary between a high and low rate.

Distribution and Space Density of Soft X-ray Emitting Polars in the Solar Neighbourhood

The ROSAT All Sky Survey (RASS) was the first one performed with an imaging telescope in the soft X-ray regime and has led to the discovery of numerous new objects whose emission is dominated by soft X-rays. Among these are white dwarfs and a subclass of the cataclysmic variables (CVs), the Polars or AM Herculis binaries. From a pre-ROSAT census of only 17, the number of known sources of this class has increased to some 55 (Beuermann and Thomas 1993, Beuermann 1997). Distances or lower limits to the distance are available for some 35 of these, based on the detection or non-detection of the TiO-Features in their optical red spectra. The derived distances range from below 100 pc up to ~ 600 pc, implying that many of these objects are located within the “Local Bubble” of low gas density in interstellar space. As the soft X-ray emission can be reasonably well represented by blackbody emission with a typical temperature of kTbb ≃ 25 eV, spectral fits to the ROSAT PSPC spectra from either the All-Sky-Survey (RASS) or from subsequent pointed ROSAT observations allow to determine the foreground absorption column density in the direction of the polars.

Radiation Hydrodynamics and Soft X-Ray Emission of Accreting Magnetic White Dwarfs

AbstractObserved cyclotron spectra of AM Herculis binaries imply a large range of mass flow rates in the accretion spot on the white dwarf. We derive this ṁ–distribution for the case of UZ For. For AM Her stars with different field strength B, such a distribution is shifted in ṁ proportional to B. This fact can account for the observed dominance of bremsstrahlung at low B and of quasi-blackbody emission at high B values.

AM Herculis Binaries

AM Herculis binaries contain mass accreting magnetic white dwarfs which appear as bright X-ray sources in the ROSAT All Sky Survey. About 52 systems are presently known which allow detailed studies of the evolution of magnetic close binaries and of fundamental plasma-physical processes in the accretion region on the white dwarf.

On the Statistics of AM Herculis Binaries

AbstractThe magnetic field and orbital period distributions of AM Herculis binaries are investigated. Our study shows that (i) there is a significant lack of very-high-field magnetic white dwarfs in binaries when compared with isolated white dwarfs, and (ii) the difference between the period distributions of AM Herculis binaries and other cataclysmic variable subclasses is statistically significant. These results imply that the evolution and the birth of AM Herculis binaries are different from those of other cataclysmic variables.

Detection of the hot white dwarf in the magnetic nova V1500 Cygni with the Hubble Space Telescope

A comprehensive investigation of the AM Herculis-like nova system V1500 Cyg is presented, featuring time-resolved spectrophotometry with the Faint Object Spectrograph (FOS) of the Hubble Space Telescope (HST), coeval optical circular spectropolarimetry and spectrophotometry, and new broadband polarimetry. A revised ephemeris for the rotating white dwarf refines our knowledge of the timescale for synchronization with the orbital period -- and for becoming a bona fide AM Herculis system -- to 170 +/- 8 yrs. Three features of the orbital-phase-dependent spectral energy distribution are keys for understanding the relative contributions of several radiation sources in this system: (1) the brightness and shape of the ultraviolet continuum, which rises abruptly at short wavelengths and shows no significant phase dependence for lambda less than 2200 A, (2) the greater than or equal to 1 mag variations in the optical continuum and emission-line fluxes, and (3) a spectral slope in the optical which is reddest when brightest. These data lead us to conclude that the hot, postnova white dwarf is the dominant source of ultraviolet flux and of the unusually large bolometric luminosity of the system, compared with typical active AM Herculis binaries. The flat mid-ultraviolet spectrum requires a radiation component in addition to the white dwarf photosphere. We consider three possible origins: (1) cyclotron emission, (2) relatively broad, optically thick accretion columns with lumps of dense gas, and/or (3) orbiting debris in a disklike geometry. Observational tests are suggested to distinguish among these alternatives.

On the rate of orbital angular momentum loss of synchronously rotating magnetic cataclysmic variables

We present calculations of the efficiency of magnetic braking in synchronously rotating magnetic binary systems, following the formulation of Li, Wu & Wickramasinghe. We show that, for the observed range of magnetic field strengths typical of AM Herculis binaries, the braking efficiency has been overestimated by a large factor in current models. Our calculations also show that the orbital evolution of the high-field AM Herculis binaries is driven by the loss of orbital angular momentum due to gravitational radiation rather than magnetic braking, if these are the oniy two competing mechanisms.

A new evolutionary model for AM Herculis binaries

Observations of AM Herculis binaries recently discovered by ROSAT and other surveys have shown that the AM Herculis binaries do not exitbit a pronounced 2-3 h period gap, which is difficult to explain in terms of the canonical magnetic braking model for the orbital evolution of these binaries. We present a new evolutionary scenario for AM Herculis binaries based on the hypothesis that magnetic braking by the stellar wind of the M-star secondary ceases to be an effective mechanism for the loss of orbital angular momentum when the rotation of the white dwarf becomes synchronized with the orbital motion

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.

The magnetic field configurations of AM Herculis binaries

We have considered equilibrium configurations of a white dwarf and a red dwarf companion in AM Herculis type binary system where the red dwarf is assumed to have an intrinsic centered dipole field, and the white dwarf an offset dipole or a centered dipole plus a quadrupole field, and the interaction is purely magnetic. The restoring torque, due to the system being perturbed from its stable equilibrium configuration, is compared with the accretion torque. The results are used to show that the observed alignment of the main cyclotron emission region relative to the red star can be explained quite simply by these models without having to invoke gravitational torques. We also provide a possible explanation of the observation that the more strongly accreting pole has the weaker field strength in the few systems where fields have been determined at both poles.

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.

Magnetic moment distribution of magnetic cataclysmic variables

A simulation study is made of the relative numbers of the AM Herculis binaries and the intermediate polars as a function of the orbital period using random variables subject to suitable constraints to describe the various parameters. It is shown that the observations can be matched by a single distribution in the magnetic moment equals 0.7 +/- 0.3. For such an ensemble, the intermediate polars are distributed in the log(Porb) - log(Ps) diagram about the critical disk line but with a larger scatter than observed.

Cyclotron emission from AM Herculis binaries - A variable magnetic field, variable temperature model

This paper examines cyclotron emission from a hot plasma flowing to the surface of a magnetic white dwarf. The two-temperature plasma free-falls along the axis of a dipole magnetic field to the white dwarf surface and cools through bremsstrahlung and optically thick cyclotron emission. It is shown that the ion temperature is constant until the magnetic field is of order of 10 MG. This result is weakly dependent on the accretion rate and the strength of the surface magnetic field. Spectra and polarization fractions are calculated for a variety of model parameters. The spectra have a broad peak at optical wavelengths for surface field strengths above about 20 MG. The emission is highly polarized at all frequencies. Cyclotron harmonic structure is often produced for low surface field strengths. This model produces the observed variation of polarization with orbital phase at both optical and infrared wavelengths. It is argued that the observed magnetic field strengths are a consequence of the plasma cooling in a 30-MG region, and that the surface field strength for some of these systems may actually be larger than the observed values. 26 refs.

An emission-line model for AM Herculis systems

view Abstract Citations (55) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An Emission-Line Model for AM Herculis Systems Ferrario, Lilia ; Wickramasinghe, D. T. ; Tuohy, I. R. Abstract The optical spectra of the AM Herculis binaries are characterized by extremely complex emission lines whose profiles can be resolved into at least three components which are formed in different regions of the accretion stream leading from the companion star toward the magnetic white dwarf. A theoretical model is presented for the radial velocity and velocity dispersion of the broad emission line component assuming that it originates mainly in the gas which is diverted out of the orbital plane and funneled onto the white dwarf surface along magnetic field lines. The model is used to locate the line-forming region in three AM Her variables: E1405-451, CW 1103+254, and EXO 033319-2554.2, using as constraints the radial velocity and velocity dispersion data. The analyses of these systems show that the material is threaded by the magnetic field in a very azimuthally extended coupling region located 0.5-0.75 of the way between the white dwarf and the inner Lagrange point. Publication: The Astrophysical Journal Pub Date: June 1989 DOI: 10.1086/167497 Bibcode: 1989ApJ...341..327F Keywords: Binary Stars; Cataclysmic Variables; Emission Spectra; Stellar Mass Accretion; White Dwarf Stars; Dwarf Novae; Polarized Radiation; Spectral Line Width; Stellar Magnetospheres; Visible Spectrum; Astrophysics; STARS: ACCRETION; STARS: BINARIES; STARS: DWARF NOVAE; STARS: MAGNETIC; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (3)