AM Herculis Binaries Research Articles

Polarized Radiation from Inhomogeneous Accretion Columns in AM Herculis Binaries

AbstractThe polarization of radiation emitted by inhomogeneous accretion columns is found to be significantly different from homogeneous columns. A good fit to the V-band circular polarization light curve of EF Eri is obtained using the two-core model with temperature &T=15keV, magnetic field B=26MG, and dimensionless plasma parameter Λ ≤ 106.

X-ray properties of AM Herculis binaries

The properties of the accretion stream and the X-ray emission region in AM Herculis binaries are reviewed. Special emphasis is given to the geometry of the accretion region and a discussion of the “soft X-ray excess”.

Mass transfer rates and the soft X-ray excess in AM Herculis binaries

Mass transfer rates and the soft X-ray excess in AM Herculis binaries

An Emission Line Model for AM Herculis Systems: Application to E1405-451

AbstractThe 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 towards the magnetic white dwarf. We present a theoretical model which localizes the formation region of the broad emission line component and provides information regarding the structure of this emitting region. In our model the particle trajectories are integrated in a Roche potential and the volume between the white dwarf and the companion has been divided into two different regimes of motion. In one region the gas escapes from the secondary near the inner Lagrange point and is accelerated along a straight line towards the white dwarf. In the other region the magnetic field is strong enought to divert the gas out of the orbital plane and to channel it towards the white dwarfs surface. The model has been used to interpret radial velocity and velocity dispersion data from the AM Herculis system E1405-451.

The attainment of synchronism in the presence of accretion in the AM Herculis binaries

Conditions are derived for which the white dwarf component in an AM Herculis binary can attain synchronism with the orbit in the presence of accretion. A stable locking mechanism must exist and, for the case of an initially over-synchronous white dwarf, there is a maximum value of the magnetic diffusivity of the secondary, |$\eta_0$|⁠, above which exact corotation cannot be attained. In the absence of a locking mechanism the primary will reach a slightly asynchronous state with a typical synodic period of a few tens of years. If η is significantly less than |$\eta_0$| the instantaneous angular velocity of the primary in this state will vary about a central value.

Polarized radiation from hot plasmas and applications to AM Herculis binaries. II - Effect of collisions and Thomson scattering

The absorption coefficients in the ordinary and extraordinary modes are calculated, for radiation emitted by magnetoactive plasmas with temperatures in the range 0.1--5 keV. The equations of radiative transfer for a homogeneous plasma, with large Faraday rotation, are solved, and simple analytic expressions for the Stokes parameters Q and V are derived in terms of the optical depths in the ordinary and extraordinary modes. The results are applied to homogeneous plasma slabs which represent a simple model for the accretion columns of the AM Herculis binaries. It is shown that the inclusion of the effects of collisions and Thomson scattering reduces the amount of fractional polarized radiation, bringing better agreement with observations. For small viewing angles with the magnetic field the circular polarization does not approach 100% as in the collisionless case, but instead approaches 0% or a value less than 100% depending on the geometry. This may provide a qualitative explanation for the standstills observed in some AM Her binaries.

AM Herculis Binaries: Particle Acceleration, Radio Emission and Synchronization

ABSTRACTIt has been suggested that the recently discovered radio emission from AM Her arises as a result of gyrosynchrotron radiation from electrons at energies ≈400 keV in the magnetosphere of the white dwarf. However, no mechanism for producing such energetic electrons was discussed. In this paper, we argue that small departures from synchronous rotation can cause the companion star to act as a unipolar inductor. This leads to high voltages being produced across the companion star, which provides the necessary acceleration mechanism. This also implies that if the magnetic white dwarf was formed with a rapid rotation, synchronization would be achieved on a time scale ~104 yr.

Two-second variability in AM Herculis binaries

AN UMa and the newly identified AM Her star E1405--451 both show excesses of optical variability in the broad frequency range from 0.4 to 0.8 Hz (2.5--1.25 s). The optical fluxes from AN UMa and E1405--451 must vary by 2.4% and 1.2% rms, respectively, in order to produce the level of noise observed in the power spectra. The noise is not produced during any one section of time or orbital phase but is approximately a constant proportion of the net optical light from E1405--451. The feature is possibly bimodal in AN UMa with a minimum of power at 0.65 Hz. Time series observations of AM Her made during both its ''bright'' and ''faint'' phases show that no such noise feature is present to less than 0.25% and 1.3% rms of the stellar light. The predictions of a model for this phenomenon, recently advanced by Langer, Chanmugam, and Shaviv and based on a thermal instability in the height of a standoff accretion shock, are tested with inconclusive results.

Polarized radiation from hot plasmas and applications to AM Herculis binaries

view Abstract Citations (129) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Polarized radiation from hot plasmas and applications to AM HER binaries. Chanmugam, G. ; Dulk, G. A. Abstract Absorption coefficients for the ordinary and extraordinary modes are calculated for cyclotron radiation from hot plasmas with temperatures between 0.2 and 20 keV. The results are applied to the accretion columns of the AM Herculis binaries. The linear and circular polarization properties of the observed polarized light can be understood if the emission is at approximately the fifth harmonic of the cyclotron frequency. The magnetic fields in these systems is deduced to be roughly 4 x 10 to the 7th gauss. Publication: The Astrophysical Journal Pub Date: March 1981 DOI: 10.1086/158736 Bibcode: 1981ApJ...244..569C Keywords: Binary Stars; High Temperature Plasmas; Magnetic Stars; Polarized Electromagnetic Radiation; Stellar Radiation; White Dwarf Stars; Absorptivity; Collisionless Plasmas; Cyclotron Radiation; Light Curve; Stellar Magnetic Fields; Astrophysics full text sources ADS | data products SIMBAD (1)

Cyclotron self-absorption in accretion columns of magnetic degenerate stars

view Abstract Citations (38) References (45) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Cyclotron self-absorption in accretion columns of magnetic degenerate stars. Chanmugam, G. ; Wagner, R. L. Abstract The maximum frequency is calculated up to which cyclotron radiation in the accretion columns of magnetic degenerate stars is self-absorbed, and the results are applied to the AM Herculis binaries. Attention is given to the emissivity for a plasma in a magnetic field, the importance of cyclotron cooling considering the effects of the strong magnetic fields in the AM Herculis binaries, and the probability that the broad-band nature of the observed polarized radiation is due to the inhomogeneity of the dipole magnetic field. Also discussed are binary X-ray systems containing magnetic neutron stars, in particular Her X-1. Publication: The Astrophysical Journal Pub Date: September 1979 DOI: 10.1086/157352 Bibcode: 1979ApJ...232..895C Keywords: Binary Stars; Cyclotron Radiation; Magnetic Stars; Neutron Stars; Self Absorption; Stellar Mass Accretion; X Ray Sources; Magnetic Dipoles; Optical Polarization; Polarized Radiation; Stellar Magnetic Fields; White Dwarf Stars; Astrophysics; Degenerate Stars:Polarization; Magnetic Stars:Polarization; Polarization:X-Ray Binaries full text sources ADS |

Cyclotron Emission from the Shock-Heated Region of AM Herculis

There has been considerable interest recently in the AM Herculis binaries, which are believed to contain a strongly magnetic white dwarf accreting matter from a lower-main-sequence companion (Tapia 1977, Chanmugam and Wagner 1977, Stockman et al. 1977). Models for these systems feature an accretion column above a magnetic pole of the white dwarf. A shock-heated region, with a temperature kT ≈ 20 keV (Raymond et al. 1979) and a height h – 106cm (Masters 1978), believed to be formed in AM Her just above the magnetic pole is likely to be responsible for the emission of the hard X-rays observed (Swank et al. 1977). It has been further suggested that this region should emit optically thick cyclotron emission in the ultraviolet because of the presence of the strong magnetic field ~ 108 gauss (Masters 1978, Lamb and Masters 1979). Such UV emission has not been observed (Raymond et al. 1979).

The effect of asynchronism on wind-driven mass transfer rates in the polars

ABSTRA C T An asynchronous magnetic white dwarf affects the rate of orbital evolution in AM Herculis binaries. An over-synchronous star leads to a positive orbital magnetic torque which reduces the rate of shrinkage of the secondary star’s Roche lobe, and hence reduces the mass transfer rate. An opposing effect occurs as a result of the orbital angular momentum loss via secondary mass transfer in the absence of an accretion disc. The modification of the magnetic braking-driven synchronous mass transfer rate is calculated for a range of degrees of asynchronism, and its effect is compared at different orbital periods.