Magnetic Degenerate Dwarfs Research Articles

Evolution of magnetic cataclysmic binaries

More than a third of all cataclysmic variables (CV’s) with established binary periods contain strongly magnetic degenerate dwarfs (Ritter 1985). The history of the manner in which the magnetic character of these systems was established suggests that many more CV’s-perhaps most-are magnetic. By comparison, only ≈ 2% of isolated degenerate dwarfs are magnetic (Angel, Borra, and Landstreet 1981; Schmidt and Liebert 1986). The magnetic CV’s divide naturally into two subclasses: the DQ Herculis stars (recently reviewed by Warner 1983, 1985) and the AM Herculis stars (recently reviewed by Liebert and Stockman 1985). Each subclass currently has about a dozen members.

Evolution of Magnetic Cataclysmic Binaries

More than a third of all cataclysmic variables (CV’s) with established binary periods contain strongly magnetic degenerate dwarfs (Ritter 1985). The history of the manner in which the magnetic character of these systems was established suggests that many more CV’s—perhaps most—are magnetic. By comparison, only ≈ 2% of isolated degenerate dwarfs are magnetic (Angel, Borra, and Landstreet 1981; Schmidt and Liebert 1986). The magnetic CV’s divide naturally into two subclasses: the DQ Herculis stars (recently reviewed by Warner 1983, 1985) and the AM Herculis stars (recently reviewed by Liebert and Stockman 1985). Each subclass currently has about a dozen members.

Extremely Magnetic Degenerate Dwarf

Extremely Magnetic Degenerate Dwarf

Synchronization of magnetic stars in binary systems

Asynchronous rotation of magnetic stars in close binary systems drives substantial field-aligned electrical currents between the magnetic star and its companion. The resulting magnetohydrodynamic torque is able to account for the heretofore unexplained synchronous rotation of the strongly magnetic degenerate dwarf component in systems like AM Her, VV Pup, AN UMa, and EF Eri as well as the magnetic A type component in systems like HD 98088 and 41 Tauri. The electric fields produced by even a small asynchronism are large and may accelerate some electrons to high energies, producing radio emission. The total energy dissipation rate in systems with degenerate dwarf spin periods as short as 1 minute may reach 10 to the 33rd ergs/s. Total luminosities of this order may be a characteristic feature of such systems.

X-ray spectra and light curves of accreting magnetic degenerate dwarfs

We report detailed calculations of the X-ray spectra and X-ray light curves of accreting magnetic 1 M/sub sun/ degenerate dwarfs. We use accretion funnel models in which plasma flows toward the dwarf along dipole magnetic field lines and accretes onto a small cap at the dwarf's magnetic pole of area 4..pi..R/sup 2//sub asterisk/ f, where R/sub asterisk/ is the stellar radius and f<<1. Above the dwarf's surface, the plasma forms a radiating shock which produces the X-rays. We find that the X-ray spectra of weakly magnetic degenerate dwarfs are qualitatively similar to the spectra of nonmagnetic dwarfs. However, because the optical depth perpendicular to the field lines in the funnel is less than the optical depth parallel to the field lines, the majority of the X-rays escape in directions perpendicular to the field lines. This has two effects: (1) For magnetic and nonmagnetic models with equivalent accretion rates per square centimeter, i.e., equal ''effective luminosities'' L/sub f/(equivalent L/f), the emergent photons in the magnetic case suffer fewer scatterings than those in the nonmagnetic case and thus show less energy degradation. (2) The beaming of the photons into directions perpendicular to the field lines leads to X-ray light curves which exhibitmore » a wide range of behaviors. The shapes of the light curves are sensitive to f and L/sub f/, to the inclination of the magnetic field axis to the dwarf's rotation axis, and to the viewing angle of the dwarf. Thus, observed X-ray light curves can be used to constrain the physical characteristics and geometry of the accreting degenerate dwarf.« less

X‐RAY SPECTRA OF ACCRETING DEGENERATE STARS*

X-ray emission from accreting nonmagnetic degenerate dwarfs is discussed. Emphasis is placed on the numerical calculations by Kylafis and Lamb (1979) with regard to the entire range of accretion rates and stellar masses. It is concluded that at moderate and high accretion rates the spectral temperature varies dramatically and the star exhibits a pronounced correlation between X-ray spectral temperature and luminosity. Also considered are the results of Lamb and Masters (1979) of calculations of high-harmonic cyclotron emission from a hot plasma, from which they developed a model of X-ray and UV emission from accreting magnetic degenerate dwarfs. It is found that the position and relative strength of the spectral components change with variations in the accretion rate and differ for different magnetic fields; in addition, magnetic degenerate dwarfs are predicted to be strong UV sources with only a few percent of the total accretion luminosity ordinarily appearing as soft and hard X-rays.

X and UV radiation from accreting magnetic degenerate dwarfs

We have carried out detailed numerical calculations of high-harmonic cyclotron emission from a hot plasma, and from these calculations we have developed at self-consistent, quantitative model of X-ray and UV emission from accreting magnetic degenerate dwards. We find that such stars are typically strong UV sources, and emit only at small fraction of their luminosity in soft and hard X-rays. The qualitative features of the X and UV spectra suffice to determine the mass, magnetic field strength, and luminosity of the source with relative confidence. Our results show that the soft and hard X-ray emission observed from AM Her, AN UMa, and 3A 0211--227 are typical of these objects.

X-Ray Emission from Degenerate Dwarfs

Accreting degenerate dwarfs comprise an important class of galactic X-ray sources. This has become evident with the recent discovery of soft and hard X-ray emission from accreting magnetic degenerate dwarfs such as AM Her, AN UMa, and 2A0311-23, and from nonmagnetic or weakly magnetic cataclysmic variables such as SS Cyg, U Gem, EX Hya, and GK Per (for an observational review, see Garmire 1979). Previously known low mass X-ray binaries such as Cyg X-2 and Sco X-1, as well as some of the “bulge” sources, may also be moderate luminosity nonmagnetic degenerate dwarfs (Kylafis and Lamb 1979; Branduardi et al. 1979). Finally, theory predicts (Fabian, Pringle and Rees 1976; Katz 1977; Kylafis and Lamb 1979) an as yet undiscovered class of low luminosity nonmagnetic degenerate dwarf X-ray sources having hard (Tobs ≳ 30–50 keV) spectra.

Synchronous rotation in magnetic X-ray binaries

AM Herculis is thought to be a binary stellar system that contains an accreting magnetic degenerate dwarf whose rotation is synchronous with the orbital period. This synchronism is remarkable, particularly because of the small moment of inertia of a degenerate dwarf and the large specific angular momentum of the accreted matter. This paper demonstrates that ohmic dissipation from the magnetic interaction of the stars is capable of bringing about exact synchronism, provided that some other process has brought the rotation period of the degenerate dwarf to the same order of magnitude as the orbital period. It is also shown that magnetostatic interaction in the synchronous state leads to oscillatory drifts in phase about exact synchronism with periods of approximately 1-10 yr. These phase drifts could manifest themselves in long-term periodic variability in the X-ray or optical properties of the source. Accretion torques could excite such oscillatory motions but need not disrupt synchronism once it has been established.

Characteristics of the AM Herculis Type Stars and Review of proposed Interpretations

Consider a close binary system where mass transfer is underway from a relatively normal star to a highly magnetic degenerate dwarf. The magnetic field is strong enough that the magnetic pressure balances the ram pressure of the infalling plasma at a distance comparable with the separation of the two stars. Under these conditions accretion takes place along the magnetic lines of force, channeling the inflow of plasma towards the magnetic polar caps. Confined to a narrow column the plasma develops a standing shock at some distance above the surface of the degenerate dwarf. Due to its gravitational energy the infalling plasma can reach very high temperatures below the shock, then very efficient cooling effects ensue. Most of our information on such binary systems will be contained in cooling mechanisms involving radiation.Current theoretical trends identify three sources of radiation linked to the accretion column: 1) electron-ion bremsstrahlung in the hot plasma below the shock, 2) thermal emission from the heated surface of the degenerate dwarf, around the magnetic polar caps, and 3) optically thick electron cyclotron emission.

A Model for Compact X-Ray Sources: Accretion by Rotating Magnetic Stars

The physics of accretion onto compact stars is considered, taking into account both the effects of stellar rotation and a stellar magnetic field. We show that far from the star the stellar magnetic field is screened by currents flowing in the accreting plasma, while close to the star the stellar field forces matter to corotate with the star. The location of the Alfven surface, where the transition between the two regimes occurs, depends on the flow pattern of the accreting matter beyond the Alfvea surface and the rotation period of the compact star, as well as the mass accretion rate and the strength of the stellar magnetic field. Three types of flow pattern are considered: radial inflow toward the compact object, orbital motion about it, and streaming motion past it. For the accretion rates of interest the radius of the Alfven surface r/sub A/, is found to be ~10/sup 7/-10/sup 8/ cm for a typical neutron star and ~ 10/sup 8/ -10/sup 10/cm for a typical magnetic degenerate dwarf: For neutron stars it is shown that inside the Alfvea surface there is comparatively little flow of matter across field lines, so that accreting matter is channeled toward the magnetic polesmore » of the star where it forms hot spots. The resulting radiation is shown to emerge from the neighborhood of the stellar surface in a strongly anisotropic angular pattern with a spectrum which depends on the details of the accretion process; for an oblique rotator, one has a natural mechanism for the production of pulsed radiation, which is not generally expected to he blackbody in character, will be in excess of 6 keV. Comparison of the above model with the observations suggests strongly that the X-ray stars in the pulsating binary X-ray sources Cen X-3 and Her X-1 are accreting neutron stars with their magnetic axes inclined at substantial angles to their axes of rotation. The implications of this interpretation for the observed X-ray spectra and pulse wave-forms are discussed. The change in the X-ray pulsation period due to accretion of matter is calculated; the calculated time scales for the spin-up which occurs when there is orbital inflow toward a star rotating ia the same sense as the orbital motion, is in excellent agreement with these observed in Her X-1 and Gn X-3. The possibility of significant optical pulsations and the likely existence of a minimum pulsation period for accreting X-ray stars are discussed. (auth)« less