We model the population of AM CVn systems in the Galaxy and discuss the detectability of these systems with optical, X-ray and gravitational-wave detectors. We concentrate on the short-period (P < 1500 s) systems, some of which are expected to be in a phase of direct-impact accretion. Using a self-consistent model for the star-formation history and radial distribution of stars in the Galaxy plus simple models for the emission of optical and X-ray radiation from the AM CVn systems and interstellar absorption, we derive the sample of short-period AM CVn systems that can be detected in the optical and/or X-ray bands. At the shortest periods, the detectable systems are all X-ray sources, some with periods as short as three minutes. At periods above 10 min, most detectable systems are optical sources. About one-third of the X-ray sources are also detectable in the optical band. We also calculate the gravitational-wave signal of the short-period AM CVn systems. We find that potentially several thousand AM CVn systems can be resolved by the gravitational-wave detector Laser Interferometer Space Antenna (LISA), comparable to the expected number of detached double white dwarfs that can be resolved. We estimate that several hundreds of the AM CVn systems resolvable by LISA are also detectable in the optical and/or X-ray bands.

Short-period AM CVn systems as optical, X-ray and gravitational-wave sources

The X-ray flux of the bright galactic bulge source GX5 - 1 shows intensity-dependent quasi-periodic oscillations between 20 and 40 Hz, appearing as a broad peak in the power spectrum whose centroid frequency, width, and integrated excess power strongly depend on the source intensity. The strength and steepness of low-frequency noise present in the power spectra below 15 Hz also depend on the source intensity. No evidence is found for coherent X-ray pulsations between 0.5 and 2000 Hz. Possible mechanisms to explain these new phenomena are discussed.

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Intensity-dependent quasi-periodic oscillations in the X-ray flux of GX5 - 1

We have used a model of magnetic accretion to investigate the rotational equilibria of magnetic cataclysmic variables (mCVs). The results of our numerical simulations demonstrate that there is a range of parameter space in the Pspin=Porb versus � 1 plane at which rotational equilibrium occurs. This has allowed us to calculate the theoretical histogram describing the distribution of mCVs as a function of Pspin=Porb. We show that this agrees with the observed distribution, assuming that the number of systems as a function of white dwarf magnetic moment is distributed approximately according to N � 1 ðÞ d� 1 / � � 1 1 d� 1 . The rotational equilibria also allow us to infer approximate values for the magnetic moments of all known intermediate polars. We predict that intermediate polars with � 1 k5 ; 10 33 Gc m 3 and Porb > 3 hr will evolve into polars, while those with � 1 P5 ; 10 33 Gc m 3 and Porb > 3 hr will either evolve into low field strength polars that are (presumably) unobservable, and possibly EUV emitters, or, if their fields are buried by high accretion rates, evolve into conventional polars, once their magnetic fields resurface when the mass accretion rate reduces. We speculate that EX Hya‐like systems may have low magnetic field strength secondaries and so avoid synchronization. Finally, we note that the equilibria we have investigated correspond to a variety of different types of accretion flow, including disklike accretion at small Pspin=Porb values, streamlike accretion at intermediate Pspin=Porb values, and accretion fed from a ring at the outer edge of the white dwarf Roche lobe at higher Pspin=Porb values.

/pdf/the-spin-periods-and-magnetic-moments-of-white-dwarfs-in-2qvxgs6mtq.pdf

The spin periods and magnetic moments of white dwarfs in magnetic cataclysmic variables

Rapid coherent and quasi-periodic oscillations in the optical and X-ray fluxes of cataclysmic variable stars are described

https://www.jstor.org/stable/10.1086/381742

Rapid oscillations in cataclysmic variables

▪ Abstract Accretion disks are important for many astrophysical phenomena, including galactic nuclei, interacting binary stars, and young stellar objects. The central issue in the theory of accretion disks is to identify the dominant mechanisms that regulate angular momentum transfer and mass flow in a variety of contexts. In the first part of this review, we described some recent advances in the study of the physical processes that may be present in accretion disks. Concurrent with these theoretical developments, the arrival of high-resolution astronomical instruments has led to explosive progress on the observational side. In many cases, the study of accretion disks has evolved from their inferred presence based on circumstantial evidence to direct imaging and detailed spectral analyses. Here, we summarize the theoretical interpretation of these data. We review the constraints that may be imposed on the efficiency and nature of angular momentum transfer processes in a variety of astrophysical contexts.

W. D. Evans

Papers

The Accretion Model of Dwarf Novae with Application To Z Chamaeleontis

An Oblique Rotator Model for DQ Herculis

A Simple Model of Fluctuations in Accretion Discs