Low Brightness State Research Articles

The Benefit of Archival Research in Investigating Dwarf Novae

Dwarf novae and nova-like stars are a sub-group of the class of cataclysmic variables. Most of these stars show high and low brightness states (maxima or outbursts and minima or quiescent states, respectively). They all are short-period interacting binaries consisting of a white dwarf primary, a late-type Roche-lobe filling secondary, and an accretion disk around the white dwarf. For details on observations of cataclysmic variables and their theoretical explanations and modelling, the interested reader is referred to recent reviews.The main problem when trying to understand these systems is the wealth of patterns presented by the whole group: no two objects are nearly identical and it is hard to say which the ‘typical’ features are. As a result of this, from an individual observation, or from observations of a few systems only, it is not possible to decide which of the observed features are characteristic of the entire class, and which are specific of individual members. The only way is to investigate many different members of the same class, arrange them according to their different physical parameters, and try to determine what the general characteristics are.

ROSAT observations of AN UMa and MR Ser: the status of the soft X-ray excess in AM Her stars

We present the results of ROSAT pointed observations of the AM Her stars AN UMa and MR Ser. AN UMa was in a high-brightness state. We examine its soft X-ray light curve, and fit its spectrum with a two-component model consisting of an absorbed soft blackbody and a harder thermal bremsstrahlung distribution. MR Ser was found to be in a low-brightness state, and had a spectrum which was well fitted with a single-component, thermal bremsstrahlung model.The question of the energy balance in AM Her stars is reviewed with a view to quantifying the incidence of the 'soft X-ray excess'. The excess has been explained as due to the deposition of energy directly into the white dwarf photosphere by the accretion stream, supplementing the energy of photons from the hot accretion column which irradiate the photosphere. We have calculated the energy balance in AN UMa and 16 other AM Her stars observed with ROSAT. We find that, although the majority of AM Her stars exhibit a soft X-ray excess, a number of objects are consistent with a model which includes radiative heating only. We examine possible factors which could determine the value of the soft X-ray excess. We find a correlation of soft X-ray excess with magnetic field strength or, alternatively, the radius at which material couples on to the magnetic field lines.

Photometry of long-period Algol binaries. IV - KU Cygni and its thick, dusty accretion disk

Five-color intermediate-band photometric observations of the totally eclipsing long-period binary KU Cygni were obtained from 1982 through 1987. Partial eclipses by the cool star of a large accretion disk around the hot gainer appear as light curve dips bordering both sides of primary stellar eclipse. Secondary eclipse is abnormally broad and deep, and occurs when the accretion disk nearly completely occults the cool loser. These circumstances permit investigation of both the emitting and the light-blocking properties of this disk. Reconciling these properties requires that the pressure-supported disk be vertically thicker than expected from purely gravitational stratification, and that the outer part of the disk be slightly contaminated by dust particles, presumably accreted from the cool loser. Existence of high and low brightness states in this system suggests variable mass accretion by the gainer, at peak rates near 10 to the -6th solar masses/yr. The general implications of such a thick disk are briefly discussed. 22 references.