scholarly journals Column Accretion on to White Dwarfs

1983 ◽  
Vol 72 ◽  
pp. 181-197
Author(s):  
A. R. King
Keyword(s):  
Electron Energy ◽  
White Dwarf ◽  
White Dwarfs ◽  
Recent Progress ◽  
Energy Transport ◽  
X Ray

The problem of column accretion on to white dwarfs in the AM Her and DQ Her systems is reviewed. Particular attention is paid to recent progress in explaining the large soft X-ray - EUV fluxes observed in these systems in terms of nonlocal electron energy transport into the white dwarf photosphere.

scholarly journals AM Herculis Binaries

1996 ◽  
Vol 165 ◽  
pp. 403-414 ◽  
Author(s):  
K. Beuermann
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Close Binaries ◽  
Physical Processes ◽  
X Ray ◽  
Sky Survey

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.

scholarly journals White Dwarf Evolution in Real Time: What Pulsating White Dwarfs Teach us About Stellar Evolution

1989 ◽  
Vol 114 ◽  
pp. 97-108 ◽  
Author(s):  
Steven D. Kawaler ◽  
Carl J. Hansen
Keyword(s):  
Real Time ◽  
Structural Properties ◽  
Stellar Evolution ◽  
White Dwarf ◽  
White Dwarfs ◽  
Recent Progress ◽  
Short Review ◽  
Observational Constraints ◽  
Made In

The variable white dwarfs repeatedly force theory to conform to their observed properties so that further progress can be made in understanding the structure and evolution of all white dwarfs. We use the term “understanding” in a loose sense here because, as we will show, both observational constraints and interpretation of the observations vis-à-vis theory contribute to uncertainties in our understanding at this time. In any case, recent progress in this field (sometimes called white dwarf seismology) has provided some fascinating insights into the evolutionary and structural properties of white dwarfs and their progenitors. This short review is our attempt to describe recent progress made in the interaction of theory with observations.

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

1997 ◽  
Vol 163 ◽  
pp. 689-689
Author(s):  
K. Beuermann ◽  
A. Fischer ◽  
Th. Rousseau
Keyword(s):  
Field Strength ◽  
Mass Flow ◽  
White Dwarf ◽  
White Dwarfs ◽  
Large Range ◽  
Radiation Hydrodynamics ◽  
Flow Rates ◽  
X Ray ◽  
Mass Flow Rates

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.

scholarly journals Time Resolved Circular Polarimetry of White Dwarf Pulsars

1982 ◽  
Vol 69 ◽  
pp. 399-401
Author(s):  
W. Krzeminski ◽  
J.D. Landstreet ◽  
I. Thompson
Keyword(s):  
Orbital Period ◽  
White Dwarf ◽  
White Dwarfs ◽  
Radiation Mechanisms ◽  
Time Resolved ◽  
X Ray ◽  
Rotation Periods ◽  
Accretion Process ◽  
Cataclysmic Binaries ◽  
Synchronous Rotation

There are two recognized subsets of cataclysmic binaries that contain white dwarfs: the AM Her stars, whose white dwarf components are sufficiently magnetized to ensure synchronous rotation with the orbital period, and the DQ Her stars, which do not maintain synchronism and probably have been spun up by mass accretion. Both groups of stars, recognized also as X-ray sources, are important as probes of the accretion process, and the radiation mechanisms in the vicinity of the white dwarf. So far, we know five objects belonging to the former group (AM Her, AN UMa, VV Pup, 2A 0311-227, and PG 1550+191), and seven falling into the latter: WZ Sge, AE Aqr, V533 Her, DQ Her, V1223 Sgr, H2252-035, and EX Hya, with the corresponding rotation periods of their white dwarf primaries of 28, 33, 64, 71, 794, 805, and 4020 s, respectively. While the main observational and theoretical efforts have been focussed on the AM Her class, the DQ Her binaries have been investigated to much lesser extent. This is probably because the DQ Her stars have been recognized only very recently as a homogeneous class.

scholarly journals Soft X-Ray Spectroscopy with Exosat

1984 ◽  
Vol 86 ◽  
pp. 59-66
Author(s):  
R. Mewe
Keyword(s):  
Absorption Edge ◽  
White Dwarf ◽  
White Dwarfs ◽  
Cataclysmic Variables ◽  
Atmospheric Model ◽  
Abundance Ratio ◽  
The Other ◽  
X Ray ◽  
Element Abundances ◽  
Medium Resolution

With the 500 and 1000 l/mm transmission gratings aboard the European x-ray Observatory SATellite (EXOSAT) we have measured medium-resolution (Δλ 5 A at 100 A) spectra of some ten objects of various categories such as isolated white dwarfs, cool stars with convective mantles, cataclysmic variables (e.g. AM Her) and a high-luminosity X-ray source (Sco X-1).The Instrument configuration was mostly such that one low-energy telescope was used as a photometer, while the other telescope was used as a spectrometer with the 500 l/mm grating inserted.The white dwarf spectra were measured between about 60 and 300 A. They show a continuum with no clear evidence of aborption and emission lines except for the He II absorption edge at 227 A in the spectrum of Feige 24. For the cooler (28 000 K) white dwarf Sirius B the emission is peaked between about 100 and 160 A and limited to about 200 A. which can be expected from atmospheric model spectra of DA white dwarfs. The soft X-ray emission of the hotter (> 60 000 K) DA white dwarfs (HZ43. Feige 24) is also interpreted in terms of photospheric emission. In the HZ43 spectrum the absorption edge is apparently absent which sets a stringent upper limit to the abundance ratio He/H of about 10−5. On the other hand the spectrum of Feige 24 shows a dominant absorption edge, implying He/H > 10−3. Moreover, here the shape of the continuum may be indicative of a stratification of element abundances in the outer atmosphere.

scholarly journals ROSAT Studies of the Composition and Structure of DA White Dwarf Atmospheres

1993 ◽  
Vol 155 ◽  
pp. 497-497
Author(s):  
C.J. Diamond ◽  
M.A. Barstow ◽  
A.E. Sansom ◽  
M.C. Marsh ◽  
S.R. Rosen ◽  
...  
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
X Ray ◽  
Composition And Structure ◽  
Sky Survey

We have made a detailed study of a sample of 28 hot DA white dwarfs detected in the ROSAT EUV and soft X-ray all-sky-survey.

scholarly journals Mass Measurement of Accreting Magnetic White Dwarfs with Hard X-Ray Spectroscopy

1998 ◽  
Vol 188 ◽  
pp. 97-100
Author(s):  
M. Ishida ◽  
R. Fujimoto
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Mass Measurement ◽  
Cataclysmic Variables ◽  
Magnetic Pole ◽  
X Ray ◽  
Secondary Star ◽  
Magnetic Cataclysmic Variables ◽  
Image Position ◽  
The Magnetic Field

Accreting magnetic white dwarfs are usually found as component stars in Magnetic Cataclysmic Variables (MCVs), in which a white dwarf with B = 105-8 G accepts mass from a late type (secondary) star via Roche Lobe overflow. Matter from the secondary is funneled by the magnetic field and concentrates on the magnetic pole(s) of the white dwarf. Since the accretion flow becomes highly supersonic, a standing shock wave is formed close to the white dwarf. The temperature of the plasma at the shock front reflects the gravitational potential and can be denoted as a function of the mass (M) and the radius (R) of the white dwarf as: Note here that the height of the shock is expected to be within 10% of the white dwarf radius, and hence neglected here.

scholarly journals Rosat - An All-Sky X-Ray and EUV Survey of White Dwarfs

1989 ◽  
Vol 114 ◽  
pp. 156-159 ◽  
Author(s):  
M.A. Barstow
Keyword(s):  
White Dwarf ◽  
Energy Band ◽  
White Dwarfs ◽  
Paper Briefly ◽  
The West ◽  
X Ray ◽  
Sky Survey ◽  
One Year ◽  
The Uk ◽  
Imaging Telescopes

AbstractThe West German astronomy satellite ROSAT comprises two imaging telescopes - an X-ray instrument covering the energy band 0.15-2keV and an EUV instrument (provided by the UK) covering the range 0.02-0.21keV. A primary aim of the mission is to perform a sensitive all-sky survey with both telescopes (two ‘colours’ in the EUV) in the first 6 months. This will be followed by a least one year of pointed observations at specific targets. This paper briefly describes the ROSAT instrumentation and discusses the likely impact of the survey on white dwarf studies.

scholarly journals White Dwarf Models of Supernovae and Cataclysmic Variables

1987 ◽  
Vol 93 ◽  
pp. 395-411
Author(s):  
K. Nomoto ◽  
M. Hashimoto
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Cataclysmic Variables ◽  
Reaction Rates ◽  
Type I ◽  
X Ray ◽  
Composition Structure ◽  
Helium Star ◽  
X Ray Binaries ◽  
Good Agreement

AbstractIf the accreting white dwarf increases its mass to the Chandrasekhar mass, it will either explode as a Type I supernova or collapse to form a neutron star. In fact, there is a good agreement between the exploding white dwarf model for Type I supernovae and observations. We describe various types of evolution of accreting white dwarfs as a function of binary parameters (i.e, composition, mass, and age of the white dwarf, its companion star, and mass accretion rate), and discuss the conditions for the precursors of exploding or collapsing white dwarfs, and their relevance to cataclysmic variables. Particular attention is given to helium star cataclysmics which might be the precursors of some Type I supernovae or ultrashort period X-ray binaries. Finally we present new evolutionary calculations using the updated nuclear reaction rates for the formation of O+Ne+Mg white dwarfs, and discuss the composition structure and their relevance to the model for neon novae.

scholarly journals Statistics of white dwarf properties in intermediate polars

2019 ◽  
Vol 15 (S357) ◽  
pp. 202-205
Author(s):  
Valery F. Suleimanov ◽  
Victor A. Doroshenko ◽  
Klaus Werner
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Luminosity Function ◽  
Cataclysmic Variables ◽  
X Ray ◽  
Intermediate Polars ◽  
The Masses

AbstractMany intermediate polars are hard X-ray sources. The theory of their hard X-ray radiation is well developed and allows us to determine white dwarf masse in this kind of cataclysmic variables. Here we present the results of determination the masses of 35 white dwarfs in the intermediate polars observed by observatories NuSTAR (10 sources) and Swift/BAT (25 sources). The corresponding mass accrerion rates and the luminosity function were also derived due to accurate distance to the sources well known now after Gaia DR2.

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