scholarly journals Cyclotron Emission from the Shock-Heated Region of AM Herculis

1979 ◽  
Vol 53 ◽  
pp. 341-342 ◽  
Author(s):  
G. Chanmugam
Keyword(s):  
Magnetic Field ◽  
Strong Magnetic Field ◽  
White Dwarf ◽  
Main Sequence ◽  
X Rays ◽  
Heated Region ◽  
Magnetic Pole ◽  
Uv Emission ◽  
Cyclotron Emission

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).

scholarly journals A new weak-field magnetic DA white dwarf in the local 20 pc volume

2019 ◽  
Vol 628 ◽  
pp. A1 ◽  
Author(s):  
J. D. Landstreet ◽  
S. Bagnulo
Keyword(s):  
Magnetic Field ◽  
White Dwarf ◽  
Field Component ◽  
Main Sequence ◽  
Mean Field ◽  
Weak Field ◽  
Line Of Sight ◽  
Limited Sample ◽  
Field Decay ◽  
The Magnetic Field

We report the discovery of a new magnetic DA white dwarf (WD), WD 0011 − 721, which is located within the very important 20 pc volume-limited sample of the closest WDs to the Sun. This star has a mean field modulus ⟨|B|⟩ of 343 kG, and from the polarisation signal we deduce a line-of-sight field component of 75 kG. The magnetic field is sufficiently weak to have escaped detection in classification spectra. We then present a preliminary exploration of the data concerning the frequency of such fields among WDs with hydrogen-rich atmospheres (DA stars). We find that 20 ± 5% of the DA WDs in this volume have magnetic fields, mostly weaker than 1 MG. Unlike the slow field decay found among the magnetic Bp stars of the upper main sequence, the WDs in this sample show no evidence of magnetic field or flux changes over several Gyr.

scholarly journals Strong Magnetic Fields in Nova Systems

1990 ◽  
Vol 122 ◽  
pp. 47-47
Author(s):  
H.S. Stockman ◽  
G.D. Schmidt
Keyword(s):  
Magnetic Field ◽  
Field Strength ◽  
Circular Polarization ◽  
Magnetic Moment ◽  
White Dwarf ◽  
Strong Magnetic Fields ◽  
General Picture ◽  
Cyclotron Emission ◽  
Accretion Shock ◽  
Independent Confirmation

The discovery of variable circular polarization in V1500 Cygni (the remnant of Nova Cygni 1975) is the strongest evidence for the presence of highly magnetic white dwarfs in nova systems. If interpreted in terms of diluted cyclotron emission from a hot accretion shock, the recent observations of Schmidt and Stockman (1990 preprint, Ap. J. 1991) of the color dependence of the circular polarization can provide a empirical lower-limit to the primary’s magnetic field strength of B > 25 × 106 gauss. Such a field strength is comparable to those observed in other magnetic variables, thus providing support for the general picture of the current and pre-nova system and the explanation for the observed search-light and period changes following the eruption. Schmidt and Stockman have also measured lengthening of the polarimetric period indicating that the system will be resynchronized within a few centuries and well before the next nova eruption. This is an independent confirmation of a significant magnetic moment for the white dwarf primary.

scholarly journals Measuring T Tauri star magnetic fields

2008 ◽  
Vol 4 (S259) ◽  
pp. 345-356 ◽  
Author(s):  
Christopher M. Johns–Krull
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Scaling Laws ◽  
Main Sequence ◽  
Critical Role ◽  
X Rays ◽  
Stellar Rotation ◽  
T Tauri ◽  
Dipole Component ◽  
Stellar Magnetic Fields

AbstractStellar magnetic fields including a strong dipole component are believed to play a critical role in the early evolution of newly formed stars and their circumstellar accretion disks. It is currently believed that the stellar magnetic field truncates the accretion disk several stellar radii above the star. This action forces accreting material to flow along the field lines and accrete onto the star preferentially at high stellar latitudes. It is also thought that the stellar rotation rate becomes locked to the Keplerian velocity near the radius where the disk is truncated. This paper reviews recent efforts to measure the magnetic field properties of low mass pre-main sequence stars, focussing on how the observations compare with the theoretical expectations. A picture is emerging indicating that quite strong fields do indeed cover the majority of the surface on these stars; however, the dipole component of the field appears to be alarmingly small. The current measurements also suggest that given their strong magnetic fields, T Tauri stars are somewhat faint in X-rays relative to what is expected from simple main sequence star scaling laws.

scholarly journals Eclipsing AM Herculis Binaries

1997 ◽  
Vol 163 ◽  
pp. 391-395
Author(s):  
Jeremy Bailey
Keyword(s):  
Magnetic Field ◽  
Strong Magnetic Field ◽  
Time Resolution ◽  
White Dwarf ◽  
Cataclysmic Variables ◽  
High Time ◽  
High Time Resolution ◽  
Companion Star ◽  
Fundamental Properties ◽  
Cyclotron Radiation

AbstractAM Herculis Binaries (or Polars) are a subclass of the Cataclysmic Variables in which the accreting white dwarf has a strong magnetic field giving rise to highly polarized cyclotron radiation from the shock heated accretion region. A number of AM Herculis binaries are now known in which the white dwarf is eclipsed by the companion star. High time resolution observations of these eclipses allow a particularly detailed study of the process of accretion onto the magnetic white dwarf. Results on a number of systems will be presented and used to derive information on the accretion structure as well as on the fundamental properties of the binaries.

scholarly journals Time Dependence in Accretion Onto Magnetic White Dwarfs

1983 ◽  
Vol 72 ◽  
pp. 199-205
Author(s):  
Steven H. Langer ◽  
G. Chanmugam ◽  
G. Shaviv
Keyword(s):  
Magnetic Field ◽  
Kinetic Energy ◽  
Orbital Period ◽  
White Dwarf ◽  
Velocity Structure ◽  
Theoretical Models ◽  
Cataclysmic Variable ◽  
X Rays ◽  
Accretion Flow ◽  
The Magnetic Field

In this talk, we consider cataclysmic variable systems containing a white dwarf with a strong magnetic field. These include systems like AM Her (see, e.g., Chiappetti, Tanzi, and Treves 1981) in which the white dwarf rotates at the orbital period and systems such as AE Aquari in which the white dwarf rotates much faster than synchronously (see Patterson et al. 1980). The magnetic field in all of these systems is strong enough to disrupt the accretion disk at a point far above the surface of the white dwarf and may prevent the formation of a disk altogether. We will present theoretical models for the temperature, density, and velocity structure of the accretion flow in the region near the surface of the white dwarf where the kinetic energy of the flow is thermalized and radiated in the form of X-rays and ultraviolet radiation. This information is required to calculate accurate model spectra, and the results also have immediate consequences for the interpretation of observations.

scholarly journals Magnetospheric accretion in the intermediate-mass T Tauri star HQ Tauri

2020 ◽  
Vol 642 ◽  
pp. A99 ◽  
Author(s):  
K. Pouilly ◽  
J. Bouvier ◽  
E. Alecian ◽  
S. H. P. Alencar ◽  
A.-M. Cody ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Strong Magnetic Field ◽  
Main Sequence ◽  
T Tauri Stars ◽  
Line Profiles ◽  
Main Sequence Stars ◽  
Intermediate Mass ◽  
Accretion Process ◽  
T Tauri ◽  
Magnetospheric Accretion

Context. Classical T Tauri stars are pre-main sequence stars surrounded by an accretion disk. They host a strong magnetic field, and both magnetospheric accretion and ejection processes develop as the young magnetic star interacts with its disk. Studying this interaction is a major goal toward understanding the properties of young stars and their evolution. Aims. The goal of this study is to investigate the accretion process in the young stellar system HQ Tau, an intermediate-mass T Tauri star (1.9 M⊙). Methods. The time variability of the system is investigated both photometrically, using Kepler-K2 and complementary light curves, and from a high-resolution spectropolarimetric time series obtained with ESPaDOnS at CFHT. Results. The quasi-sinusoidal Kepler-K2 light curve exhibits a period of 2.424 d, which we ascribe to the rotational period of the star. The radial velocity of the system shows the same periodicity, as expected from the modulation of the photospheric line profiles by surface spots. A similar period is found in the red wing of several emission lines (e.g., HI, CaII, NaI), due to the appearance of inverse P Cygni components, indicative of accretion funnel flows. Signatures of outflows are also seen in the line profiles, some being periodic, others transient. The polarimetric analysis indicates a complex, moderately strong magnetic field which is possibly sufficient to truncate the inner disk close to the corotation radius, rcor ∼ 3.5 R⋆. Additionally, we report HQ Tau to be a spectroscopic binary candidate whose orbit remains to be determined. Conclusions. The results of this study expand upon those previously reported for low-mass T Tauri stars, as they indicate that the magnetospheric accretion process may still operate in intermediate-mass pre-main sequence stars, such as HQ Tauri.

scholarly journals Extension of the Radio Spectrum of AE Aqr to the Sub-millimetric Range

1995 ◽  
Vol 151 ◽  
pp. 268-271
Author(s):  
Meil Abada-Simon ◽  
Tim S. Bastian ◽  
Jay A. Bookbinder ◽  
Monique Aubier ◽  
Gordon Bromage ◽  
...  
Keyword(s):  
White Dwarf ◽  
Main Sequence ◽  
Ultra Violet ◽  
Radio Spectrum ◽  
Cataclysmic Variable ◽  
X Rays ◽  
Dipole Axis ◽  
Γ Rays ◽  
Γ Ray ◽  
Magnetic Poles

AE Aquarii is a magnetic cataclysmic variable containing a white dwarf and a K3-K7 star which lies slightly above the main sequence. The white dwarf is the most rapidly rotating known (Prot ≃ 33.08 s, Patterson 1979), and it is the most strongly asynchronous with its revolution (Porb = 9.88 hr). The white dwarf accretes matter from the K star, which approximately fills the Roche lobe. AE Aqr exhibits flares in the soft X-rays, the ultra-violet, and almost continuously in the visible and the radio regimes. Rapid optical and TeV γ-ray bursts have also been discovered, which are modulated with the period of the white dwarf and at half of this period (de Jager & Meintjes 1993). This modulation, also found in X-rays, is interpreted as the accretion of matter onto the white dwarf’s magnetic poles. The strength of the white dwarf’s magnetic field is not well-determined, it is estimated to be ∼ 6.104 - 105 G (Lamb & Patterson 1983, Cropper 1986) at the white dwarf’s surface. Eracleous et al. (1994) recently suggested that the magnetic dipole axis lies close to the equatorial plane (∼ 20°). De Jager et al. (1994) discovered a rapid spin down of the white dwarf leading to a spin down power which exceeds the accretion power. They suggest that a significant fraction of the spin down power may be converted to the acceleration of particles, which may explain the radio and the γ-ray emissions. Both the characteristics of the optical flares and the existence of TeV γ-rays suggest a relation with the non-thermal radio flares.

scholarly journals Breaking the 100 MG Barrier: The First High Field Magnetic CV

1997 ◽  
Vol 163 ◽  
pp. 409-412
Author(s):  
Paula Szkody ◽  
Gary D. Schmidt ◽  
Paul S. Smith ◽  
Andrew Silber ◽  
D.W. Hoard ◽  
...  
Keyword(s):  
Magnetic Field ◽  
High Magnetic Field ◽  
White Dwarf ◽  
High Field ◽  
Optical Spectra ◽  
Cataclysmic Variable ◽  
High State ◽  
Optical Photometry ◽  
X Ray ◽  
Cyclotron Emission

AbstractWe present results from IUE and optical spectra, optical photometry and circular polarimetry during high and low states of the highly luminous soft X-ray cataclysmic variable AR UMa that identifies the primary in this system as a white dwarf with a magnetic field of 230 MG. The high magnetic field likely threads accretion blobs all the way from the secondary to below the surface of the white dwarf, resulting in a lack of polarised cyclotron emission and an extreme soft-X-ray luminosity during the high state.

scholarly journals The EUV Excess in Magnetic Cataclysmic Variables

1996 ◽  
Vol 152 ◽  
pp. 325-329
Author(s):  
John K. Warren ◽  
Koji Mukai
Keyword(s):  
Magnetic Field ◽  
Orbital Period ◽  
White Dwarf ◽  
Preliminary Analysis ◽  
Cataclysmic Variables ◽  
X Rays ◽  
System Parameters ◽  
Magnetic Capture ◽  
Magnetic Cataclysmic Variables ◽  
Capture Radius

We present preliminary analysis of EUVE pointed data of 8 magnetic cataclysmic variables. Blackbody temperatures, luminosities, and interstellar columns have been better constrained. Using these luminosities we look for correlations between the EUV excess (over optical and hard X-rays) and various system parameters. While it appears there is no correlation between the EUV excess and the system inclination and orbital period, correlations are suggested between the EUV excess and the longitude of the accretion spot, the colatitude of the accretion spot, the white dwarf magnetic field, and the magnetic capture radius.

scholarly journals The Magnetic Field of the Intermediate Polar RE 0751+14

1996 ◽  
Vol 158 ◽  
pp. 183-183
Author(s):  
H. Väth
Keyword(s):  
Magnetic Field ◽  
White Dwarf ◽  
Accretion Rate ◽  
Shock Structure ◽  
Light Curves ◽  
X Rays ◽  
Strong Magnetic Fields ◽  
X Ray ◽  
Spin Period ◽  
The Magnetic Field

Piirola, Hakala & Coyne (1993) modeled the optical/IR light curve of RE 0751+14 assuming a uniform shock structure and neglecting the hard X-ray emission. In this paper, we model the light curves at optical/IR and hard X-ray wavelengths and include the effects of the shock structure.We base our model on accretion onto a white dwarf with a displaced magnetic dipole for a range of likely white dwarf masses. We find that the observed intensity variations of X-rays and in the I band over one spin period largely determine the position of the emission regions. Furthermore, the observed maximum X-ray flux constrains the specific accretion rate. We deduce that the magnetic field at the pole is likely to be in the range 9 .. .21 MG, which is consistent with the estimates of Piirola et al. (1993). It had been proposed previously that there must exist asynchronous rotators with sufficiently strong magnetic fields such that the binaries will evolve into AM Her binaries (Chanmugam & Ray 1984; King, Frank & Ritter 1985). With this deduced high magnetic field RE 0751+14 is the most likely example of such a system known to date.

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