Search

Accretion-driven spin-up of a magnetized white dwarf in a close binary system is discussed. We address a situation in which the magnetic field of the white dwarf is screening during the accretion phase and re-generating due to the field diffusion through the accreted material after it. We find this scenario to be effective for a formation of massive pulsar-like white dwarfs., Comment: 3 pages, to appear in Proc. of Thirteenth Marcel Grossmann Meeting - MG13

The most striking feature of the close binary system AE Aqr is its unique photometric behaviour - irregular alternations of quiet and active phases without noticeable transition periods. Although extensive photometric observations of AE Aqr have been carried out for more than forty years (see Bruch 1991 and references therein), the mechanism of its flaring activity is still poorly understood and the star - although generally classified as a cataclysmic variable - does not fit well into any of the subclasses. According to us, much insight into this peculiar object can be gained by an investigation of the connections between the flaring activity of AE Aqr and the strictly periodic oscillations at 33 s , identified as the spin period of the primary. The latter have been seen in optical (Patterson 1979) and UV light (Eracleous et al. 1994), in X-rays (Patterson et al. 1980) and in TeV γ rays (Meintjes et al. 1992). There are contradictory reports about the relation between their amplitude and the flaring activity in AE Aqr: Patterson (1979) noted that the amplitude of the pulsations increases during active phases, while Eracleous et al. (1994) observed that their characteristics are not strongly affected by the flares.

The spin-down power of the white dwarf in the close binary AE Aquarii significantly exceeds the bolometric luminosity of the system. The interpretation of this phenomenon in terms of the gravitational-wave emitter mechanism has been recently suggested by Choi & Yi. The basic assumption of their interpretation is that the spatially limited blobs or mounds of the mass ��m ~ 10^{-3} M_sun, are present at the magnetic poles of the white dwarf. We show that the mounds of this mass can be confined by the magnetic field of the white dwarf only if the dipole magnetic moment of the star exceeds 4x10^{37} G cm^3. Under these conditions, however, the magnetodipole losses of the white dwarf would exceed the evaluated spin-down power 6 orders of magnitude. On this basis we discard a possibility that the observed rapid braking of the white dwarf in AE Aquarii can be explained in terms of the mechanism proposed by Choi & Yi., 6 pages, published in ApJ, 576, L57

Study of observed spin evolution of long-period X-ray pulsars challenges quasi-spherical and Keplerian disk accretion scenarios. It suggests that the magnetospheric radius of the neutron stars is substantially smaller than Alfven radius and the spin-down torque applied to the star from accreting material significantly exceeds the value predicted by the theory. We show that these problems can be avoided if the fossil magnetic field of the accretion flow itself is incorporated into the accretion model. The initially spherical flow in this case decelerates by its own magnetic field and converts into a non-Keplerian disk (magnetic slab) in which the material is confined by its intrinsic magnetic field ("levitates") and slowly moves towards the star on a diffusion timescale. Parameters of pulsars expected within this magneto-levitation accretion scenario are evaluated., to appear in Proc. of 4th High Energy Phenomena in Relativistic Outflows (HEPRO IV) meeting, Heidelberg, July 23-26, 2013 (International Journal of Modern Physics: Conference Series, 2014)

Analysis of line profiles allows the geometric and kinematic characteristics of gaseous inhomogeneities to be determined. In addition, multi-colour polarimetry makes it possible to study the structure of the circumstellar medium close to the star, and to diagnose the magnetic field inside a moving inhomogeneity. The influence of the magnetic field on the polarization parameters was investigated in the framework of a simple model of a gaseous jet rotating in the equatorial plane of the star (Beskrovnaya & Pogodin 1998), with allowance for the Faraday rotation of the polarization plane due to electron scattering in the magnetized media (Gnedin & Silant’ev 1984). This method was used to study circumstellar peculiarities of a well-known Herbig Ae star, AB Aur. We investigated variations of the Stokes parameters observed on 1994 January 9, when the rotating jet was travelling close to the line-of-sight (as determined from analysis of the spectroscopic data; for details see Beskrovnaya et al. 1995). The observed locus of the Stokes parameters on the (q, u)-plane is in good agreement with the results of calculations for i = 60° and B = 0. Another type of cyclic phenomena can be noticed in the polarimetric variations of HD 163296 (for details see Beskrovnaya et al. 1998a). Sinusoid-like curves with a period of 7d.5 are clearly seen in both Stokes parameters in the V-band, while in the I-band sinusoidal variations are seen only in the q-parameter. This effect can be explained by assuming that the rotating gaseous condensation is magnetized, since the influence of the magnetic field is stronger at longer wavelengths. A multi-component picture of circumstellar inhomogeneities moving with different periods at different distances from the star has also been revealed from spectral and polarimetric analysis of HD 36112 (Beskrovnaya et al. 1998b).

The 33s.08 oscillation and its second harmonic in the optical light of AE Aqr, detected by Patterson (1979; ApJ 234, 978) and being due to hot magnetic pole caps rotating in and out of sight, are considered a persistent characteristic of the system. Photometry performed over a time base of four weeks in July-August 1993 shows, however, that the oscillations have vanished below a detectable limit (see also Bruch et ah 1994; IBVS 3996). Power spectra of the light curves show neither a coherent signal at 33s.08 or 15s.54 nor QPO around these periods. Tests with tracer signals allowed us to put a conservative upper limit of 0m.005 to the amplitude of any coherent signal in the data. We investigated archive light curves of AE Aqr of 45 nights distributed (unevenly) over the years 1978 – 1992 and found the oscillations to be absent only in one the these nights. A comparable case may be V533 Her where Robinson and Nather (1983; ApJ 373, 255) reported the disappearance of the 63 s periodicity.

Simultaneous high-resolution spectroscopy in Ha and UBVRI polarimetric observations are proposed as an effective method for the search for circumstellar inhomogeneities in A0-type Herbig stars. The new results for AB Aur are presented as a successful example of the use of this method. The analysis of about 100 CCD Ha profiles (R = 30 000) and more than 150 polarimetric measurements obtained in January, 1994 allowed to discover a long-lived stream-like inhomogeneity in the circumstellar gaseous envelope.

The most striking feature of the close binary system AE Aqr is its unique photometric behaviour - irregular alternations of quiet and active phases without noticeable transition periods. Although extensive photometric observations of AE Aqr have been carried out for more than forty years (see Bruch 1991 and references therein), the mechanism of its flaring activity is still poorly understood and the star - although generally classified as a cataclysmic variable - does not fit well into any of the subclasses.According to us, much insight into this peculiar object can be gained by an investigation of the connections between the flaring activity of AE Aqr and the strictly periodic oscillations at 33s, identified as the spin period of the primary. The latter have been seen in optical (Patterson 1979) and UV light (Eracleous et al. 1994), in X-rays (Patterson et al. 1980) and in TeV γ rays (Meintjes et al. 1992). There are contradictory reports about the relation between their amplitude and the flaring activity in AE Aqr: Patterson (1979) noted that the amplitude of the pulsations increases during active phases, while Eracleous et al. (1994) observed that their characteristics are not strongly affected by the flares.