Comparisons of MHD propeller model with observations of cataclysmic variable AE Aqr

Abstract

We have developed a numerical MHD model of the propeller candidate star AE Aqr using axisymmetric magneto-hydrodynamic (MHD) simulations. We suggest that AE Aqr is an intermediate polar-type star, where the magnetic field is relatively weak and an accretion disc may form around the white dwarf. The star is in the propeller regime, and many of its observational properties are determined by the disc-magnetosphere interaction. Comparisons of the characteristics of the observed versus modelled AE Aqr star show that the model can explain many observational properties of AE Aqr. In a representative model, the magnetic field of the star is B ≈ 3.3 × 10 5 G and the time-averaged accretion rate in the disc is 5.5 × 10 16 g/s. Most of this matter is ejected into conically-shaped winds. The numerical model explains the rapid spin-down of AE Aqr through the outflow of angular momentum from the surface of the star to the wind, corona and disc. The energy budget in the outflows, 9 × 10 33 erg/s, is sufficient for explaining the observed flaring radiation in different wavebands. The time scale of ejections into the wind matches the short time scale variability in the light curves of AE Aqr.

High-dispersion spectroscopy of AE Aquarii -- II. Evidence of material orbiting the primary star

“…Also, the narrow component of the Balmer emission lines observed by Reinsch & Beuermann (1994) is in anti-phase to the absorption lines of the red dwarf, which suggests its origin is near the primary star. Furthermore, the presence of an accretion disc with a variable nature cannot be discarded, as suggested by the axisymmetric magnetohydrodynamics simulations put forward by Blinova et al (2019).…”

“…These authors, with proper caution, interpreted this as possible evidence that the material transferred from the secondary was being ejected by the rapidly-rotating magnetic field of the WD, which acted as a propeller at the time of their observations. Indeed, numerical magnetohydrodynamics simulations (Isakova et al 2016;Blinova et al 2019) predict that, when in a propeller regime, an oscillating and rather unstable disc of a transient nature, is able to form around the WD of AE Aqr, which would be difficult to detect in Doppler tomograms. So as was succinctly put by Wynn et al (1997): AE Aqr is likely to alternate between phases of disc accretion, in which the white dwarf spins up, and propeller states in which it spins down.…”