On the Rapid Spin‐down and Low‐Luminosity Pulsed Emission from AE Aquarii

Abstract

AE Aqr is an unusual close binary system with a very short white dwarf spin period, a high spin-down rate, a relatively low quiescent luminosity, and clear pulse signals. The exact nature of the large spin-down power has not been well explained mainly because the observed luminosities in various energy ranges are much lower than the spin-down power. We consider an unconventional picture of AE Aqr in which an accreting white dwarf, modeled as a magnetic dipole whose axis is misaligned with the spin axis, is rapidly spun down via gravitational radiation emission and therefore the spin-down power is not directly connected to any observable electromagnetic emission. The rapid spin-down is caused by the nonaxisymmetric polar mounds of accreted material slowly spreading away from the magnetic poles over the surface of the star. The effectiveness of the spin-down driven by the gravitational radiation depends on complex diffusion and thermonuclear burning of the accretion mound material. The accretion proceeds at high altitudes toward the magnetic poles of the white dwarf, while a large fraction of the inflowing material is ejected in a propeller-like manner. Based on the observed quiescent X-ray and UV emission, the magnetic field strength is estimated as ~1 × 105η G and the mass accretion rate as ~1 × 1015η g s-1, where ηX < 1 is the X-ray radiative efficiency. A large fraction of the accreted mass is flung out by the propeller action, and ~50% of the accreted material arrives at the magnetic poles. The electromagnetic dipole emission is expected at the level of ~1 × 1029η ergs s-1, which suggests that, for ηX ~ 0.1, the observed radio luminosity could be well accounted for by dipole radiation.