Propeller spin-down and the non-thermal emission from AE Aquarii

To explain the observed multiwavelength emission from the cataclysmic varia ble star AE Aquarii, we propose a model based upon the assumption that the compact component of this binary consists of a white dwarf with a surface f ield strength of 1 MG, rotating through a clumpy ring near the circularizat ion radius. We argue that MHD instabilities like the Kelvin-Helmholtz insta bilities will result in the white dwarf field mixing with the gas in this r egion and becoming highly sheared. The Poynting flux pumped into the region , and hence angular momentum transferred in the process, is enough to accel erate material centrifugally from the system. We also show that magnetic re connection in this region will result in runaway electron acceleration to e nergies of 300 MeV or more, which will radiate through the synchrotron proc ess up to frequencies of the order of nu similar to 10(15) Hz. We show that , because the radiation is emitted in a gaseous medium, the spectrum will b e suppressed below the so-called Razin-Tsytovich frequency, which is nu(R)s imilar to 2000 GHz, which agrees with the observations. Magnetic shear in t he ejection region will also induce field-aligned currents in the magnetosp here. Huge potential differences (double layers) can be generated in the ci rcuit where the gas density becomes very low. This will occur close to the white dwarf. In this region potentials of the order of Phi(parallel to)=300 TV (TV = teravolt) can be generated. Synchrotron losses on electrons will restrict them to energies of the order of E less than or equal to 50 keV. A s protons and ions are not affected by synchrotron losses, energies in exce ss of 1 TeV are possible. Very high-energy (VHE) gamma-ray emission can be produced if these proton and ion beams collide with the clumpy gas in, or o utside, the ejection zone. We show that pulsed VHE gamma-ray emission is po ssible, if the gas particle density of the target matter N-g> 10(13) cm(-3) , which is compatible with the typical gas particle densities N(g)similar t o 10(16) cm(-3) of the blobs inside the clumpy accretion stream from the se condary star. We show that the VHE gamma-ray bursts that were reported by t wo groups can be explained in terms of an exploding double layer, which wil l result in a cataclysmic burst of VHE protons and ions over short time-sca les. The total power released in the process can be P(gamma,b)similar to 10 (34) erg s(-1), which is equal to the spin-down power of the white dwarf, a nd is enough to explain the reported VHE gamma-ray bursts. This implies tha t the whole reservoir of spin-down power has gone into the production of VH E particles and gamma-ray emission.