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Astron. Astrophys. 322, 242-255 (1997)

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8. Conclusions

If the fast spinning white dwarf in AE Aqr has a surface field of 100 T magnetic pumping is a serious candidate for the particle acceleration needed to explain the observed radio outbursts. We have studied the various physical conditions that are required. The most efficient radiator of strong magnetic oscillations is a stream of gas blobs from the secondary into the magnetosphere of the white dwarf. Further an important ingredient for magnetic pumping to work is to put a sufficient amount of dilute plasma onto the magnetic field lines, and to keep the particles trapped long enough for the accelerator to work. To have efficient pumping a scattering process is required such that the pitch angle scattering time matches the oscillation time of the field. For simplicity we have limited ourselves to oscillations at the spin period, although perturbations exist also at higher frequencies. We have just assumed that a suitable plasma instability takes care of the scattering, and more work is needed to demonstrate that such an instability occurs. Note that plasma instabilities caused by loss-cone distributions can also lead to radio emission. However, a clear signature of a coherent emission process is lacking in the majority of the radio observations in AE Aqr. On particular magnetic field lines the MHD oscillations lead to particle acceleration also by the processes of transit-time damping and resonant heating, and these processes deserve further study. The injection energy appears to be no problem, both because of the initial particle energy in the gravitational well of the white dwarf and because of the rapid rotation which gives a high initial pickup energy to ionized atoms. The accelerated particles are not the origin of the X-rays seen in AE Aqr. Finally our proposal of magnetic pumping of a particle trap anchored in the white dwarf and its subsequent expulsion seems to explain the pertinent radio observations of AE Aqr.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998