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

Magnetic pumping in the cataclysmic variable AE Aquarii

small.htm J. Kuijpers ^{1, 2, 3}, L. Fletcher ^{* 1, 2}, M. Abada-Simon ^{1, 2}, K.D. Horne ⁴, M.A. Raadu ⁵, G.Ramsay ^{1, 2} and D. Steeghs <sup>1, 2, 4

1</sup> Sterrekundig Instituut, Universiteit Utrecht, Postbus 80 000, 3508 TA Utrecht, The Netherlands
² CHEAF (Centrum voor Hoge Energie Astrofysica), Postbus 41882, 1009 DB Amsterdam
³ Dep. of Experimental High Energy Physics, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen
⁴ School of Physics and Astronomy, The University, St. Andrews, Fife KY16 9SS, Scotland, UK
⁵ Division of Plasma Physics, Alfvén Laboratory, The Royal Institute of Technology, S-10044 Stockholm, Sweden

Received 10 June 1996 / Accepted 12 November 1996

Abstract

We propose that the radio outbursts of the cataclysmic variable AE Aqr are caused by eruptions of bubbles of fast particles from a magnetosphere surrounding the white dwarf. We investigate the acceleration process of magnetic pumping in the magnetosphere which is periodically driven both by the relative motion with the companion and with the infalling spray of gas at the spin frequency of the white dwarf. As the accretion rate is relatively low, the conversion of spin energy into acceleration (rather than heating) of electrons and protons can be efficient. The accelerated particles are trapped in the white dwarf magnetosphere until their total energy content becomes comparable to that of the trapping magnetic field structure and a MHD instability sets in. Synchrotron radiation is emitted in the expelled expanding plasmoid at radio and down to millimetric wavelengths. We find that there is sufficient energy transferred from the rotation energy of the rapidly-spinning white dwarf to fast particles by magnetic pumping to explain quiescent and flaring radio emissions.

Key words: acceleration of particles magnetic fields turbulence cataclysmic variables stars: individual: AE Aqr radio continuum: stars

* Present affiliation: ESA Space Science Dep., ESTEC, Postbus 299, 2200 AG Noordwijk

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Contents

• 1. Introduction
• 2. Magnetic powering
  • 2.1. Power radiated in MHD waves
  • 2.2. Power radiated by the companion
  • 2.3. Power radiated by infalling gas
• 3. Invariants and acceleration
• 4. Acceleration by magnetic pumping
  • 4.1. Acceleration by standing magnetic compressions.
  • 4.2. Acceleration by travelling magnetic disturbances.
• 5. Energy losses
  • 5.1. Coulomb and synchrotron losses
  • 5.2. Particle losses and trapping.
• 6. Emission from AE Aquarii
  • 6.1. Quiescent radio emission
  • 6.2. Flare radio emission
  • 6.3. Hard X-ray emission by trapped and precipitating particles
• 7. Discussion
  • 7.1. Pickup acceleration
  • 7.2. Transit-time damping
  • 7.3. Resonant heating
  • 7.4. Magnetic flares and reconnection
  • 7.5. Flares on other magnetised binaries?
• 8. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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