First detections of the cataclysmic variable AE Aquarii in the near to far infrared with ISO and IRAS: Investigating the various possible thermal and non-thermal contributions

¹ LESIA/CNRS UMR8109, Observatoire de Paris, 92195 Meudon, France; e-mail: meil.abada-simon@obspm.fr
² Instituto de Astrofísica de Canarias, Tenerife, Spain
³ Astrophysics Group, Keele University, UK
⁴ University of Central Lancashire, Preston, UK
⁵ University of Amsterdam, The Netherlands
⁶ MERLIN/VLBI National Facility, Jodrell Bank Observatory, Macclesfield, UK
⁷ Potchefstroom University, South Africa
⁸ Nobeyama Radio Observatory, Japan
⁹ Osservatorio Astronomico di Capodimonte, Napoli, Italy
¹⁰ Université D. Diderot/Paris 7, Paris, France
¹¹ LUTH/CNRS UMR8102, Observatoire de Paris, Meudon, France
¹² Cavendish Laboratory, Cambridge, UK
¹³ Mullard Space Science Lab, University College London, UK
¹⁴ ISO Data Centre, Astrophysics Division, ESA, Madrid, Spain
¹⁵ California Institute of Technology, Pasadena, USA

Received: 12 November 2001
Accepted: 16 December 2004

Abstract

We have used ISO to observe the Magnetic Cataclysmic Variable AE Aquarii in the previously unexplored range from 4.8 μm up to 170 μm in the framework of a coordinated multi-wavelength campaign from the radio to optical wavelengths. We have obtained for the first time a spectrum between 4.8 and 7.3 μm with ISOCAM and ISOPHOT-P: the major contribution comes from the secondary star spectrum, with some thermal emission from the accretion stream, and possibly some additional cyclotron radiation from the post-shock accretion material close to the magnetised white dwarf. Having reprocessed ISOPHOT-C data, we confirm AE Aqr detection at m and we have re-estimated its upper limit at 170 μm. In addition, having re-processed IRAS data, we have detected AE Aqr at 60 μm and we have estimated its upper limits at 12, 25, and 100 μm. The literature shows that the time-averaged spectrum of AE Aqr increases roughly with frequency from the radio wavelengths up to m; our results indicate that it seems to be approximately flat between ~761 and m, at the same level as the 3σ upper limit at 170 μm; and it then decreases from m to m. Thermal emission from dust grains or from a circum-binary disc seems to be very unlikely in AE Aqr, unless such a disc has properties substantially different from those predicted recently. Since various measurements and the usual assumptions on the source size suggest a brightness temperature below 10⁹ K at mm, we have reconsidered also the possible mechanisms explaining the emission already known from the submillimetre to the radio. The complex average spectrum measured from m to the radio must be explained by emission from a plasma composed of more than one “pure” non-thermal electron energy distribution (usually assumed to be a power-law): either a very large volume (diameter ≥ 80 times the binary separation) could be the source of thermal bremsstrahlung which would dominate from m to the ~millimetre, with, inside, a non-thermal source of synchrotron which dominates in radio; or, more probably, an initially small infrared source composed of several distributions (possibly both thermal, and non-thermal, mildly relativistic electrons) radiates gyro-synchrotron and expands moderately: it requires to be re-energised in order to lead to the observed, larger, radio source of highly relativistic electrons (in the form of several non-thermal distributions) which produce synchrotron.