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

Abstract

We have used ISO to observe the Magnetic Cataclysmic Variable AE Aquarii in the previously unexplored range from 4.8 µ mu p to 170µm in the framework of a coordinated multi-wavelength campaign from the radio to optical wave- lengths. 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 re- processed ISOPHOT-C data, we confirm AE Aqr detection at 90 µm 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 ∼761 µm; our results indicate that it seems to be approximately flat between ∼761 and ∼90 µm, at the same level as the 3σ upper limit at 170 µm; and it then decreases from ∼90 µ mt o∼7 µm. 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 10 9 Ka tλ ≤ 3.4 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 ∼7 µm 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 ∼10 µ mt o 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.