Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005a%26a...433.1063a&link_type=abstract
Astronomy and Astrophysics, Volume 433, Issue 3, April III 2005, pp.1063-1077
Astronomy and Astrophysics
Astrophysics
4
Stars: Novae, Cataclysmic Variables, Infrared: Stars, Radio Continuum: Stars, Stars: Flare, Radiation Mechanisms: Thermal, Radiation Mechanisms: Non-Thermal
Scientific paper
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 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~ μm to ˜ 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 109 K at λ ≤ 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 ~μm 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.
Abada-Simon Meil
Casares Jorge
de Jager O.
de Martino Domitilla
Evans Aaron
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