2004A&A...420..527V

We report on the June 2000 long (100ks) BeppoSAX exposure that unveiled a new very high energy component of the X-ray spectrum of η Car above 10keV extending to at least 50keV. We find that the 2-150keV spectrum is best reproduced by a thermal + non-thermal model. The thermal component dominates the 2-10keV spectral range with kT_h=5.5±0.3keV and logNH_h=22.68±0.01. The spectrum displays a prominent iron emission line centred at 6.70keV. Its equivalent width of 0.94 keV, if produced by the thermal source, gives a slightly sub-solar iron abundance ([Fe/H]=-0.15±0.02). The high energy tail above 10keV is best fitted by a power law with a photon index of 2.42±0.04. The integrated 13-150keV luminosity of ∼12L_☉ is comparable to that of the 2-10keV thermal component (19L_☉). The present result can be explained, in the η Car binary star scenario, by Comptonisation of low frequency radiation by high energy electrons, probably generated in the colliding wind shock front, or in instabilities in the wind of the S Dor primary star. It is possible that the high energy tail had largely weakened near the minimum of the 5.53yr cycle. Probably, the thermal component has a longer recovery time like that of the highest excitation optical emission lines. Both features can be associated with the large absorption measured by BeppoSAX at phase 0.05.