The high energy X-ray tail of η Car revealed by BeppoSAX

We report on the June 2000 long (100 ks) BeppoSAX exposure that unveiled a new very high energy component of the X-ray spectrum of eta Car above 10 keV extending to at least 50 keV. We find that the 2-150 keV spectrum is best reproduced by a thermal + non-thermal model. The thermal component dominates the 2-10 keV spectral range with kT(h) = 5.5 +/- 0.3 keV and log NHh = 22.68 +/- 0.01. The spectrum displays a prominent iron emission line centred at 6.70 keV. 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 10 keV is best fitted by a power law with a photon index of 2.42 +/- 0.04. The integrated 13-150 keV luminosity of similar to12 L. is comparable to that of the 2-10 keV thermal component (19 L.). The present result can be explained, in the eta 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 5.53 yr 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.