The high energy X-ray tail of $\mathsf{\eta}$ Car revealed by BeppoSAX

Abstract

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 η 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 kTh = 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 ~12 is comparable to that of the 2–10 keV thermal component (19 ). 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 5.53 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.