Detection of a Hot Binary Companion of η Carinae

We report the detection of a hot companion of η Carinae using high-resolution spectra (905-1180 Å) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. Observations were obtained at two epochs of the 2024 day orbit: 2003 June, during ingress to the 2003.5 X-ray eclipse, and 2004 April, several months after egress. These data show that essentially all the far-UV flux from η Car shortward of Lyα disappeared at least 2 days before the start of the X-ray eclipse (2003 June 29), implying that the hot companion, η Car B, was also eclipsed by the dense wind or extended atmosphere of η Car A. Analysis of the far-UV spectrum shows that η Car B is a luminous, hot star. The N II λλ1084-1086 emission feature suggests that it may be nitrogen-rich. The observed far-UV flux levels and spectral features, combined with the timing of their disappearance, is consistent with η Car being a massive binary system.