Increasing X-ray emissions and periodic outbursts from the massive star η Carinae

Eta Carinae is one of the most massive, luminous and unstable stars known. The basic nature of this star is poorly understood, despite much study1,2. It is a fluctuating source of hard X-rays3,4,5, indicative of gas at an unusually high temperature (60 million kelvin): the mechanism for producing this gas has yet to be established, but may be related to strong shocks in a dense stellar wind. We have monitored the hard X-ray emission (2–10 keV) from η Car over the past 1.5 years, in order to better understand the nature of these emissions and their variations. We show here that there has been an overall increase in the mean X-ray flux which has accelerated since January 1997, and that there are also small-scale periodic outbursts that occur every 85 days. It has recently been argued6,7 that η Car is in fact a binary stellar system whose components are approaching periastron near 1 January 1998. If this is indeed the case, then it is plausible that the hard X-ray emission is produced by shocks associated with the collision of the winds from the two stars, in which case the X-ray flux should increase through periastron, and rapidly decline thereafter. Continued monitoring will test this prediction.