3D modelling of the colliding winds in η Carinae – evidence for radiative inhibition

The X-ray emission from the supermassive star η Car is simulated using a 3D model of the wind–wind collision. In the model the intrinsic X-ray emission is spatially extended and energy dependent. Absorption due to the unshocked stellar winds and the cooled post-shock material from the primary LBV star is calculated as the intrinsic emission is ray traced along multiple sightlines through the 3D spiral structure of the circumstellar environment. The observable emission is then compared to available X-ray data, including the light curve observed by the Rossi X-ray Timing Explorer (RXTE) and spectra observed by XMM–Newton. The orientation and eccentricity of the orbit are explored, as are the wind parameters of the stars and the nature and physics of their close approach. Our modelling supports a viewing angle with an inclination of ≃42°, consistent with the polar axis of the Homunculus nebula, and the projection of the observer's line of sight on to the orbital plane has an angle of ≃0°–30° in the prograde direction on the apastron side of the semimajor axis. However, there are significant discrepancies between the observed and model light curves and spectra through the X-ray minimum. In particular, the hard flux in our synthetic spectra is an order of magnitude greater than observed. This suggests that the hard X-ray emission near the apex of the wind–wind collision region (WCR) ‘switches off’ from periastron until two months afterwards. Further calculations reveal that radiative inhibition significantly reduces the pre-shock velocity of the companion wind. As a consequence the hard X-ray emission is quenched, but it is unclear whether the long duration of the minimum is due solely to this mechanism alone. For instance, it is possible that the collapse of the WCR on to the surface of the companion star, which would be aided by significant inhibition of the companion wind, could cause an extended minimum as the companion wind struggles to re-establish itself as the stars recede. For orbital eccentricities, e≲ 0.95, radiative braking prevents a wind collision with the companion star's surface. Models incorporating a collapse/disruption of the WCR and/or reduced pre-shock companion wind velocities bring the predicted emission and the observations into much better agreement.