The periodic spectroscopic events in \eta Carinae are now well established and occur near the periastron passage of two massive stars in a very eccentric orbit .
Several mechanisms have been proposed to explain the variations of different spectral features , such as an eclipse by the wind-wind collision boundary , a shell ejection from the primary star or accretion of its wind onto the secondary .
All of them have problems explaining all the observed phenomena .
To better understand the nature of the cyclic events , we performed a dense monitoring of \eta Carinae with 5 Southern telescopes during the 2009 low excitation event , resulting in a set of data of unprecedented quality and sampling .
The intrinsic luminosity of the He ii \lambda 4686 emission line ( L \sim 310 ~ { } L _ { \odot } ) just before periastron reveals the presence of a very luminous transient source of extreme UV radiation emitted in the wind-wind collision ( WWC ) region .
Clumps in the primary ’ s wind probably explain the flare-like behavior of both the X-ray and He ii \lambda 4686 light-curves .
After a short-lived minimum , He ii \lambda 4686 emission rises again to a new maximum , when X-rays are still absent or very weak .
We interpret this as a collapse of the WWC onto the “ surface ” of the secondary star , switching off the hard X-ray source and diminishing the WWC shock cone .
The recovery from this state is controlled by the momentum balance between the secondary ’ s wind and the clumps in the primary ’ s wind .