Gamma-ray follow-up studies on η Carinae

Aims. Observations of high-energy gamma-rays recently revealed a persistent source in spatial coincidence with the binary system eta Carinae. Since modulation of the observed gamma-ray flux on orbital time scales has not been reported so far, an unambiguous identification was hitherto not possible. Particularly the observations made by the Fermi Large Area Telescope (LAT) posed additional questions regarding the actual emission scenario. Analyses show two energetically distinct components in the gamma-ray spectrum, which are best described by an exponentially cutoff power-law function (CPL) at energies below 10 GeV and a power-law (PL) component dominant at higher energies. Methods. The increased exposure in conjunction with the improved instrumental response functions of the LAT now allow us to perform a more detailed investigation of location, spectral shape, and flux time history of the observed gamma-ray emission. Results. We detect a weak but regular flux decrease over time. This can be understood and interpreted in a colliding-wind binary scenario for orbital modulation of the gamma-ray emission. We find that the spectral shape of the gamma-ray signal agrees with a single emitting particle population in combination with significant absorption by gamma-gamma pair production. Conclusions. We are able to report on the first unambiguous detection of GeV gamma-ray emission from a colliding-wind massive star binary. Studying the correlation of the flux decrease with the orbital separation of the binary components allows us to predict the behaviour up to the next periastron passage in 2014.