Hard X-ray identification of Eta Carinae and steadiness close to periastron

Abstract

Context: The colliding-wind binary Eta Car exhibits soft X-ray thermal emission that varies strongly around periastron, and non-thermal emission seen in hard X-rays and gamma-rays. Aims: To definitively identify Eta Car as the source of the hard X-ray emission, to examine how changes in the 2-10 keV band influence changes in the hard X-ray band, and to understand more clearly the mechanisms producing the non-thermal emission using new INTEGRAL observations obtained close to periastron. Methods: A Chandra observation encompassing the ISGRI error circle was analysed, and all other soft X-ray sources (including the outer shell of Eta Car itself) were discarded as likely counter-parts. New hard X-ray images of Eta Car were studied close to periastron, and compared to previous observations far from periastron. Results: The INTEGRAL component, when represented by a power law (with a photon index of 1.8), would produce more emission in the Chandra band than observed from any point source in the ISGRI error circle apart from Eta Car, as long as the hydrogen column density to the ISGRI source is lower than 1E24 cm^{-2}. Such sources are rare, thus the ISGRI emission is very likely to be associated with Eta Car. The eventual contribution of the outer shell to the non-thermal component also remains fairly limited. Close to periastron, a 3-sigma detection is achieved for the hard X-ray emission of Eta Car, with a flux similar to the average value far from periastron. Conclusions: Assuming a single absorption component for both the thermal and non-thermal sources, this detection can be explained with a hydrogen column density that does not exceed 6E23 cm^{-2} without resorting to an intrinsic increase in the hard X-ray emission. The energy injected in hard X-rays (averaged over a month) appears rather constant as close as a few stellar radii, well within the acceleration region of the wind.