Abstract

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

Jean-Christophe Leyder (USRA/NASA-GSFC)

Roland Walter (ISDC/Observatoire de Genève); Gregor Rauw (Institut d'Astrophysique et de Géophysique, Liège)

The colliding-wind binary Eta Carinae exhibits soft X-ray thermal emission that varies strongly around the periastron passage. It is known to exhibit non-thermal emission, thanks to its detection in hard X-rays and in gamma-rays. To strengthen the identification of Eta Carinae with the hard X-ray source, we analyzed a long Chandra observation encompassing the INTEGRAL/ISGRI error circle, and all other soft X-ray sources (including the outer shell of Eta Carinae itself) were discarded as likely counter-parts. To expand the knowledge of the physical processes governing the X-ray lightcurve, we studied new hard X-ray images of Eta Carinae close to periastron, and compared to previous observations far from periastron. We found that 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 Carinae, as long as the hydrogen column density to the ISGRI source is lower than 10^{24} cm^{-2}. Sources with such a high absorption are very rare, thus the hard X- ray emission is very likely to be associated with Eta Carinae. The eventual contribution of the outer shell to the non-thermal component also remains fairly limited. Close to periastron passage, a 3-sigma detection is achieved for the hard X-ray emission of Eta Carinae, with a flux similar to the average value far from periastron. Assuming a single absorption component for both the thermal and non-thermal sources, this 3-sigma detection can be explained with a hydrogen column density that does not exceed 6*10^{23} 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 timescale) appears to be rather constant at least as close as a few stellar radii, well within the acceleration region of the wind.