Abstract:
The hottest central stars of planetary nebulae (PNe) can be expected to be X-ray emitters. Since the Wein tail of their photospheric emission decays sharply with energy, no X-ray emission above 0.5 keV can be expected from the central stars of PNe. Therefore, the Chandra discovery of hard (0.5-3.0 keV) X-ray sources at the central stars of PNe came as a surprise. This hard X-ray emission can be attributed to the coronal emission from a late-type companion or to accretion onto a compact source. Thus, the detection of hard X-ray emission from a PN central star was immediately envisioned to be a simple and reliable tool for the detection of binary central stars of PNe. Things have turned to be not that simple because the temporal and spectral properties of the hard X-ray emission from central stars of PNe are not conclusive as for its origin, but also because there is not a one-to-one correlation between hard X-ray emission an binarity. Other possible origins for the hard X-ray emission from some central stars of PNe have to be considered.