Rosat observations of X-ray emission from planetary nebulae

We have searched the entire ROSAT archive for useful observations to study X-ray emission from Galactic planetary nebulae (PNs). The search yields a sample of 63 PNs, which we call the ROSAT PN sample. About 20%-25% of this sample show X-ray emission; these include 13 definite detections and three possible detections (at a 2 σ level). All X-ray sources in these PNs are concentrated near the central stars. Only A30, BD + 30°3639, and NGC 6543 are marginally resolved by the ROSAT instruments. Three types of X-ray spectra are seen in PNs. Type 1 consists of only soft X-ray emission (<0.5 keV), peaks at 0.1-0.2 keV, and can be fitted by blackbody models at temperatures 1-2 × 10⁵ K. Type 2 consists of harder X-ray emission, peaks above 0.5 keV, and can be fitted by thin plasma emission models at temperatures of a few times 10⁶ K. Type 3 is a composite of a bright type 1 component and a fainter type 2 component. Unresolved soft sources with type 1 spectra or the soft component of type 3 spectra are most likely photospheric emission from the hot central stars. Absorption cross sections are large for these soft-energy photons; therefore, only large, tenuous, evolved PNs with hot central stars and small absorption column densities have been detected. The origin of hard X-ray emission from PNs is uncertain. PNs with type 2 spectra are small, dense, young nebulae with relatively cool (≪ 10⁵ K) central stars, while PNs with type 3 X-ray spectra are large, tenuous, evolved nebulae with hot central stars. The hard X-ray luminosities are also different between these two types of PNs, indicating perhaps different origins of their hard X-ray emission. Future Chandra and XMM observations with high spatial and spectral resolution will help to understand the origin of hard X-ray emission from PNs.