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 s ample of 63 PNs, which we call the ROSAT PN sample. About 20%-25% of this s ample show X-ray emission; these include 13 definite detections and three p ossible detections (at a 2 sigma level). All X-ray sources in these PNs are concentrated near the central stars. Only A30, BD + 30 degrees 3639, and N GC 6543 are marginally resolved by the ROSAT instruments. Three types of X-ray spectra are seen in PNs. Type I consists of only soft X-ray emission (<0.5 keV), peaks at 0.1-0.2 keV, and can be fitted by black body models at temperatures 1-2 x 10(5) K. Type 2 consists of harder X-ray emission, peaks above 0.5 keV, and can be fitted by thin plasma emission mo dels at temperatures of a few times 10(6) K. Type 3 is a composite of a bri ght type 1 component and a fainter type 2 component. Unresolved soft sources with type I 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; therefor e, only large, tenuous, evolved PNs with hot central stars and small absorp tion column densities have been detected. The origin of hard X-ray emission from PNs is uncertain. PNs with type 2 sp ectra are small, dense, young nebulae with relatively cool (<< 10(5) K) cen tral 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 diffe rent between these two types of PNs, indicating perhaps different origins o f 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.