ROSAT Observations of X‐Ray Emission from Planetary Nebulae

We have searched the entire ROSAT archive for useful observations to studyX-ray emission from Galactic planetary nebulae (PNs). The search yields asample of 63 PNs, which we call the ROSAT PN sample. About 20-25% of thissample show X-ray emission; these include 13 definite detections and threepossible detections (at a 2-sigma level). All X-ray sources in these PNs areconcentrated near the central stars. Only A 30, BD+30 3639, and NGC 6543 aremarginally resolved by the ROSAT instruments. Three types of X-ray spectra areseen in PNs. Type 1 consists of only soft X-ray emission (<0.5 keV), peaks at0.1-0.2 keV, and can be fitted by blackbody models at temperatures 1-2 10^5 K.Type 2 consists of harder X-ray emission, peaks at >0.5 keV, and can be fittedby thin plasma emission models at temperatures of a few 10^6 K. Type 3 is acomposite 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 3spectra 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 absorptioncolumn densities have been detected. The origin of hard X-ray emission from PNsis uncertain. PNs with Type 2 spectra are small, dense, young nebulae withrelatively cool (<<10^5 K) central stars, while PNs with Type 3 X-ray spectraare large, tenuous, evolved nebulae with hot central stars. The hard X-rayluminosities are also different between these two types of PNs, indicatingperhaps different origins of their hard X-ray emission. Future Chandra and XMMobservations with high spatial and spectral resolution will help to understandthe origin of hard X-ray emission from PNs.Comment: To be published in The Astrophysical Journal Supplement Series. 21 pages, 7 figures, 5 table