ITAL]ROSAT[/ITAL] X-Ray Observations of Two Planetary Nebulae: NGC 1535 and NGC 3587

Abstract

X-ray emission from planetary nebulae (PNs) may originate from the central stars or from the shocked fast stellar winds. These two origins may be distinguished by the spatial extent and the spectral properties of the X-ray emission. Using ROSAT Position Sensitive Proportional Counter (PSPC) observations, Leahy et al. reported X-ray emission from the PNs NGC 1535 and NGC 3587. However, a careful comparison between optical and X-ray images of NGC 1535 shows that the X-ray emission is coincident with a star outside the optical boundary of the nebula. Previous reports of X-ray emission from NGC 1535 are therefore spurious. The PSPC image of NGC 3587 shows three peaks, two projected within and one outside the optical shell boundary. To improve the spatial resolution, we have obtained a 35 ks ROSAT High Resolution Imager observation of NGC 3587. The HRI image shows clearly that the three peaks are point sources: RX J111447.9+550106, RX J111450.9+550208, and RX J111504.9+550141. RX J111447.9+550106 is projected at the position of the central star, and its PSPC energy distribution shows a very soft X-ray spectrum; therefore, RX J111447.9+550106 most likely represents the photospheric emission from the 110,000 K central star. RX J111450.9+550208 and RX J111504.9+550141 have spectral properties different from those of RX J111447.9+550106 and are probably unrelated background sources.