Two-temperature X-ray emission from the planetary nebula NGC 7293

Abstract

ROSAT PSPC observations of the planetary nebula NGC are reported here. This planetary nebula is here the first discovered to show clearly two components in its X-ray spectrum. A two-component model consisting of a blackbody and a Raymond-Smith thermal plasma is fitted to the observed ROSAT PSPC spectrum. This results in a temperature of T1 = 1.4 × 105 K for the blackbody component and a temperature T2 = 8.7 × 106 K for the hot plasma component, at a hydrogen column density NH = 1.4 × 1020 cm-2. The temperature of the blackbody component is consistent with the helium Zanstra temperature of the central star, indicating that it may be attributed to the photosphere of the central star. The high-temperature component is possibly from a corona around the central star, which may be related to a strong convection in the star. An alternative explanation is that the hot plasma resides in a hot bubble predicted by the interacting wind model. A lower limit of the electron density in the hot plasma is found to be ∼ 10 cm-3.