Abstract.We have observed the stellar wind blown bubble NGC 6888 with the ROSAT HRI. A map and a catalogue of X-ray filaments is derived, the typical filament being a few 0.1 pc in extent with an HRI count rate of a few 10 −4 s −1 . We show that this filamentary structure can qualitatively be modelled as local count rate enhancements due to denser gas near evaporating cool clumps which in turn are seen in the optical and probably are produced by instabilities after the passage of the primary shock front of the expanding bubble.

We present ASCA SIS observations of the wind-blown bubble NGC 6888. Owing to the higher sensitivity of the SIS for higher energy photons compared to the ROSAT PSPC, we are able to detect a T $ 8 ; 10 6 K plasma component in addition to the T $ 1:3 ; 10 6 K component previously detected in PSPC observations. No significant temperature variations are detected within NGC 6888. García-Segura & Mac Low's analytical models of WR bubbles constrained by the observed size, expansion velocity, and mass of the nebular shell underpredict the stellar wind luminosity and cannot reproduce simultaneously the observed X-ray luminosity, spectrum, surface brightness profile, and SIS count rate of NGC 6888's bubble interior. The agreement between observations and expectations from models may be improved if one or more of the following ad hoc assumptions are made: (1) the stellar wind luminosity was weaker in the past, (2) the bubble is at a special evolutionary stage and the nebular shell has recently been decelerated to 1 2 of its previous expansion velocity, and (3) the heat conduction between the hot interior and the cool nebular shell is suppressed. Chandra and XMM-Newton observations with high spatial resolution and high sensitivity are needed to accurately determine the physical conditions of NGC 6888's interior hot gas for critical comparisons with bubble models.

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Abstract. We present ASCA SIS observations of the wind-blown bubble NGC 6888. Because the ASCA SIS is sensitive to higher energy photons and has a higher spectral resolution compared to the ROSAT PSPC, we are able to detect a T a 8 x 10 6 K plasma component besides the T « 1.5 x 10 6 K component known from previous PSPC observations. The existence of a high-temperature component, the observed limb-brightened X-ray surface brightness profile, and the observed level of X-ray surface brightness cannot be satisfactorily explained by currently available models. Reducing heat conduction at the contact discontinuity may raise the central temperature and produce a limb-brightening; however, the expected X-ray surface brightness is still considerably higher than the observed surface brightness.