X-Ray Emission from Wind-blown Bubbles. III. ASCA SIS Observations of NGC 6888

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. Garc?a-Segura & Mac Lowʹs 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 Xray luminosity, spectrum, surface brightness profile, and SIS count rate of NGC 6888ʹs 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 1/2of 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 6888ʹs interior hot gas for critical comparisons with bubble models.