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 \sim 8 \times 10 ^ { 6 } K plasma component in addition to the T \sim 1.3 \times 10 ^ { 6 } K component previously detected in PSPC observations .
No significant temperature variations are detected within NGC 6888 .
García-Segura & Mac Low ’ s ( 1995 ) analytical models of WR bubbles constrained by the observed size , expansion velocity , and mass of the nebular shell under-predict the stellar wind luminosity , and can not 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 can 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 determine accurately the physical conditions NGC 6888 ’ s interior hot gas for critical comparisons with bubble models .