Suzaku Observations of the Prototype Wind-Blown Bubble NGC 6888

TL;DR

This study analyzes Suzaku X-ray observations of NGC 6888, revealing a complex hot bubble with both cool and hot plasma components, and suggests that the X-ray emitting gas originates from the optical nebula with implications for thermal conduction and energy exchange processes.

Contribution

First detailed Suzaku X-ray spectral analysis of NGC 6888, combining observational data with theoretical models to understand plasma temperatures and origins.

Findings

X-ray spectra are predominantly soft with some hard X-ray emission.

The plasma has a cool component (<0.5 keV) and a hotter component (>2.0 keV).

X-ray emitting gas shares chemical abundances with the optical nebula.

Abstract

We present an analysis of the Suzaku observations of the prototype wind-blown bubble NGC 6888 which is based both on use of standard spectral models and on a direct comparison of theoretical models with observations. The X-ray spectra of NGC 6888 are soft and most of the X-rays are in the (0.3 - 1.5 keV) energy range. But, hard X-rays (1.5 - 4.0 keV) are also detected (~10% of the observed flux). The corresponding spectral fits require a relatively cool plasma with kT < 0.5 keV but much hotter plasma with temperature kT > 2.0 keV is needed to match the observed hard X-ray emission. We find no appreciable temperature variations within the hot bubble in NGC 6888. The derived abundances (N, O, Ne) are consistent with those of the optical nebula. This indicates a common origin of the X-ray emitting gas and the outer cold shell: most of the X-ray plasma (having non-uniform spatial distribution: clumps) has flown into the hot bubble from the optical nebula. If the electron thermal conduction is efficient, this can naturally explain the relatively low plasma temperature of most of the X-ray emitting plasma. Alternatively, the hot bubble in NGC 6888 will be adiabatic and the cold clumps are heated up to X-ray temperatures likely by energy exchange between the heavy particles (hot ions diffusing into the cold clumps).