The origin of the local 1/4-keV X-ray flux in both charge exchange and a hot bubble

The solar neighbourhood is the closest and most easily studied sample of the Galactic interstellar medium, an understanding of which is essential formodels of star formation and galaxy evolution.Observations of an unexpectedly intense diffuse flux of easily absorbed 1/4-kiloelectronvolt X-rays ^1,2, coupled with the discovery that interstellar space within about a hundredparsecs of the Sun is almost completely devoid of cool absorbing gas³, led to a picture of a 'local cavity' filled with X-ray-emitting hot gas, dubbed the local hot bubble^4-6. This model was recently challenged by suggestions that the emission could instead be readily produced within the Solar System by heavy solar-wind ions exchanging electrons with neutralHandHein interplanetary space^7-11, potentially removing themajor piece of evidence for the local existence of million-degree gas within the Galactic disk^12-15. Here we report observations showing that the total solarwind charge-exchange contribution is approximately 40 per cent of the 1/4-keV flux in the Galactic plane. The fact that the measured flux is not dominated by charge exchange supports the notion of a million-degree hot bubble extending about a hundred parsecs from the Sun.