Journal Article

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

Uprety, Y., Ursino, E., Morgan, K., Thomas, N. E., McCammon, D., Porter, F. S., Galeazzi, M., Chiao, M., Cravens, T., Collier, M. R., Lallement, R., Koutroumpa, D., Robertson, I. P., Walsh, B. M., Snowden, S. L., Kuntz, K. D., Lepri, S. T.

2014

Overview

The contribution of solar-wind ions exchanging electrons with helium and hydrogen near the Sun is shown to be only about 40 per cent of the 1/4-keV X-ray flux observed in the Galactic plane; this supports the existence of a local ‘hot bubble’ filled with X-ray-emitting gas, accounting for the rest of the flux. Dual source for local X-rays Observations of an unexpectedly intense diffuse flux of easily absorbed 1/4-keV X-rays, coupled with the discovery that interstellar space within about a hundred parsecs of the Sun is almost completely devoid of cool absorbing gas, have led to a picture of a local cavity, or hot bubble, filled with X-ray-emitting hot gas. Suggestions that the emission could instead be produced within the Solar System by charge exchange between heavy solar-wind ions and neutral hydrogen and helium have raised questions over this model. Massimiliano Galeazzi et al . report observations showing that solar wind charge exchange contributes about 40% of the 1/4-keV flux in the Galactic plane. The fact that the measured flux is not dominated by charge exchange supports models that include a million-degree hot bubble extending about a hundred parsecs from the Sun. The solar neighbourhood is the closest and most easily studied sample of the Galactic interstellar medium, an understanding of which is essential for models 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 hundred parsecs of the Sun is almost completely devoid of cool absorbing gas 3 , led to a picture of a ‘local cavity’ filled with X-ray-emitting hot gas, dubbed the local hot bubble 4 , 5 , 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 neutral H and He in interplanetary space 7 , 8 , 9 , 10 , 11 , potentially removing the major piece of evidence for the local existence of million-degree gas within the Galactic disk 12 , 13 , 14 , 15 . Here we report observations showing that the total solar-wind 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.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/33/34/865 / 639/33/525 / Analysis / Astronomy / Astrophysics / Fluctuations / Humanities and Social Sciences / Ions / letter / multidisciplinary / Physics / Plasma / Science / Sciences of the Universe / Solar and Stellar Astrophysics / Solar system / Sun