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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 for models of star formation and galaxy evolution. Observations of an unexpectedly intense diffuse flux of easily absorbed 1/4-kiloelectronvolt X-rays1,2, coupled with the discovery that interstellar space within about a hundred parsecs 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 bubble4,5,6. 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 space7,8,9,10,11, potentially removing the major piece of evidence for the local existence of million-degree gas within the Galactic disk12,13,14,15. 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.

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Figure 1: The He focusing cone.
Figure 2: The DXL scan path.
Figure 3: Neutral atom column density for DXL and ROSAT.
Figure 4: Fit to DXL and ROSAT data.


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We thank the personnel at NASA’s Wallops Flight Facility and the White Sands Military Range for their support of payload development, integration and launch, and the technical personnel at the University of Miami, NASA’s Goddard Space Flight Center and the University of Michigan for their support of the instrument’s development. This work was supported by NASA award numbers NNX11AF04G and NNX09AF09G. D.K. and R.L. acknowledge financial support for their activity through the programme ‘Soleil Héliosphère Magnétosphère’ of the French space agency CNES, and the National Program ‘Physique Chimie du Milieu Interstellaire’ of the Institut National des Sciences de l'Univers (INSU). M.C. and N.E.T. are employed through the Center for Research and Exploration in Space Science and Technology (CRESST) and the University of Maryland, Baltimore County, Baltimore, Maryland, USA.

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Authors and Affiliations



Y.U., N.E.T., M.G., D.M., M.R.C., F.S.P. and S.T.L. contributed to hardware development. Y.U., N.E.T., M.G., D.M., M.R.C., F.S.P., M.C., D.K., K.D.K. and K.M. contributed to launch operations. M.G., D.M., Y.U., N.E.T., M.R.C., D.K., K.D.K., K.M., T.C., I.P.R., S.L.S., E.U. and B.M.W. contributed to data reduction and analysis. D.K. and R.L. prepared the neutral integral distributions. All authors discussed the results and commented on the manuscript.

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Correspondence to M. Galeazzi.

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Extended data figures and tables

Extended Data Figure 1 DXL count rates versus time during flight.

Count rate of Counter-I (red) and Counter-II (black) as a function of time during launch. The DXL observation started off the cone, moved towards the nose of the cone (first scan) and back (second scan) at about 0.7 degrees per second, then performed an Earth scan at about 10 degrees per second to measure the instrument background (fast scan), returned to the nose of the cone, and then performed another scan off the cone (third scan) and back to the nose (fourth scan), for a total of four slow scans along the cone and one fast scan. The gradient up and down the He focusing cone is evident. The error bars are s.e.m.

Source data

Extended Data Figure 2 The DXL detector response.

Effective grasp (solid angle times area) for the DXL Counter-I (red) and Counter-II (black), compared with the ROSAT 1/4-keV (R12 band) effective area (blue).

Source data

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Galeazzi, M., Chiao, M., Collier, M. et al. The origin of the local 1/4-keV X-ray flux in both charge exchange and a hot bubble. Nature 512, 171–173 (2014).

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