Letter | Published:

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

Nature volume 512, pages 171173 (14 August 2014) | Download Citation


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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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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Author notes

    • B. M. Walsh

    Present address: Space Sciences Laboratory, University of California, Berkeley, California 94720, USA.


  1. Department of Physics, University of Miami, Coral Gables, Florida 33124, USA

    • M. Galeazzi
    • , Y. Uprety
    •  & E. Ursino
  2. NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA

    • M. Chiao
    • , M. R. Collier
    • , F. S. Porter
    • , S. L. Snowden
    • , N. E. Thomas
    •  & B. M. Walsh
  3. Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045, USA

    • T. Cravens
    •  & I. P. Robertson
  4. Université Versailles St Quentin; Sorbonne Universités, UPMC Université Paris 06; CNRS/INSU, LATMOS-IPSL, Guyancourt 78280, France

    • D. Koutroumpa
  5. The Henry A. Rowland Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA

    • K. D. Kuntz
  6. GEPI Observatoire de Paris, CNRS UMR 8111, Université Paris Diderot, 92190, Meudon, France

    • R. Lallement
  7. Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA

    • S. T. Lepri
  8. Department of Physics, University of Wisconsin, Madison, Wisconsin 53706, USA

    • D. McCammon
    •  & K. Morgan


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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.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to M. Galeazzi.

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