Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Discrete sources as the origin of the Galactic X-ray ridge emission


An unresolved X-ray glow (at energies above a few kiloelectronvolts) was discovered about 25 years ago and found to be coincident with the Galactic disk—the Galactic ridge X-ray emission1,2. This emission3,4,5,6,7,8,9,10 has a spectrum characteristic of a 108 K optically thin thermal plasma, with a prominent iron emission line at 6.7 keV. The gravitational well of the Galactic disk, however, is far too shallow to confine such a hot interstellar medium; instead, it would flow away at a velocity of a few thousand kilometres per second, exceeding the speed of sound in the gas. To replenish the energy losses requires a source of 1043 erg s-1, exceeding by orders of magnitude all plausible energy sources in the Milky Way11. An alternative is that the hot plasma is bound to a multitude of faint sources12, which is supported by the recently observed similarities in the X-ray and near-infrared surface brightness distributions13,14 (the latter traces the Galactic stellar distribution). Here we report that at energies of 6–7 keV, more than 80 per cent of the seemingly diffuse X-ray emission is resolved into discrete sources, probably accreting white dwarfs and coronally active stars.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: The Chandra image in the 0.5–7 keV energy band.
Figure 2: GRXE spectrum and its resolved fraction.
Figure 3: Fraction of resolved X-ray emission around the 6.7 keV iron emission line as a function of the limiting source flux/luminosity.


  1. 1

    Worrall, D. M. et al. HEAO 1 measurements of the Galactic ridge. Astrophys. J. 255, 111–121 (1982)

    ADS  Article  Google Scholar 

  2. 2

    Warwick, R. S. et al. The Galactic ridge observed by EXOSAT. Nature 317, 218–221 (1985)

    ADS  Article  Google Scholar 

  3. 3

    Koyama, K. et al. Thermal X-ray emission with intense 6.7-keV iron line from the Galactic ridge. Publ. Astron. Soc. Jpn 38, 121–131 (1986)

    CAS  ADS  Google Scholar 

  4. 4

    Yamauchi, S. & Koyama, K. The 6.7 keV iron line distribution in the Galaxy. Astrophys. J. 404, 620–624 (1993)

    CAS  ADS  Article  Google Scholar 

  5. 5

    Yamauchi, S. et al. Unresolved X-ray emission from the Galactic ridge with ASCA. Publ. Astron. Soc. Jpn 48, L15–L20 (1996)

    ADS  Article  Google Scholar 

  6. 6

    Hands, A. D. P. et al. X-ray source populations in the Galactic plane. Mon. Not. R. Astron. Soc. 351, 31–56 (2004)

    CAS  ADS  Article  Google Scholar 

  7. 7

    Ebisawa, K. et al. Chandra deep X-ray observation of a typical Galactic plane region and near-infrared identification. Astrophys. J. 635, 214–242 (2005)

    CAS  ADS  Article  Google Scholar 

  8. 8

    Koyama, K. et al. Iron and nickel line diagnostics for the Galactic Center diffuse emission. Publ. Astron. Soc. Jpn 59, 245–255 (2007)

    Article  Google Scholar 

  9. 9

    Ebisawa, K. et al. Spectral study of the Galactic ridge X-ray emission with Suzaku. Publ. Astron. Soc. Jpn 60, 223–230 (2008)

    Article  Google Scholar 

  10. 10

    Yamauchi, S. et al. Iron emission lines on the Galactic ridge observed with Suzaku. Preprint at <> (2008)

  11. 11

    Tanaka, Y. ASCA observation of X-ray emission from the Galactic ridge. Astron. Astrophys. 382, 1052–1060 (2002)

    ADS  Article  Google Scholar 

  12. 12

    Worrall, D. M. & Marshall, F. E. Stellar contributions to the hard X-ray Galactic ridge. Astrophys. J. 267, 691–697 (1983)

    CAS  ADS  Article  Google Scholar 

  13. 13

    Revnivtsev, M. et al. Origin of the Galactic ridge X-ray emission. Astron. Astrophys. 452, 169–178 (2006)

    CAS  ADS  Article  Google Scholar 

  14. 14

    Revnivtsev, M., Molkov, S. & Sazonov, S. Map of the Galaxy in the 6.7-keV emission line. Mon. Not. R. Astron. Soc. 373, L11–L15 (2006)

    ADS  Article  Google Scholar 

  15. 15

    Park, S. et al. Characteristics of diffuse X-ray line emission within 20 parsecs of the Galactic Center. Astrophys. J. 603, 548–559 (2004)

    CAS  ADS  Article  Google Scholar 

  16. 16

    Sazonov, S. et al. X-ray luminosity function of faint point sources in the Milky Way. Astron. Astrophys. 450, 117–128 (2006)

    CAS  ADS  Article  Google Scholar 

  17. 17

    Weisskopf, M. C. et al. An overview of the performance and scientific results from the Chandra X-Ray Observatory. Publ. Astron. Soc. Jpn 114, 1–24 (2002)

    ADS  Article  Google Scholar 

  18. 18

    Giacconi, R. et al. Evidence for X-rays from sources outside the Solar System. Phys. Rev. Lett. 9, 439–443 (1962)

    ADS  Article  Google Scholar 

  19. 19

    Hickox, R. C. & Markevitch, M. Absolute measurement of the unresolved cosmic X-ray background in the 0.5–8 keV band with Chandra. Astrophys. J. 645, 95–114 (2006)

    ADS  Article  Google Scholar 

  20. 20

    Moretti, A. et al. The resolved fraction of the cosmic X-ray background. Astrophys. J. 588, 696–703 (2003)

    ADS  Article  Google Scholar 

  21. 21

    Dutra, C. M. et al. Extinction within 10° of the Galactic Centre using 2MASS. Mon. Not. R. Astron. Soc. 338, 253–262 (2003)

    ADS  Article  Google Scholar 

  22. 22

    Indebetouw, R. et al. The wavelength dependence of interstellar extinction from 1.25 to 8.0 μm using GLIMPSE data. Astrophys. J. 619, 931–938 (2005)

    CAS  ADS  Article  Google Scholar 

  23. 23

    Sunyaev, R. A., Markevitch, M. & Pavlinsky, M. The center of the Galaxy in the recent past—a view from GRANAT. Astrophys. J. 407, 606–610 (1993)

    ADS  Article  Google Scholar 

  24. 24

    Koyama, K. et al. ASCA view of our Galactic Center: remains of past activities in X-rays? Publ. Astron. Soc. Jpn 48, 249–255 (1996)

    CAS  ADS  Article  Google Scholar 

  25. 25

    Vikhlinin, A. et al. Chandra temperature profiles for a sample of nearby relaxed galaxy clusters. Astrophys. J. 628, 655–672 (2005)

    CAS  ADS  Article  Google Scholar 

  26. 26

    Chandra. Proposers’ Observatory Guide <> (23 January 2009)

  27. 27

    Hellier, C., Mukai, K. & Osborne, J. P. Iron Kα linewidths in magnetic cataclysmic variables. Mon. Not. R. Astron. Soc. 297, 526–530 (1998)

    CAS  ADS  Article  Google Scholar 

  28. 28

    Huenemoerder, D., Canizares, C. & Schulz, N. X-ray spectroscopy of II Pegasi: coronal temperature structure, abundances, and variability. Astrophys. J. 559, 1135–1146 (2001)

    CAS  ADS  Article  Google Scholar 

Download references


M.R. thanks M. Markevitch for his help with the Chandra instrumental background. This research was supported by the DFG Cluster of Excellence “Origin and Structure of the Universe”, by NASA Chandra grant GO8-9132A, by the OFH-17 programme of the Russian Academy of Sciences, and by grants RFFI 07-02-01004 and RFFI 07-02-00961.

Author information



Corresponding author

Correspondence to M. Revnivtsev.

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Revnivtsev, M., Sazonov, S., Churazov, E. et al. Discrete sources as the origin of the Galactic X-ray ridge emission. Nature 458, 1142–1144 (2009).

Download citation

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing