The Origin of the Extragalactic X-Ray Background

  • G. Setti
Conference paper
Part of the Astrophysics and Space Science Library book series (ASSL, volume 164)


Although the shape of the spectrum in the 3–100 keV interval is suggestive of an optically thin bremsstrahlung at ~ 40 keV, it is well known that the interpretation in terms of a hot intergalactic gas (IGG) requires a rather extreme energy supply and a gas density conflicting with the baryon density upper limit derived from primordial nucleosynthesis calculations. A summary discussion of the estimated contributions from the integrated X-ray emission of known classes of extragalactic discrete sources at a reference energy of 2 keV is given. Although these estimates are still uncertain, the subtraction of a “minimum” contribution drastically modifies the 40 keV thermal shape, prima facie evidence of a hot IGG. AGNs are the main contributors. Low luminosity AGNs at redshift z = 1−2 may in fact saturate the 2 keV XRB, but their observed hard X-ray spectra are on the average unlike (much too steep) that of the XRB. This has led a number of authors to postulate new classes of sources and some exotic models which are briefly summarized. However, if a recently proposed unified scheme of AGNs holds, then the bulk of the XRB intensity can be explained independently of the observed spectral differences and with a mild cosmological evolution.


Cosmic Microwave Background Active Galactic Nucleus Radio Galaxy Cosmological Evolution Surface Brightness Fluctuation 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


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  1. Anderson, S.F., and Margon, B. 1987, Astrophys. J., 314, 111.ADSCrossRefGoogle Scholar
  2. Barcons, X., and Lapiedra, R. 1985, Astrophys. J., 289, 33.ADSCrossRefGoogle Scholar
  3. Barcons, X. 1987, Astrophys. J., 313, 547.ADSCrossRefGoogle Scholar
  4. Barcons, X., and Fabian, A.C. 1988, Mon. Not. R. astr. Soc., 230, 189.ADSGoogle Scholar
  5. Barthel, P.D. 1989, Astrophys. J., 336, 606.ADSCrossRefGoogle Scholar
  6. Boesgaard, A.M., and Steigman, G. 1985, Ann. Rev. Astr. Astrophys., 23, 319.ADSCrossRefGoogle Scholar
  7. Boldt, E. 1987, Phy. Reports, 146, No. 4, 215.ADSCrossRefGoogle Scholar
  8. Boldt, E. 1988, NASA Report #88-037.Google Scholar
  9. Boldt, E., and Leiter, D. 1987, Astrophys. J., 322, L1.ADSCrossRefGoogle Scholar
  10. Bookbinder, J., Cowie, L.L., Krolik, J.H., Ostriker, J.P., and Rees, M.J. 1980, Astrophys. J., 237, 647.ADSCrossRefGoogle Scholar
  11. Canizares, C.R., and White, J.L. 1989, Astrophys. J., 339, 27.ADSCrossRefGoogle Scholar
  12. Cheng, E.S., Saulson, P.R., Wilkinson, D.T., and Corey, B.E. 1979, Astrophys. J., 232, L139.ADSCrossRefGoogle Scholar
  13. Cowsik, R., and Kobetich, E.J. 1972, Astrophys. J., 177, 585.ADSCrossRefGoogle Scholar
  14. Daly, R.A. 1987, Astrophys. J., 322, 20.ADSCrossRefGoogle Scholar
  15. Danese, L., De Zotti, G., Fasano, G., and Franceschini, A. 1986, Astrophys. J., 161, 1.Google Scholar
  16. De Zotti, G., Boldt, E.A., Cavaliere, A., Danese, L., Franceschini, A., Marshall, F.E., Swank, J.H., and Szymkowiak, A.E. 1982, Astrophys. J., 253, 47.ADSCrossRefGoogle Scholar
  17. Fabian, A.C., Done, C., and Ghisellini, G. 1988, Mon. Not. R. astr. Soc., 232, 21P.ADSGoogle Scholar
  18. Field, G.B., and Perrenod, S.C. 1977, Astrophys. J., 215, 717.ADSCrossRefGoogle Scholar
  19. Fomalont, E.B., Kellermann, K.I., and Wall, J.V. 1984, Astrophys. J., 277, L23.ADSCrossRefGoogle Scholar
  20. Giacconi, R., Gursky, H., Paolini, F., and Rossi, B. 1962, Phys. Rev. Letters, 9, 439.ADSCrossRefGoogle Scholar
  21. Giacconi, R., et al. 1979, Astrophys. J., 234, L1.ADSCrossRefGoogle Scholar
  22. Giacconi, R., and Zamorani, G. 1987, Astrophys. J., 313, 20.ADSCrossRefGoogle Scholar
  23. Gioia, I.M., Maccacaro, T., Schild, R.E., Stocke, J.T., Liebert, J.W., Danziger, I.J., Kunth, D., and Lub, J. 1984, Astrophys. J., 283, 495.ADSCrossRefGoogle Scholar
  24. Griffiths, R.E. 1989, NATO-ASI on The Epoch of Galaxy Formation, eds. C.S. Frenk et al. (Kluwer, Dordrecht), p. 235.Google Scholar
  25. Gruber, D.E., Rothschild, R.E., Matteson, J.L., and Kinzer, R.L. 1984, in X-Ray and UV Emission from Active Galactic Nuclei, eds. W. Brinkmann and J. Trumper, MPE Report 184, p. 129.Google Scholar
  26. Guilbert, P.W., and Fabian, A.C. 1986, Mon. Not. R. astr. Soc., 220, 439.ADSGoogle Scholar
  27. Hamilton, T.T., and Helfand, D.J. 1987, Astrophys. J., 318, 93.ADSCrossRefGoogle Scholar
  28. Inoue, H. 1989, in Big Bang, Active Galactic Nuclei and Supernovae, (Universal Ac. Press, Tokyo), p. 301.Google Scholar
  29. Knoke, J.E., Partridge, R.B., Ratner, M.I., and Shapiro, I.I. 1984, Astrophys. J., 284, 479.ADSCrossRefGoogle Scholar
  30. Leiter, D., and Boldt, E. 1982, Astrophys. J., 260, 1.ADSCrossRefGoogle Scholar
  31. Maccacaro, T., Gioia, I.M., Wolter, A., Zamorani, G., and Stocke, J.T. 1988, Astrophys. J., 326, 680.ADSCrossRefGoogle Scholar
  32. Makishima, K., and Ohashi, T. 1989, in Big Bang, Active Galactic Nuclei and Supernovae, (Universal Ac. Press, Tokyo), p. 371.Google Scholar
  33. Morisawa, K., and Takahara, F. 1989, P.A.S.J., to be published.Google Scholar
  34. Mushotzky, R.F. 1984, in COSPAR/IAU Symp., High-Energy Astrophysics and Cosmology, eds. G.F. Bignami and R.A. Sunyaev, Adv. Space Res., 3, p. 157.Google Scholar
  35. Ostriker, J.P., Thompson, C., and Witten, E. 1986, Phys. Letters, B180, 231.ADSGoogle Scholar
  36. Picchiotti et al. 1982, Astrophys. J., 253, 485.ADSCrossRefGoogle Scholar
  37. Reeves, H., Richer, J., Sato, K., and Terasawa, N. 1989, preprint.Google Scholar
  38. Schmidt, M., and Green, R.F. 1986, Astrophys. J., 305, 68.ADSCrossRefGoogle Scholar
  39. Setti, G., and Woltjer, L. 1982, in Astrophysical Cosmology, eds. H.A. Bruck, G.V. Coyne and M.S. Longair (Pontificia Academia Scientiarum, Vatican City), p. 315.Google Scholar
  40. Setti, G. 1984, in X-Ray and UV Emission from Active Galactic Nuclei, eds. W. Brinkmann and J. Trumper, MPE Report 184, p. 243.Google Scholar
  41. Setti, G. 1985, in Non-thermal and Very High Temperature Phenomena in X-ray Astronomy, eds. G.C. Perola and M. Salvati (Istituto Astronomico, Universita “La Sapienza”, Rome), p. 159.Google Scholar
  42. Setti, G. 1987, in IAU Symp. No. 124, Observational Cosmology, eds. A. Hewitt, G. Burbidge and L.Z. Fang (Reidei, Dordrecht), p. 579.CrossRefGoogle Scholar
  43. Setti, G., and Woltjer, L. 1989, Astron. Astrophys., 224, L21.ADSGoogle Scholar
  44. Shafer, R.A. 1983, NASA Tech. Mem. 85029.Google Scholar
  45. Shafer, R.A., and Fabian, A.C. 1983, in IAU Symp. No. 104, Early Evolution of the Universe and its Present Structure, eds. G.O. Abell and G. Chincarini (Reidel, Dordrecht), p. 333.CrossRefGoogle Scholar
  46. Smoot, G.F., Gorenstein, M.V., and Muller, R.A. 1977, Phys. Rev. Letters, 39, 898.ADSCrossRefGoogle Scholar
  47. Tananbaum, H., Avni, Y., Green, R.F., Schmidt, M., and Zamorani, G. 1986, Astrophys. J., 305, 57.ADSCrossRefGoogle Scholar
  48. Taylor, G.B., and Wright, E.L. 1989, Astrophys. J., 339, 619.ADSCrossRefGoogle Scholar
  49. Turner, T.J., and Pounds, K.A. 1989, Mon. Not. R. astr. Soc., submitted.Google Scholar
  50. Tyson, J.A. 1984, in IAU Colloq. 78, Astronomy with Schmidt Type Telescopes, ed. M. Capaccioli (Reidei, Dordrecht), p. 489.Google Scholar
  51. Wilkes, B.J., and Elvis, M. 1987, Astrophys. J., 323, 243.ADSCrossRefGoogle Scholar
  52. Worral, D.M., Mushotzky, R.F., Boldt, E.A., Holt, S.S., and Serlemitsos, P.J. 1979, Astrophys. J., 232, 683.ADSCrossRefGoogle Scholar
  53. Zamorani, G., et al. 1981, Astrophys. J., 245, 357.ADSCrossRefGoogle Scholar

Copyright information

© Kluwer Academic Publishers 1990

Authors and Affiliations

  • G. Setti
    • 1
  1. 1.European Southern ObservatoryGarching bei MünchenFederal Republic of Germany

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