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Resolving the extragalactic hard X-ray background

Nature volume 404, pages 459–464 (2000)Cite this article

Abstract

The origin of the hard (2–10 keV) X-ray background has been a mystery for over 35 years. Most of the soft X-ray background has been resolved into individual sources (mainly quasars), but these sources do not have the spectral energy distribution required to match the spectrum of the X-ray background as a whole. Here we report the results of a deep survey, using the Chandra satellite, in which the detected hard X-ray sources account for at least 75 per cent of the hard X-ray background. The mean X-ray spectral energy distribution of these sources is in good agreement with that of the background. Moreover, most of those hard X-ray sources are associated unambiguously with either the nuclei of otherwise normal bright galaxies, or with optically faint sources. The latter could be active nuclei in dust-enshrouded galaxies or a population of quasars at extremely high redshift.

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Figure 1: Integral number counts per square degree of X-ray sources in the SSA13 field versus flux for the hard (a) and the soft (b) energy bands.
Figure 2: I-band images 9″ × 9″ of the X-ray sources of Table 1.
Figure 3: Keck LRIS spectra of three of the X-ray-detected bright galaxies whose redshifts have been determined.
Figure 4: I-band magnitudes versus X-ray photon indices.

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Acknowledgements

We thank J. Halpern and G. Hasinger for comments which improved the first draft of this Article; we also thank the CXC, L. Van Speybroeck, M. Weisskopf and the MSFC team, M. Bautz, G. Garmire, and the ACIS team for building and operating the observatory. We acknowledge the use of HEASARC software. A.J.B. was supported by the Hubble and Chandra fellowship programs.

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Authors and Affiliations

  1. NASA Goddard Space Flight Center , Code 662, Greenbelt, 20771, Maryland, USA

    R. F. Mushotzky & K. A. Arnaud

  2. Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, 96822, Hawaii, USA

    L. L. Cowie & A. J. Barger

  3. Astronomy Department, University of Maryland, College Park, 20742, Maryland, USA

    K. A. Arnaud

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  1. R. F. Mushotzky
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Correspondence to L. L. Cowie.

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Mushotzky, R., Cowie, L., Barger, A. et al. Resolving the extragalactic hard X-ray background. Nature 404, 459–464 (2000). https://doi.org/10.1038/35006564

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