Evolution of the X-ray emission of radio-quiet quasars

Brandon C. Kelly, Jill Bechtold, Aneta Siemiginowska, Tom Aldcroft, MaŁgorzata Sobolewska

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47 Scopus citations


We report new Chandra observations of seven optically faint, z ∼ 4 radio-quiet quasars (RQQs). We have combined these new observations with previous Chandra observations of RQQs to create a sample of 174 sources. These sources have 0.1 < z < 4.7, and 1044 ergs s-1 < νLν (2500 Å) < 1048 ergs s-1. The X-ray detection fraction is 90%. We find that the X-ray loudness of RQQs decreases with UV luminosity and increases with redshift. The model that is best supported by the data has a linear dependence of optical to X-ray ratio αox on cosmic time and a quadratic dependence of αox on log LUV, where αox becomes X-ray quiet more rapidly at higher log LUV. We find no significant evidence for a relationship between the X-ray photon index Fx and the UV luminosity, and we find marginally significant evidence that the X-ray continuum flattens with increasing z (2 σ). The Τx-z anticorrelation may be the result of X-ray spectral curvature, redshifting of a Compton reflection component into the observed Chandra band, and/or redshifting of a soft excess out of the observed Chandra band. Using the results for Τx, we show that the αox-z relationship is unlikely to be a spurious result caused by redshifting of the observable X-ray spectral region. A correlation between αox and z implies evolution of the accretion process. We present a qualitative comparison of these new results with models for accretion disk emission.

Original languageEnglish (US)
Pages (from-to)116-134
Number of pages19
JournalAstrophysical Journal
Issue number1 I
StatePublished - Mar 1 2007



  • Accretion, accretion disks
  • Quasars: general
  • Ultraviolet: galaxies
  • X-rays: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Kelly, B. C., Bechtold, J., Siemiginowska, A., Aldcroft, T., & Sobolewska, M. (2007). Evolution of the X-ray emission of radio-quiet quasars. Astrophysical Journal, 657(1 I), 116-134. https://doi.org/10.1086/510876