Observational constraints on the dependence of radio-quiet quasar X-ray emission on black hole mass and accretion rate

Brandon C. Kelly, Jill Bechtold, Jonathan R. Trump, Marianne Vestergaard, Meta Siemiginowska

Research output: Contribution to journalArticlepeer-review

82 Scopus citations


In this work we use a sample of 318 radio-quiet quasars (RQQs) to investigate the dependence of the ratio of optical/UV flux to X-ray flux, αox, and the X-ray photon index, ΓX , on black hole mass, UV relative to Eddington luminosity, and X-ray relative to Eddington luminosity. Our sample is drawn from the literature, with X-ray data from ROSAT and Chandra, and optical data mostly from the SDSS; 153 of these sources have estimates of ΓX from Chandra. We estimate MBH using standard estimates derived from the HβMg II, and C IV broad emission lines. Our sample spans a broad range in black hole mass (106 ≲ M BH/M ≲ 1010), redshift (0 < z < 4.8), and luminosity (1043 ≲ λLλ (2500 Å)[ergs s-1] ≲ 1048). We find that αox increases with increasing MBH and L UV/LEdd and decreases with increasing LX/L Edd. In addition, we confirm the correlation seen in previous studies between ΓX and MBH and both LUV/L Edd and LX/LEdd; however, we also find evidence that the dependence of ΓX of these quantities is not monotonic, changing sign at MBH ∼ 3 × 108 M . We argue that the αox correlations imply that the fraction of bolometric luminosity emitted by the accretion disk, as compared to the corona, increases with increasing accretion rate relative to the Eddington rate, ṁ. In addition, we argue that the ΓX trends are caused by a dependence of X-ray spectral index on ṁ. We discuss our results within the context of accretion models with Comptonizing corona and discuss the implications of the αox correlations for quasar feedback. To date, this is the largest study of the dependence of RQQ X-ray parameters on black hole mass and related quantities, and the first to attempt to correct for the large statistical uncertainty in the broad-line mass estimates.

Original languageEnglish (US)
Pages (from-to)355-373
Number of pages19
JournalAstrophysical Journal, Supplement Series
Issue number2
StatePublished - Jun 2008


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

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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