| Title | Evolution of the X-ray Emission of Radio-quiet Quasars |
| Authors | Kelly, Brandon C.; Bechtold, Jill; Siemiginowska, Aneta; Aldcroft, Tom; Sobolewska, MaÅgorzata |
| Bibcode | 2007ApJ...657..116K Search ADS ↗ |
| Abstract | 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 0.144 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 logLUV, where αox becomes X-ray quiet more rapidly at higher logLUV. We find no significant evidence for a relationship between the X-ray photon index Γx and the UV luminosity, and we find marginally significant evidence that the X-ray continuum flattens with increasing z (2 σ). The Γx-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 Γx, 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. |
| Objects | 174 Objects Search NED ↙ |
