The X-ray Energy Dependence of the Relation Between Optical and X-ray Emission in Quasars
Abstract
We develop a new approach to the well-studied anti-correlation between the optical-to-X-ray spectral index, αox, and the monochromatic optical luminosity, l opt. By cross-correlating the Sloan Digital Sky Survey DR5 quasar catalog with the XMM-Newton archive, we create a sample of 327 quasars with X-ray signal-to-noise ratio (S/N) > 6, where both optical and X-ray spectra are available. This allows αox to be defined at arbitrary frequencies, rather than the standard 2500 Å and 2 keV. We find that while the choice of optical wavelength does not strongly influence the αox-l opt relation, the slope of the relation does depend on the choice of X-ray energy. The slope of the relation becomes steeper when αox is defined at low (~1 keV) X-ray energies. This change is significant when compared to the slope predicted by a decrease in the baseline over which αox is defined. The slopes are also marginally flatter than predicted at high (~10 keV) X-ray energies. Partial correlation tests show that while the primary driver of αox is l opt, the Eddington ratio correlates strongly with αox when l opt is taken into account, so accretion rate may help explain these results. We combine the αox-l opt and Γ-L bol/L Edd relations to naturally explain two results: (1) the existence of the Γ-lx relation as reported in Young et al., and (2) the lack of a Γ-l opt relation. The consistency of the optical/X-ray correlations establishes a more complete framework for understanding the relation between quasar emission mechanisms. We also discuss two correlations with the hard X-ray bolometric correction, which we show correlates with both αox and Eddington ratio. This confirms that an increase in accretion rate correlates with a decrease in the fraction of up-scattered disk photons.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 2010
- DOI:
- 10.1088/0004-637X/708/2/1388
- arXiv:
- arXiv:0911.0474
- Bibcode:
- 2010ApJ...708.1388Y
- Keywords:
-
- accretion;
- accretion disks;
- galaxies: active;
- Astrophysics - High Energy Astrophysical Phenomena;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- 27 pages preprint style, 10 figures. New material added to section 4.3 (Fig. 11) regarding hard X-ray bolometric corrections. Accepted for publication in Astrophysical Journal