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.
The X-ray energy dependence of the relation between optical and X-ray emission in quasars / Young, M.; Elvis, M.; Risaliti, G.. - In: THE ASTROPHYSICAL JOURNAL. - ISSN 0004-637X. - ELETTRONICO. - 708:(2010), pp. 1388-1397. [10.1088/0004-637X/708/2/1388]
The X-ray energy dependence of the relation between optical and X-ray emission in quasars
Risaliti, G.
2010
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.I documenti in FLORE sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.