Can X-Ray Observations Improve Optical-UV-based Accretion-rate Estimates for Quasars?

Abstract

Current estimates of the normalized accretion rates of quasars ( L / L Edd ) rely on measuring the velocity widths of broad optical-UV emission lines (e.g., H β and Mg ii λ 2800). However, such lines tend to be weak or inaccessible in the most distant quasars, leading to increasing uncertainty in L / L Edd estimates at z > 6. Utilizing a carefully selected sample of 53 radio-quiet quasars that have H β and C iv λ 1549 spectroscopy as well as Chandra coverage, we searched for a robust accretion-rate indicator for quasars, particularly at the highest-accessible redshifts ( z ∼ 6–7). Our analysis explored relationships between the H β -based L / L Edd , the equivalent width (EW) of C iv , and the optical-to-X-ray spectral slope ( α ox ). Our results show that EW(C iv ) is the strongest indicator of the H β -based L / L Edd parameter, consistent with previous studies, although significant scatter persists particularly for sources with weak C iv lines. We do not find evidence for the α ox parameter improving this relation, and we do not find a significant correlation between α ox and H β -based L / L Edd . This absence of an improved relationship may reveal a limitation of our sample. X-ray observations of additional luminous sources, found at z ≳ 1, may allow us to mitigate the biases inherent in our archival sample and test whether X-ray data could improve L / L Edd estimates. Furthermore, deeper X-ray observations of our sources may provide accurate measurements of the hard-X-ray power-law photon index (Γ), which is considered an unbiased L / L Edd indicator. Correlations between EW(C iv ) and α ox with a Γ-based L / L Edd may yield a more robust prediction of a quasar normalized accretion rate.