Can X-ray Observations Improve Optical-UV-based Accretion-Rate Estimates for Quasars?

TL;DR

This study investigates whether X-ray observations can enhance optical-UV methods for estimating quasar accretion rates, focusing on spectral indicators like C IV EW and X-ray spectral slope, especially at high redshifts.

Contribution

It identifies C IV EW as a strong indicator of accretion rate and explores the potential of X-ray data to improve these estimates at high redshifts.

Findings

C IV EW correlates with Hβ-based L/L_Edd but with scatter.

X-ray spectral slope a_ox does not improve the accretion rate relation.

Additional X-ray data may help refine accretion rate estimates.

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$\beta$ and Mg II $\lambda2800$). 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$\beta$ and C IV $\lambda1549$ spectroscopy as well as {\sl Chandra} coverage, we searched for a robust accretion-rate indicator for quasars, particularly at the highest-accessible redshifts ($z \sim 6-7$). Our analysis explored relationships between the H$\beta$-based L/L_Edd, the equivalent width (EW) of C IV, and the optical-to-X-ray spectral slope (a_ox). Our results show that EW(C IV) is the strongest indicator of the H$\beta$-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 a_ox parameter improving this relation, and we do not find a significant correlation between a_ox and H$\beta$-based L/L_Edd. This absence of an improved relationship may reveal a limitation in our sample. X-ray observations of additional luminous sources, found at $z \gtrsim 1$, 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 ($\Gamma$), which is considered an unbiased L/L_Edd indicator. Correlations between EW(C IV) and a_ox with $\Gamma$-based L/L_Edd may yield a more robust prediction of a quasar normalized accretion rate.