paper

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

arXiv:2203.16671 · doi:10.3847/1538-4357/ac5f58

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 and Mg II ). However, such lines tend to be weak or inaccessible in the most distant quasars, leading to increasing uncertainty in L/L_Edd estimates at . Utilizing a carefully selected sample of 53 radio-quiet quasars that have H and C IV spectroscopy as well as {\sl Chandra} coverage, we searched for a robust accretion-rate indicator for quasars, particularly at the highest-accessible redshifts (). Our analysis explored relationships between the H-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-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-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 , 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 (), which is considered an unbiased L/L_Edd indicator. Correlations between EW(C IV) and a_ox with -based L/L_Edd may yield a more robust prediction of a quasar normalized accretion rate.

arXiv comment13 pages (aastex62), 4 figures. Accepted for publication in ApJ

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