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$\beta$ and Mg II $\lambda 2800$). 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$\beta$ and C IV $\lambda 1549$ 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.

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

References in corpus (21)

• Wilkinson Microwave Anisotropy Probe (WMAP) Three Year Results: Implications for Cosmology
• A Catalog of Quasar Properties from SDSS DR7
• The X-ray-to-Optical Properties of Optically-Selected Active Galaxies Over Wide Luminosity and Redshift Ranges
• The Sloan Digital Sky Survey Quasar Catalog: Sixteenth Data Release
• The X-ray Properties of the Most-Luminous Quasars from the Sloan Digital Sky Survey
• The Pan-STARRS1 distant z>5.6 quasar survey: more than 100 quasars within the first Gyr of the universe
• The Hard X-ray Spectrum as a Probe for Black-Hole Growth in Radio-Quiet Active Galactic Nuclei
• The Hard X-ray Spectral Slope as an Accretion-Rate Indicator in Radio-Quiet Active Galactic Nuclei
• The Radius-Luminosity Relationship Depends on Optical Spectra in Active Galactic Nuclei
• X-ray Insights into the Nature of PHL 1811 Analogs and Weak Emission-Line Quasars: Unification with a Geometrically Thick Accretion Disk?
• Three new VHS-DES Quasars at 6.7 < z < 6.9 and Emission Line Properties at z > 6.5
• Revealing the Accretion Physics of Supermassive Black Holes at Redshift z~7 with Chandra and Infrared Observations
• Detection of Rest-frame Optical Lines from X-shooter Spectroscopy of Weak Emission Line Quasars
• On the Observational Difference Between the Accretion Disk-Corona Connections among Super- and Sub-Eddington Accreting Active Galactic Nuclei
• Weak Emission Line Quasars in the Context of a Modified Baldwin Effect
• The correlations between optical/UV broad lines and X-ray emission for a large sample of quasars
• Variability and the X-ray/UV ratio of Active Galactic Nuclei
• Black Hole Mass Estimates and Rapid Growth of Supermassive Black Holes in Luminous $z\sim$ 3.5 Quasars
• What controls the UV-to-X-ray continuum shape in quasars?
• Characterizing Quasar CIV Emission-line Measurements from Time-resolved Spectroscopy
• Placing High-Redshift Quasars in Perspective: a Catalog of Spectroscopic Properties from the Gemini Near Infrared Spectrograph -- Distant Quasar Survey