C iv wind properties of the SDSS-V X-ray selected quasars: strong optical-to-UV emission is key regardless of X-ray strength

Abstract

ABSTRACT We present an investigation of the rest-frame optical/UV and X-ray properties for a sample of 3027 X-ray selected quasars between $1.5 \le z \le 3.5$ detected in the deepest Spectrum Roentgen Gamma/eROSITA data available and observed by the fifth iteration of the Sloan Digital Sky Survey (SDSS-V). We parametrize the C iv $\lambda 1549$ emission line to infer the strength of accretion disc winds and perform X-ray spectral fitting. The X-ray spectral properties – namely, the 2 keV monochromatic luminosity ($L_{2, \text{keV}}$) and spectral slope – are not strongly correlated with wind strength. Despite this result, the X-ray selected sample is shifted towards lower C iv blueshifts and higher equivalent widths than the optically selected sample observed in previous SDSS surveys, and matching in optical luminosity, redshift, and Eddington ratio does not reduce these differences. We estimate the far-UV luminosity using the He ii $\lambda 1640$ line luminosity and define the slopes between this and the 2500 Å monochromatic luminosity ($L_{2500}$) and $L_{2, \text{keV}}$ ($\alpha _\text{ouv}$ and $\alpha \text{uvx}$, respectively) in a similar manner to the familiar $\alpha \text{ox}$ parameter, which tracks the spectral slope between $L{2500}$ and $L{2, \text{keV}}$. The quantity $\alpha _\text{ouv}$ is more strongly correlated with wind strength in our sample than $\alpha _\text{ox}$. We show that the correlation between $\alpha _\text{ox}$ and wind strength is driven by the relationship between the optical luminosity and wind strength. Our results are consistent with a radiation line-driven wind, whereby the ionizing far-UV photons must not over-ionize the gas. The hard X-ray photons are few enough in number to have a negligible effect on the ionization state of the material.