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

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 $1.5 \leq z \leq 3.5$ 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 CIV$\lambda1549$ emission line to infer the strength of accretion disc winds and perform X-ray spectral fitting. The X-ray spectral properties -- namely, the 2keV monochromatic luminosity (L$_\text{2keV}$) and spectral slope -- are not strongly correlated with wind strength. Despite this result, the X-ray selected sample is shifted towards lower CIV 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 HeII$\lambda1640$ line luminosity and define the slopes between this and the 2500A monochromatic luminosity ($L_{2500}$) and L$_\text{2keV}$ ($α_\text{ouv}$ and $α_\text{uvx}$, respectively) in a similar manner to the familiar $α_\text{ox}$ parameter, which tracks the spectral slope between $L_{2500}$ and L$_\text{2keV}$. The quantity $α_\text{ouv}$ is more strongly correlated with wind strength in our sample than $α_\text{ox}$. We show that the correlation between $α_\text{ox}$ 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 ionising far-UV photons must not over-ionise the gas. The hard X-ray photons are few enough in number to have a negligible effect on the ionisation state of the material.

Figures (18)

• Figure 1: Civ emission space for the SDSS-IV quasar sample presented in rankine_bal_2020rankine_placing_2021 (grey) and the subset of this sample that was part of the CORE SDSS quasar targeting programmes (blue; see Section \ref{['sec:civ']}). The contours encircle 11.8, 39.3, 67.5 and 86.4 per cent of the objects in each sample. The marginalised distributions have been renormalized. The Civ distance reference line is the black curve, with the direction of increasing Civ distance indicated. The optical selection that constitutes the CORE sample does not bias the SDSS-IV comparison sample towards a particular part of the parameter space. The X-ray selected SDSS-V sample (orange; discussed in Section \ref{['sec:civ']}) is shifted towards lower blueshifts and larger Civ EWs but spectra with significant blueshifts are still apparent in the sample.
• Figure 2: Rest-frame quasar spectrum composites generated using the SDSS pipeline redshifts (light blue and light orange) and updated redshifts from this work (dark blue and dark orange) for quasars with redshift differences $-1500 \leq \Delta v \leq -500$ km s$^{-1}$ (blue) and $500 \leq \Delta v \leq 1500$ km s$^{-1}$ (orange). The distribution of $\Delta v$ is presented in the inset figure. Most notably, the peak of the Mgii$\,\lambda$2800 emission line is shifted towards 2800 Å when using the corrected redshifts and the Ciii] $\lambda$1909 is in better alignment.
• Figure 3: Example of our Bayesian X-ray modelling procedure. This is an eRASS:5 spectrum with a total of 253 photon counts and is for an AGN with SDSS fibre coordinates (hms) RA, Dec = 04:18:26.22, -03:18:41.9. In the upper left panel we show the spectral data in counts and the best-fitting model (dark blue, solid line). The model consists of two main components: the source model (TBabs*zTBabs*powerlaw, green dotted) and the model for the background (green dash-dot). The confidence regions for the model are shown for 1$\sigma$ (orange) and 99 per cent (yellow) level confidence. The bottom left panel shows the error-normalised residuals of the model. In the right panels we show the constraints on models parameters that we we are able to derive from the posterior distributions. The panels on the right show the posteriors for the intrinsic absorption log(N$_\textrm{H}$ [$10^{22}$$\textrm{cm}^{-2}]$) and the power-law index $\Gamma$. The inset panel in the left plot shows the posterior for the derived 2 keV monochromatic luminosity ($L_{2\,\textrm{keV}}$ [$10^{42}$ erg s$^{-1}$ keV$^{-1}$]). We show the posterior constraints on the model parameters, specifically for the intrinsic absorption log(N$_\textrm{H}$ [$10^{22}$$\textrm{cm}^{-2}]$) in the upper right panel, the power-law index $\Gamma$ in the lower right panel, and the derived 2 keV monochromatic luminosity ($L_{2\,\textrm{keV}}$ [$10^{42}$ erg s$^{-1}$ keV$^{-1}$]) as inset in the upper left panel. Solid lines show the means of the distributions and dashed lines indicate the 1$\sigma$ intervals. The photon index and 2keV luminosity are typically better constrained than the absorption.
• Figure 4: Optical (monochromatic luminosity at 2500 Å rest-frame wavelength, top) and X-ray (monochromatic luminosity at rest-frame 2 keV, bottom) luminosities as a function of redshift for the X-ray selected SDSS-V quasar sample divided into eFEDS (blue) and eRASS (orange). The optically selected SDSS-IV DR14 sample's $L_{2500}$ distribution is displayed in grey for comparison. The 1D distributions show the stacked SDSS-V samples. The eFEDS and eRASS nominal flux limits (calculated in Section \ref{['sec:xray_fit']}), converted to 2 keV luminosity limits assuming an X-ray spectrum with photon index $\Gamma=2$, are indicated as the dashed and dotted lines, respectively.
• Figure 5: $\alpha_\text{ox}$ as a function of $L_{2500}$ for the same subsamples as Fig. \ref{['fig:lum']}. The grey regions demarcate the eFEDS and eRASS flux limits for $\Gamma=2$, converted to average $\alpha_\text{ox}$ limits for our samples. The slopes and normalisations differ between the eFEDS and eRASS samples due to the different X-ray flux limits and area coverage, thus volume. The relations from timliniii_what_2021 and lusso_x-ray_2010 are indicated by the black dashed and dot-dashed lines, respectively.