The correlations between optical/UV broad lines and X-ray emission for a large sample of quasars

TL;DR

This study analyzes a large sample of quasars to explore the correlations between optical/UV broad emission lines and X-ray emission, revealing significant relationships and differences in X-ray weakness between typical quasars and weak-line quasars.

Contribution

It provides the largest dataset to date linking optical/UV emission line properties with X-ray emission in quasars, and compares typical quasars with weak-line quasars using new correlations.

Findings

Significant correlations between $ ext{α}_{ox}$ and emission-line parameters.

Weak-line quasars show weaker X-ray emission than typical quasars.

The observed behaviors support the disk-wind and X-ray shielding models.

Abstract

We present Chandra observations of 2106 radio-quiet quasars in the redshift range 1.7<z<2.7 from the Sloan Digital Sky Survey (SDSS), through data release fourteen (DR14), that do not contain broad absorption lines (BAL) in their rest-frame UV spectra. This sample adds over a decade worth of SDSS and Chandra observations to our previously published sample of 139 quasars from SDSS DR5 which is still used to correlate X-ray and optical/UV emission in typical quasars. We fit the SDSS spectra for 753 of the quasars in our sample that have high-quality (exposure time $\gtrapprox$10 ks and off-axis observation angle <10 arcmin) X-ray observations, and analyze their X-ray-to-optical SED properties ($\alpha_{ox}$ and $\Delta\alpha_{ox}$) with respect to the measured CIV and MgII emission-line rest-frame equivalent width (EW) and the CIV emission-line blueshift. We find significant correlations (at the >99.99% level) between $\alpha_{ox}$ and these emission-line parameters, as well as between $\Delta\alpha_{ox}$ and CIV EW. Slight correlations are found between $\Delta\alpha_{ox}$ and CIV blueshift, MgII EW, and the CIV EW to MgII EW ratio. The best-fit trend in each parameter space is used to compare the X-ray weakness ($\Delta\alpha_{ox}$) and optical/UV emission properties of typical quasars and weak-line quasars (WLQs). The WLQs typically exhibit weaker X-ray emission than predicted by the typical quasar relationships. The best-fit relationships for our typical quasars are consistent with predictions from the disk-wind quasar model. The behavior of the WLQs compared to our typical quasars can be explained by an X-ray "shielding" model.