Are Optically-Selected Quasars Universally X-Ray Luminous? X-Ray/UV Relations in Sloan Digital Sky Survey Quasars

TL;DR

This study investigates the X-ray and UV spectral properties of SDSS quasars at redshifts 1.7 to 2.7, finding that X-ray weakness is rare and correlates with specific UV emission line features, challenging the idea of universal X-ray luminosity.

Contribution

It provides the first large-scale analysis of X-ray/UV relations in SDSS quasars, revealing the rarity of X-ray weak quasars and their spectral characteristics.

Findings

X-ray weak quasars are less than 2% of the sample.

X-ray luminosity varies by a factor of 2 over years.

X-ray brightness correlates with UV emission line properties.

Abstract

We analyze archived Chandra and XMM-Newton X-ray observations of 536 Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) quasars (QSOs) at 1.7 <= z <= 2.7 in order to characterize the relative UV and X-ray spectral properties of QSOs that do not have broad UV absorption lines (BALs). We constrain the fraction of X-ray weak, non-BAL QSOs and find that such objects are rare; for example, sources underluminous by a factor of 10 comprise $\la$2% of optically-selected SDSS QSOs. X-ray luminosities vary with respect to UV emission by a factor of $\la$2 over several years for most sources. UV continuum reddening and the presence of narrow-line absorbing systems are not strongly associated with X-ray weakness in our sample. X-ray brightness is significantly correlated with UV emission line properties, so that relatively X-ray weak, non-BAL QSOs generally have weaker, blueshifted CIV$\lambda$1549 emission and broader CIII]$\lambda$1909 lines. The CIV emission line strength depends on both UV and X-ray luminosity, suggesting that the physical mechanism driving the global Baldwin effect is also associated with X-ray emission.