Connecting the X-ray properties of weak-line and typical quasars: testing for a geometrically thick accretion disk

Abstract

We present X-ray and multiwavelength analyses of 32 weak emission-line quasars (WLQs) selected in a consistent and unbiased manner. New Chandra 3.1-4.8 ks observations were obtained for 14 of these WLQs with C IV rest-frame equivalent widths (REWs) of 5-15 Å, and these serve as an X-ray observational "bridge" between previously studied WLQs with C IV REW ≲ 5 Å and more-typical quasars with C IV REW ≈ 15-100 Å. We have identified and quantified a strong dependence of the fraction of X-ray weak quasars upon C IV REW; this fraction declines by a factor of ≈ 13 (from ≈ 44% to ≈ 3%) for C IV REW ranging from 4 to 50 Å, and the rate of decline appears particularly strong in the 10-20 Å range. The dependence broadly supports the proposed "shielding" model for WLQs, in which a geometrically and optically thick inner accretion disk, expected for a quasar accreting at a high Eddington ratio, both prevents ionizing EUV/X-ray photons from reaching the high-ionization broad emission-line region and also sometimes blocks the line of sight to the central X-ray-emitting region. This model is also supported by the hard average spectral shape of X-ray weak WLQs (with a power-law effective photon index of Γ _eff=1.19^{+0.56}_{-0.45}). Additionally, we have examined ultraviolet (UV) continuum/emission-line properties that might trace X-ray weakness among WLQs, confirming that red UV continuum color is the most-effective tracer.