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 $\lesssim$ 5 Å and more-typical quasars with C IV REW $\approx$ 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 $\approx 13$ (from $\approx 44$% to $\approx 3$%) for C IV REW ranging from 4-50 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 ${\Gamma }_{\mathrm{eff}}=1.19_{-0.45}^{+0.56}$). Additionally, we have examined UV continuum/emission-line properties that might trace X-ray weakness among WLQs, confirming that red UV continuum color is the most-effective tracer.

21 pages, 12 figures. Accepted for publication in MNRAS

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