Connecting the X-ray properties of weak-line and typical quasars: testing for a geometrically thick accretion disk. Issue 4 (27th July 2018)

• Record Type: Journal Article
• Authors: Ni, Q; Brandt, W N; Luo, B; Hall, P B; Shen, Yue; Anderson, S F; Plotkin, R M; Richards, Gordon T; Schneider, D P; Shemmer, O; Wu, Jianfeng
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 480:Issue 4(2018)
• Page Start: 5184
• Page End: 5202
• Publication Date: 2018-07-27
• Subjects: galaxies: active -- galaxies: nuclei -- quasars: general -- X-rays: galaxies; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/sty1989
• Languages: English
• ISSNs: 0035-8711

Abstract

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 $\Gamma _{\rm eff}=1.19^{+0.56}_{-0.45}$ ). 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.