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

Q. Ni, W. N. Brandt, B. Luo, P. B. Hall, Yue Shen, S. F. Anderson, R. M. Plotkin, Gordon T. Richards, D. P. Schneider, O. Shemmer, Jianfeng Wu

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21 Scopus citations

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 A, and these serve as an X-ray observational "bridge"between previously studied WLQs with C iv REW 5 A and more-typical quasars with C iv REW ≈ 15-100 A. 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 A, and the rate of decline appears particularly strong in the 10-20 A 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.

Original languageEnglish (US)
Pages (from-to)5184-5202
Number of pages19
JournalMonthly Notices of the Royal Astronomical Society
Volume480
Issue number4
DOIs
StatePublished - Nov 1 2018

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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