X-RAY INSIGHTS INTO THE NATURE OF PHL 1811 ANALOGS AND WEAK EMISSION-LINE QUASARS: UNIFICATION WITH A GEOMETRICALLY THICK ACCRETION DISK?

Abstract

We present an X-ray and multiwavelength study of 33 weak emission-line\nquasars (WLQs) and 18 quasars that are analogs of the extreme WLQ, PHL 1811, at\nz ~ 0.5-2.9. New Chandra 1.5-9.5 ks exploratory observations were obtained for\n32 objects while the others have archival X-ray observations. Significant\nfractions of these luminous type 1 quasars are distinctly X-ray weak compared\nto typical quasars, including 16 (48%) of the WLQs and 17 (94%) of the PHL 1811\nanalogs with average X-ray weakness factors of 17 and 39, respectively. We\nmeasure a relatively hard ($\\Gamma=1.16_{-0.32}^{+0.37}$) effective power-law\nphoton index for a stack of the X-ray weak subsample, suggesting X-ray\nabsorption, and spectral analysis of one PHL 1811 analog, J1521+5202, also\nindicates significant intrinsic X-ray absorption. We compare composite SDSS\nspectra for the X-ray weak and X-ray normal populations and find several\noptical-UV tracers of X-ray weakness; e.g., Fe II rest-frame equivalent width\nand relative color. We describe how orientation effects under our previously\nproposed "shielding-gas" scenario can likely unify the X-ray weak and X-ray\nnormal populations. We suggest that the shielding gas may naturally be\nunderstood as a geometrically thick inner accretion disk that shields the broad\nline region from the ionizing continuum. If WLQs and PHL 1811 analogs have very\nhigh Eddington ratios, the inner disk could be significantly puffed up (e.g., a\nslim disk). Shielding of the broad emission-line region by a geometrically\nthick disk may have a significant role in setting the broad distributions of C\nIV rest-frame equivalent width and blueshift for quasars more generally.\n