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A POPULATION OF X-RAY WEAK QUASARS: PHL 1811 ANALOGS AT HIGH REDSHIFT
Journal article   Open access

A POPULATION OF X-RAY WEAK QUASARS: PHL 1811 ANALOGS AT HIGH REDSHIFT

Jianfeng Wu, W. N. Brandt, Patrick B. Hall, Robert R. Gibson, Gordon T. Richards, Donald P. Schneider, Ohad Shemmer, Dennis W. Just and Sarah J. Schmidt
The Astrophysical journal, v 736(1), pp jQuery1323905986708='48'-jQuery1323905986708='48'
20 Jul 2011
DOI: https://doi.org/10.1088/0004-637X/736/1/28
url
https://doi.org/10.1088/0004-637x/736/1/28View
Published, Version of Record (VoR)Open Access (License Unspecified),  Open
url
https://doi.org/10.1088/0004-637X/736/1/28View
Published, Version of Record (VoR) Open

Abstract

Astronomy & Astrophysics Physical Sciences Science & Technology
We report the results from Chandra and XMM-Newton observations of a sample of 10 type 1 quasars selected to have unusual UV emission-line properties (weak and blueshifted high-ionization lines; strong UV Fe emission) similar to those of PHL 1811, a confirmed intrinsically X-ray weak quasar. These quasars were identified by the Sloan Digital Sky Survey at high redshift (z approximate to 2.2); eight are radio quiet while two are radio intermediate. All of the radio-quiet PHL 1811 analogs, without exception, are notably X-ray weak by a mean factor of approximate to 13. These sources lack broad absorption lines and have blue UV/optical continua, supporting the hypothesis that they are intrinsically X-ray weak like PHL 1811 itself. However, their average X-ray spectrum appears to be harder than those of typical quasars, which may indicate the presence of heavy intrinsic X-ray absorption. Our sample of radio-quiet PHL 1811 analogs supports a connection between an X-ray weak spectral energy distribution and PHL 1811-like UV emission lines; this connection provides an economical way to identify X-ray weak type 1 quasars. The fraction of radio-quiet PHL 1811 analogs in the radio-quiet quasar population is estimated to be less than or similar to 1.2%. We have investigated correlations between relative X-ray brightness and UV emission-line properties (e. g., C IV equivalent width and blueshift) for a sample combining our radio-quiet PHL 1811 analogs, PHL 1811 itself, and typical type 1 quasars. These correlation analyses suggest that PHL 1811 analogs may have extreme wind-dominated broad emission-line regions. Observationally, the radio-quiet PHL 1811 analogs appear to be a subset (approximate to 30%) of radio-quiet weak-line quasars (WLQs). The existence of a subset of quasars in which high-ionization "shielding gas" covers most of the broad emission-line region (BELR), but little more than the BELR, could potentially unify the PHL 1811 analogs and WLQs. The two radio-intermediate PHL 1811 analogs are X-ray bright. X-ray spectral analyses and consideration of their multiwavelength properties suggest that one of them has jet-dominated X-ray emission, while the nature of the other remains unclear.

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