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We report the results from Chandra and XMM-Newton observations of a sample of10 type 1 quasars selected to have unusual UV emission-line properties (weakand blueshifted high-ionization lines; strong UV Fe emission) similar to thoseof PHL 1811, a confirmed intrinsically X-ray weak quasar. These quasars wereidentified by the Sloan Digital Sky Survey at high redshift (z~2.2); eight areradio quiet while two are radio intermediate. All of the radio-quiet PHL 1811analogs are notably X-ray weak by a mean factor of ~13. These sources lackbroad absorption lines and have blue UV/optical continua, suggesting they areintrinsically X-ray weak. However, their average X-ray spectrum appears to beharder than those of typical quasars, which may indicate the presence of heavyintrinsic X-ray absorption. Our radio-quiet PHL 1811 analogs support aconnection between an X-ray weak spectral energy distribution and PHL 1811-likeUV emission lines; this connection provides an economical way to identify X-rayweak type 1 quasars. The fraction of radio-quiet PHL 1811 analogs in theradio-quiet quasar population is estimated to be < 1.2%. We have investigatedcorrelations between relative X-ray brightness and UV emission-line propertiesfor a sample combining radio-quiet PHL 1811 analogs, PHL 1811, and typical type1 quasars. These correlation analyses suggest that PHL 1811 analogs may haveextreme wind-dominated broad emission-line regions. Observationally,radio-quiet PHL 1811 analogs appear to be a subset (~30%) of radio-quietweak-line quasars. The existence of a subset of quasars in whichhigh-ionization "shielding gas" covers most of the BELR, but little more thanthe BELR, could potentially unify the PHL 1811 analogs and WLQs. The tworadio-intermediate PHL 1811 analogs are X-ray bright. One of them appears tohave jet-dominated X-ray emission, while the nature of the other remainsunclear.

A Population of X-ray Weak Quasars: PHL 1811 Analogs at High Redshift