The X-ray Properties of the Most-luminous Quasars from the Sloan Digital Sky Survey

Utilizing 21 new Chandra observations as well as archival Chandra, ROSAT, and XMM-Newton data, we study the X-ray properties of a representative sample of 59 of the most optically luminous quasars in the Universe (Mi ≈ −29.3 to −30.2) spanning a redshift range of z ≈ 1.5–4.5. Our full sample consists of 32 quasars from the Sloan Digital Sky Survey (SDSS) Data Release 3 (DR3) quasar catalog, two additional objects in the DR3 area that were missed by the SDSS selection criteria, and 25 comparably luminous quasars at z∼4. This is the largest X-ray study of such luminous quasars to date. By jointly fitting the X-ray spectra of our sample quasars, excluding radio-loud and broad absorption line (BAL) objects, we find a mean X-ray powerlaw photon index of Γ = 1.92+0.09 −0.08 and constrain any neutral intrinsic absorbing material to have a mean column density of NH ∼ 2× 1021 cm−2. We find, consistent with other studies, that Γ does not change with redshift, and we constrain the amount of allowed Γ evolution for the most-luminous quasars. Our sample, excluding radio-loud and BAL quasars, has a mean X-ray-to-optical spectral slope of αox = −1.80±0.02, as well as no significant evolution of αox with redshift. We also comment upon the X-ray properties of a number of notable quasars, including an X-ray weak quasar with several strong narrow absorption-line systems, a mildly radio-loud BAL quasar, and a well-studied gravitationally lensed quasar. Subject headings: Galaxies: Active: Nuclei — Galaxies: Active: Optical/UV/X-ray — Galaxies: Active: Evolution — Methods: Statistical