X-ray lighthouses of the high-redshift universe: Probing the most luminous z > 4 palomar digital sky survey quasars with Chandra

We present the results from exploratory Chandra observations of nine high-redshift (z = 4.09-4.51) optically selected quasars. These quasars, taken from the Palomar Digital Sky Survey (DPOSS), are among the optically brightest and most luminous z > 4 quasars known (M_B ≈ -28.4 to -30.2). All have been detected by Chandra in exposure times of ≈ 5-6 ks, tripling the number of highly luminous quasars (M_B < -28.4) with X-ray detections at z > 4. These quasars' average broadband spectral energy distributions are characterized by steeper (more negative) α_ox values (〈α_ox〉 = -1.81 ± 0.03) than those of lower luminosity, lower redshift samples of quasars. We confirm the presence of a significant correlation between the ultraviolet magnitude and soft X-ray flux previously found for z > 4 quasars. The joint ≈ 2-30 keV restframe X-ray spectrum of the nine quasars is well parameterized by a simple power-law model whose photon index, Γ ≈ 2.0 ± 0.2, is consistent with those of lower redshift quasars. No evidence for significant amounts of intrinsic absorption has been found (N_H ≲ 8.8 × 10²¹ cm^-2 at 90% confidence). In general, our results show that z ≈ 4.1-4.5 quasars and local quasars have reasonably similar X-ray and broadband spectra (once luminosity effects are taken into account), suggesting that the accretion mechanisms in these objects are similar. We also present near-simultaneous optical spectra for these quasars obtained with the Hobby-Eberly Telescope; this is the first time optical spectra have been published for seven of these objects. The objects presented in this paper are among the best z > 4 targets for X-ray spectroscopy with XMM-Newton and next-generation, large-area X-ray telescopes. These will detect or constrain iron Kα emission lines down to rest-frame equivalent widths of ≈50 eV and intrinsic column densities down to N_H values roughly a few times 10²¹ cm^-2 at z ≥ 4. We also present 45 new ROSAT upper limits for z ≥ 4 quasars and a likely (3 σ) HRI detection of the blazar GB 1713+2148 at z = 4.01.