INFRARED TO X-RAY SPECTRAL ENERGY-DISTRIBUTIONS OF HIGH-REDSHIFT QUASARS

We have observed 14 quasars with z>2.8 with the ROSAT-PSPC, and detect ed 12 of them, including the z=4.11 quasar 0000-263. We present the fi rst x-ray spectrum of a radio quiet quasar with z>3, 1946+768. Its x-r ay spectrum is consistent with a power law with spectral index alpha(E ) = 1.8(+2.1/-1.4) and no evidence for absorption in excess of the gal actic column [alpha(E) = 1.00(+0.28/-0.32) assuming N(H) = N(H)(Gal)]. A PSPC hardness ratio is used to constrain the x-ray spectral propert ies of the quasars for which there were less than 100 photons detected . For the radio quiet quasars, [alpha(E)] almost-equal-to 1.2, if one assumes that there is no absorption in excess of the galactic column. We combine the x-ray data with new ground based optical and near-IR ob tained at the Steward 2.3 m and Multiple Mirror Telescope, and data fr om the literature. The energy distributions are compared to those of l ow redshift objects. For the radio quiet quasars with z>2.5, the mean (alpha(ox)] approximately 1.8. This is larger than the mean for quasar s, with z<2.5, but consistent with the expected value for quasars with the high optical luminosities of the objects in this sample. For the radio-loud quasars, [alpha(ox)] approximately 1.4, independent of reds hift. This is smaller than the expected value for the optically lumino us, high redshift objects in this sample, if they are mostly GHz peake d radio sources and hence comparable to steep-spectrum, compact radio sources at lower redshift. Finally, we compare the spectral energy dis tributions of two representative objects to the predicted spectrum of a thin accretion disk in the Kerr geometry, and discuss the uncertaint ies in deriving black hole masses and mass accretion rates.