We have observed 14 quasars with z>2.8 with the ROSAT-PSPC, and detected 12 of them, including the z=4.11 quasar 0000-263. We present the first x-ray spectrum of a radio quiet quasar with z>3, 1946+768. Its x-ray spectrum is consistent with a power law with spectral index αE = 1.8+2.1-1.4 and no evidence for absorption in excess of the galactic column [αE=1.00+0.28-0.32 assuming NH=NH(Gal)]. A PSPC hardness ratio is used to constrain the x-ray spectral properties of the quasars for which there were less than 100 photons detected. For the radio quiet quasars, 〈αE〉≈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 spectrophotometry obtained at the Steward 2.3 m and Multiple Mirror Telescope, and data from the literature. The spectral energy distributions are compared to those of low redshift objects. For the radio quiet quasars with z>2.5, the mean 〈αox〉∼1.8. This is larger than the mean for quasars 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, 〈αox〉∼1.4, independent of redshift. This is smaller than the expected value for the optically luminous, high redshift objects in this sample, if they are mostly GHz peaked radio sources and hence comparable to steep-spectrum, compact radio sources at lower redshift. Finally, we compare the spectral energy distributions of two representative objects to the predicted spectrum of a thin accretion disk in the Kerr geometry, and discuss the uncertainties in deriving black hole masses and mass accretion rates.
|Original language||English (US)|
|Number of pages||21|
|State||Published - Aug 1 1994|
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science