ROSAT observations of an optical quasar survey field

Abstract

Deep (T∼35 ksec) pointed ROSAT observations of a 2.2°×2.2° optical quasar survey field (149 quasars; mlim=20.5; Crampton et al. 1989) have yielded a detection rate (3 σ) of ∼60% (86 quasars; limiting sensitivity ∼5 ⋅ 10−15 erg cm−2 s−1 keV−1 at 1 keV). 46 quasars were bright enough to perform spectral power law fits. The mean energy power law index drops from ∼1.4 at z=0 to ∼0.9 at z≳2. This is interpreted as being due to a break in the spectrum between a soft, thermal accretion disk and a hard power law component, occurring at a source frame energy around 1 keV. Mean accretion disk model parameters are derived (M=5 ⋅ 108 M⊙, Ṁ=0.65 ⋅ MEdd., αvisc. =0.5), using a geometrically thin α‐accretion disk model (Dorrer et al., 1992, and references therein). The αox distribution and the optical number‐redshift relation is modeled, using the accretion disk parameters as determined from the X‐ray spectral data and assuming a constant comoving volume density (H0=100 km/s Mpc, q0=0.5) and statistical orientation of t...