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; m_lim = 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). See Fig. 1 for the distribution of the ROSAT PSPC source count rates and Fig. 2a, b for the fraction of quasars detected in X-rays as a function of redshift and optical magnitude. 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 (Fig. 4; only the 20 brightest sources are plotted). This is interpreted as being due to a break in the spectrum between a soft, thermal accretion disk and a hard power law component, occuring at a source frame energy around 1 keV (Fig. 5). Mean accretion disk model parameters are derived (M = 5.10⁸ M_⊙, Ṁ = 0.65 M_Edd. , α_visc. = 0.5) using an optically thin α-accretion disk model (Dörrer et al., 1992 and references therein). Model predictions for the decline of the X-ray spectral index with redshift are plotted in Fig. 4. The α_ox distribution (Fig. 3; dotted line: X-ray upper limits) and the optical number-redshift relation (Fig. 6; dotted line: X-ray 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 (H ₀ = 100 km/s Mpc, q ₀ = 0.5) and statistical orientation of the inclination angles of the model source population.