What controls the UV-to-X-ray continuum shape in quasars?

ABSTRACT We present an investigation of the interdependence optical-to-X-ray spectral slope (?ox), He ii equivalent-width (EW), and monochromatic luminosity at 2500 Å (L2500). The values ?ox EW are indicators strength/shape quasar ionizing continuum, from ultraviolet (UV; 1500–2500 Å), through extreme (EUV; 300–50 to X-ray (2 keV) regime. For this investigation, we measure 206 radioquiet quasars devoid broad absorption lines that have high-quality observations UV 2 keV X-rays. sample spans wide redshift (?0.13–3.5) (log(L2500) ? 29.2–32.5 erg s?1 Hz?1) ranges. recover well-known ?ox–L2500 EW–L2500 anticorrelations, find a similarly strong correlation between EW, thus overall shape UV, EUV, regime is largely set by luminosity. A significant ?ox– remains after removing contribution L2500 each quantity, emission in EUV X-rays also directly tied. This relations surprising, since expected be formed three physically distinct regions. Our results indicate presence redshift-independent physical mechanism couples continuum these different regions, controls