Soft X-ray and ultraviolet emission relations in optically selected AGN samples

Using a sample of 228 optically selected active galactic nuclei (AGNs) in the 0.01-6.3 redshift range with a high fraction of X-ray detections (81%-86%), we study the relation between rest-frame UV and soft X-ray emission and its evolution with cosmic time. The majority of the AGNs in our sample (155 objects) have been selected from the Sloan Digital Sky Survey (SDSS) in an unbiased way, rendering the sample results representative of all SDSS AGNs. The addition of two heterogeneous samples of 36 high-redshift and 37 low-redshift AGNs further supports and extends our conclusions. We confirm that the X-ray emission from AGNs is correlated with their UV emission and that the ratio of the monochromatic luminosity emitted at 2 keV compared to 2500 A decreases with increasing luminosity (α _ox = -0.136l _UV + 2.616, where l _UV is in log units) but does not change with cosmic time. These results apply to intrinsic AGN emission, as we correct or control for the effects of the host galaxy, UV/X-ray absorption, and any X-ray emission associated with radio emission in AGNs. We investigate a variety of systematic errors and can thereby state with confidence that (1) the α _ox- l _UV anticorrelation is real and not a result of accumulated systematic errors and (2) any α _ox dependence on redshift is negligible in comparison. We provide the best quantification of the α _ox-l _UV relation to date for normal radio-quiet AGNs; this should be of utility for researchers pursuing a variety of studies.