Emission Line-Ultraviolet to X-Ray Continuum Correlations: Constraints on the Anisotropy of the Ionizing Continuum in Active Galactic Nuclei

Abstract

Anisotropic emission of the ionizing continuum is a general prediction of the accretion disk models. In this paper, we present the results of correlation analysis of the UV emission line and UV to X-ray continuum properties for a large sample of broad emission line AGNs observed with ROSAT, IUE and HST. We find strong correlations between the CIV/$Ly\alpha$ ratio, the equivalent width of CIV, and the UV to soft X-ray spectral slope. The results are in good agreement with the photoionization calculation, suggesting that the overall ionizing continuum can well match the observed UV to soft X-ray spectrum. These results are consistent with the assumption of isotropic ionizing continuum shape. Our analysis suggests a small range for the ``big blue bump'' cutoff energy for the objects in this sample, consistent with the similar results of Laor et al. (1997) and Walter & Fink (1993) based on the continuum properties . The mean UV-to-X-ray spectral slope is similar to the soft X-ray spectral slope. This similarity also holds for radio-loud and radio-quiet objects separately. This suggests that the two might be drawn from the same distribution. The two spectral slopes are only weakly correlated. The UV to X-ray spectral index is correlated with absolute optical magnitude. This result confirms the earlier suggestion that the ionizing continua are softer for higher luminosity objects.