A UV to X-Ray View of Soft Excess in Type 1 Active Galactic Nuclei. II. Broadband Correlations

Abstract

The physical origin of soft X-ray excess (SE) is a long lasting question, with two prevailing theories -- ``warm corona'' and ``ionized reflection'' -- dominating the discussion. In the warm corona scenario, SE originates from upscattered disk photons and should therefore correlate strongly with UV emission. Conversely, in the ionized reflection scenario, SE arises from the illumination of the accretion disk by the hot corona and should primarily correlate with the hard X-ray primary continuum (PC). In this second paper of the series, we investigate the correlations among SE, UV and PC, leveraging a sample of 59 unobscured type 1 AGNs compiled in \citet{Chen+2025a}. Our extensive analysis reveals a strong intrinsic correlation between SE and UV that remains robust after controlling for PC ($p_\mathrm{null}\lesssim 10^{-7}$). In contrast, the correlation between SE and PC is weaker but still statistically significant ($p_\mathrm{null}\lesssim 5\times 10^{-2}$). These findings suggest that, in addition to ionized reflection -- a natural outcome of the hot corona illuminating the disk -- a warm corona component is essential, and may even dominate, in producing the soft excess. Additionally, we report a mild anti-correlation between SE strength ($q$) and PC photon index ($\Gamma_\mathrm{PC}$) ($p_\mathrm{null}=10^{-2}$), suggesting a potential competition between the warm and hot coronae. Finally, we find that the $\Gamma_\mathrm{PC}$ values we derived with SE properly incorporated exhibit a much weaker correlation with $\lambda_\mathrm{Edd}$ ($p_\mathrm{null}=2\times 10^{-2}$) than previously reported in the literature. This highlights the critical role of accurately modeling SE in studies of the $\Gamma_\mathrm{PC}$--$\lambda_\mathrm{Edd}$ relation.