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

TL;DR

The study addresses the origin of the soft X-ray excess in Type 1 AGNs by examining broadband, model-independent luminosity correlations among SE, UV, and the hard X-ray primary continuum. Using a robust sample of 59 AGNs and multiple correlation techniques (bivariate, normalization-based, partial correlations, and multilinear regression), the authors find a strong intrinsic SE–UV link that persists after accounting for the primary continuum, while the SE–PC connection is present but weaker. This pattern supports a hybrid SE scenario in which a warm corona dominates SE production with a secondary contribution from ionized disk reflection, and it highlights corona–disk coupling as a key driver of SE. The results also show that accurately modeling SE affects inferences about accretion physics, such as the Γ_PC–λ_Edd relation, emphasizing the need to include SE in analyses of AGN spectral energy distributions and corona properties.

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 ($Γ_\mathrm{PC}$) ($p_\mathrm{null}=10^{-2}$), suggesting a potential competition between the warm and hot coronae. Finally, we find that the $Γ_\mathrm{PC}$ values we derived with SE properly incorporated exhibit a much weaker correlation with $λ_\mathrm{Edd}$ ($p_\mathrm{null}=2\times 10^{-2}$) than previously reported in the literature. This highlights the critical role of accurately modeling SE in studies of the $Γ_\mathrm{PC}$--$λ_\mathrm{Edd}$ relation.

Figures (8)

• Figure 1: Normalized UV-to-X-ray SEDs for our sample. The absorption corrected best-fit X-ray model spectrum (model 3) is shifted to the rest frame, scaled to the simultaneous rest frame 2500Å flux, and color coded based on $\alpha_\mathrm{oX}$ (the power-law slope of the dashed color lines).
• Figure 2: Relation between SE strength ($q$) and UV--PC luminosity ratio ($L_\mathrm{UV}/L_\mathrm{PC,0.5-10}$). Shown are the Spearman’s $\rho$, null hypothesis probability $p_\mathrm{null}$, OLS slope $\beta$, and OLS bisector slope $\beta^\mathrm{bi}$. The shaded region marks the $1\sigma$ confidence interval of the bisector fit. Unless noted otherwise, the same legend format applies to all scatter plots in this paper.
• Figure 3: Relation between $\alpha_\mathrm{oX}$ and soft X-ray spectral slope ($\alpha_\mathrm{0.5-2}$, purple) or hard X-ray spectral slope ($\alpha_\mathrm{2-10}$, green). A 1:1 reference line is shown in gray for comparison.
• Figure 4: Luminosity-luminosity correlation for SE vs. UV (left), SE vs. PC (middle), and PC vs. UV (right). Model 3 is used to derive SE and PC luminosities.
• Figure 5: Intrinsic SE--UV and SE--PC relations as revealed by luminosity-ratio correlations. The top three panels (in purple) illustrate the intrinsic correlation between SE and UV luminosities, with both scaled by PC luminosity, under different SE models (blackbody, power-law, and cut-off power-law). The bottom three panels (in green) similarly show the intrinsic correlation between SE and PC luminosities, scaled by UV luminosity. The results highlight a strong intrinsic link between SE and UV luminosities ($p_\mathrm{null}\sim2.0e-7$), even after removing PC contributions. The correlation between SE and PC is also statistically significant ($p_\mathrm{null}\sim6.0e-3$) but is generally weaker than the SE--UV correlation.