Unveiling energy pathways in AGN accretion flows with the warm corona model for the soft excess

The soft excess in active galactic nuclei (AGNs) may arise through a combination of relativistic reflection and the effects of a warm corona at the surface of the accretion disc. Detailed examination of the soft excess can therefore constrain models of the transport and dissipation of accretion energy. Here, we analyse 34 XMM-Newton observations from 14 type 1 AGNs with the REXCOR spectral model that self-consistently combines emission from a warm corona with relativistic reflection assuming a lamppost corona. The model divides accretion energy between the disc, the warm corona, and the lamppost. The XMM-Newton observations span a factor of 188 in Eddington ratio (λ_obs) and 350 in black hole mass, and we find that a warm corona is a significant contributor to the soft excess for 13 of the 14 AGNs with a mean warm corona heating fraction of 0.51. The REXCOR fits reveal that the fraction of accretion energy dissipated in the lamppost is anticorrelated with λ_obs. In contrast, the relationship between λ_obs and both the optical depth and the heating fraction of the warm corona appears to transition from an anticorrelation to a correlation at λ_obs,t ≈ 0.15. Therefore, at least one other physical process in addition to the accretion rate is needed to explain the evolution of the warm corona. Overall, we find that a warm corona appears to be a crucial depository of accretion energy in AGNs across a broad range of λ_obs and black hole mass.