Accretion mechanism

X-ray emission is a common characteristic of AGN and is originated from the Comptonization of optical/UV photons from the accretion disk by a corona of hot electrons situated above the supermassive black hole. This process generates a power-law X-ray spectrum, typically with a photon index of Γ ≈ 1.8–2 and a high-energy cut-off at a few hundred keV. This cut-off is directly linked to the temperature and optical depth of the hot electron plasma that produces the power-law emission.

Observations have revealed a correlation between the photon index Γ and the Eddington ratio (L_bol/L_Edd) In general, higher Eddington ratios are associated with steeper (softer) X-ray spectra, meaning higher values of Γ. This trend suggests that as the accretion rate increases, the disk becomes more efficient at cooling the corona via inverse Compton scattering, leading to softer emission. Conversely, lower Eddington ratios, often linked to Advection dominated accretion flows (ADAF), result in harder spectra (lower Γ). This relationship provides valuable insight into the coupling between accretion physics and coronal properties in AGN.

Studies of X-ray binaries (XRBs), particularly those containing stellar-mass black holes, have provided valuable analogies for understanding accretion processes in AGN. Both systems share similar physical components—an accretion disk, a hot corona, and in many cases, relativistic jets—despite their vastly different mass scales. The observed spectral states and variability patterns in XRBs, which evolve on human-observable timescales, offer a scaled-down laboratory to probe the physics of accretion and feedback. Transitions between high/soft and low/hard states in XRBs have been compared to changes in AGN accretion modes, suggesting that accretion efficiency and geometry may follow similar principles across the mass scale. However, whether these analogies fully extend to AGN remains an open question, particularly due to differences in the surrounding environment and timescale of variability.

Despite significant progress, key questions remain regarding the fundamental mechanisms driving accretion in AGN. A particularly intriguing open question is how accretion processes evolve across different luminosity regimes and how this evolution affects the observed spectral properties. In this context, the X-ray reflection component becomes a critical diagnostic. By studying the reflection features in detail, we can better constrain the geometry and physical state of the circumnuclear material, which in turn can refine measurements of the photon index. Improved constraints on Γ are essential to establish more accurate correlations with intrinsic parameters such as the accretion rate or Eddington ratio, and to disentangle the contributions of different emission and absorption mechanisms across AGN populations.