Photometric SED Fitting of High-Redshift AGN via Markov Chain Monte Carlo Analysis

<p>Active galactic nuclei (AGN) at z > 3 offer key insights into early supermassive black hole<br>(SMBH) growth. Blazar-type AGN, with relativistic jets aligned toward us, provide well-sampled<br>UV–optical spectral energy distributions (SEDs), enabling accretion disk modeling. We fit SEDs<br>of six high-z blazars using the Shakura–Sunyaev thin disk model and Markov Chain Monte Carlo<br>(MCMC), both with and without dust extinction. The black hole mass MBH and mass accretion<br>rate Mdot are treated as free parameters.<br>Without extinction, we find MBH ∼ (2.6–11.4) × 10E9 Sun Masses, Mdot ∼ 2.7–60 Sun masses/year, and Eddington ratios λ ∼ 0.04–0.23. With extinction, masses decrease to ∼ (1.3–5.0) × 10E9 sun masses. Accretion rates increase significantly, with λ reaching up to 0.79 and deviations in Eddington ratio from<br>literature values up to +279%.<br>We find mass discrepancies up to 100% compared to published values, and up to 800% when<br>compared to spectroscopic estimates. Assuming continuous Eddington-limited growth from<br>z = 20, the inferred seed masses span ∼ 10E5–10E9 sun masses (no extinction) and can drop to sub-solar<br>values with extinction. For each source, possible formation scenarios for these seeds are discussed<br>in context.<br>Estimated disk temperatures fall in the far-UV, with Tmax ∼ (2–3.5) × 10E4 K in the no-extinction<br>case, and rise to ∼ (9–10) × 10E4 K with extinction, consistent with hotter, UV-peaking disks.<br>These results demonstrate the effectiveness of MCMC-based SED fitting in constraining black<br>hole masses, accretion rates, and growth histories of high-redshift AGN, while underscoring the<br>need for physically motivated modeling.</p>