2025ApJ...985..179S

There is general consensus that active galactic nuclei (AGNs) derive their radiating power from a supermassive black hole (SMBH) that accretes matter. Yet, their precise powering mechanisms and the resulting growth of the SMBH are poorly understood, especially for AGNs at high redshift. Blazars are AGNs pointing their jet toward the observer, thus being detectable from radio through gamma rays at high redshift due to Doppler boosting. The blazar MG3 J163554+3629 is located at redshift z = 3.65 and it is a flat spectrum radio quasar (FSRQ). In this work, we show the results of the modeling of its spectral energy distribution (SED) from radio to gamma rays with a one-zone leptonic model. We estimate the uncertainties through a Markov Chain Monte Carlo approach. As a result, we infer the black hole mass ${M}_{{\rm{B}}{\rm{H}}}=1.{1}_{-0.1}^{+0.2}\times 1{0}^{9}{M}_{\odot }$ and a modest magnetic field of $B=6.5{6}_{-0.09}^{+0.13}\times 1{0}^{-2}\,{\rm{G}}$ in line with the Compton dominance observed in high-redshift FSRQs. The emitting region is outside the broad line region but within the region of the dust torus radius. The rather small accretion efficiency of η = 0.083 is not solely inferred through the SED modeling but also through the energetics. An evolution study suggests that in an Eddington-limited accretion process the SMBH did not have time enough to grow from an initial seed mass of ∼10⁶M_⊙ at z ≈ 30 into a mass of M_BH ≈ 10⁹M_⊙ at z = 3.65. Faster mass growth might be obtained in a super-Eddington process throughout frequent episodes. Alternative scenarios propose that the existence of the jet itself can facilitate a more rapid growth.