Characterising SMSS J2157-3602, the most luminous known quasar, with accretion disc models

We develop an accretion disc (AD)-fitting method, utilizing thin and slim disc models and Bayesian inference with the Markov-Chain Monte Carlo approach, testing it on the most luminous known quasar, SMSS J215728.21-360215.1, at redshift z = 4.692. With a spectral energy distribution constructed from near-infrared spectra and broad-band photometry, the AD models find a black hole mass of (MAD/M⊙) = 10.31+0.17-0.14 with an anisotropy-corrected bolometric luminosity of (Lbolerg, s-1) = 47.87 ± 0.10 and derive an Eddington ratio of 0.29+0.11-0.10 as well as a radiative efficiency of 0.09+0.05-0.03. Using the near-infrared spectra, we estimate the single-epoch virial black hole mass estimate to be log (MSE/M·) = 10.33 ± 0.08, with a monochromatic luminosity at 3000 Å of L(3000 AA), s-1 = 47.66 ± 0.01. As an independent approach, AD fitting has the potential to complement the single-epoch virial mass method in obtaining stronger constraints on properties of massive quasar black holes across a wide range of redshifts.