Comptonization in Super‐Eddington Accretion Flow and Growth Timescale of Supermassive Black Holes

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Abstract
Super-Eddington accretion onto black holes (BHs) may occur at ultraluminous compact X-ray sources in nearby galaxies, Galactic microquasars, and narrow-line Seyfert 1 galaxies (NLS1s). Effects of electron scattering (opacity and Comptonization) and the relativistic correction (gravitational redshift and transverse Doppler effect) on the emergent spectra from super-Eddington accretion flows onto nonrotating BHs are examined for 101.5 and 106.5 M☉ BH masses (MBH). With [≡ /(LEdd/c2)] ≥ 100 (where is the accretion rate), the spectral hardening factor via electron scattering is 2.3-6.5. As a result of the -sensitive hardening factor, the color temperature of the innermost radiation is not proportional to L0.25, differing from the simplest standard accretion disk. The model is applied to optical-soft X-ray emission from NLS1s. We pick up one NLS1, namely, PG 1448+273 with an inferred MBH of 106.4 M☉, among the highest candidates. The broadband spectral distribution is successfully reproduced by the model with an extremely high (=1000) and the viscosity parameter α of 0.01. This implies that this object, as well as some other highest systems, is really young: the inferred age, MBH/, is about 106 yr. We also briefly discuss the distribution of for transient and highly variable NLS1s, finding that those are located at 3 300. Such a moderately high accretion rate is indicative of thermal instability. Furthermore, for a possible type 2 counterpart of NLS1s, NGC 1068, is found to be similar to for NLS1s.