Comptonization in super-eddington accretion flow and growth timescale of supermassive black holes

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 10(1.5) and 10(6.5) M-. BH masses (M-BH). With (m) over dot\= (M) over dot/(L-Edd/c(2))] greater than or equal to 100 (where (M) over dot M is the accretion rate), the spectral hardening factor via electron scattering is less than or similar to 2.3 - 6.5. As a result of the (m) over dot-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 10(6.4) M-., among the highest (m) over dot candidates. The broadband spectral distribution is successfully reproduced by the model with an extremely high (m) over dot (= 1000) and the viscosity parameter alpha of 0.01. This implies that this object, as well as some other highest (m) over dot systems, is really young: the inferred age, M-BH/(M) over dot, is about 10(6) yr. We also briefly discuss the distribution of (m) over dot for transient and highly variable NLS1s, finding that those are located at 3 less than or similar to (m) over dot less than or similar to 300. Such a moderately high accretion rate is indicative of thermal instability. Furthermore, (m) over dot for a possible type 2 counterpart of NLS1s, NGC 1068, is found to be similar to (m) over dot for NLS1s.