2003ApJ...593...69K

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{dot}[≡M{dot}/(L_Edd/c²)]≥100 (where M{dot} is the accretion rate), the spectral hardening factor via electron scattering is ≲2.3-6.5. As a result of the m{dot}-sensitive hardening factor, the color temperature of the innermost radiation is not proportional to L^0.25, 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 M_BH of 10^6.4 M_☉, among the highest m{dot} candidates. The broadband spectral distribution is successfully reproduced by the model with an extremely high m{dot} (=1000) and the viscosity parameter α of 0.01. This implies that this object, as well as some other highest m{dot} systems, is really young: the inferred age, M_BH/M{dot}, is about 10⁶ yr. We also briefly discuss the distribution of m{dot} for transient and highly variable NLS1s, finding that those are located at 3≲m{dot}≲300. Such a moderately high accretion rate is indicative of thermal instability. Furthermore, m{dot} for a possible type 2 counterpart of NLS1s, NGC 1068, is found to be similar to m{dot} for NLS1s.