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Context. Scatter around the relationship between central black hole masses in active galactic nuclei (AGNs) obtained by reverberationmapping methods and host-galaxy bulge velocity dispersion indicates that the masses are uncertain typically by a factor of about three. Aims. In this paper, we try to identify the sources and systematics of this uncertainty. Methods. We characterize the broad Hβ emission-line profiles by the ratio of their full-width at half maximum (FWHM) to their line dispersion, i.e., the second moment of the line profile. We use this parameter to separate the reverberation-mapped AGNs into two populations, the first with narrower Hβ lines that tend to have relatively extended wings, and the second with broader lines that are relatively flat-topped. The first population is characterized by higher Eddington ratios than the second. Within each population, we calibrate the black-hole mass scale by comparison of the reverberation-based mass with that predicted by the bulge velocity dispersion. We also use the distribution of ratios of the reverberation-based mass to the velocity-dispersion mass prediction in a comparison with a "generalized thick disk" model in order to see if inclination can plausibly account for the observed distribution. Results. We find that the line dispersion is a less biased parameter in general than FWHM for black hole mass estimation, although we show that it is possible to empirically correct for the bias introduced by using FWHM to characterize the emission-line width. We also argue that inclination effects are apparent only in some small subset of the reverberation-based mass measurements; it is primarily the objects with the narrowest emission lines that seem to be most strongly affected. Conclusions. Our principal conclusion is that the Hβ profile is sensitive primarily to Eddington ratio, but that inclination effects play a role in some cases.

Aperiodic optical variability is a common property of Active Galactic Nuclei (AGNs), though its physical origin is still open to question. To study the origin of the optical -ultraviolet variability in AGN, we compare light curves of two models to observations of quasar 0957+561 in terms of a structure function analysis. In the starburst (SB) model, random superposition of supernovae in the nuclear starburst region produce aperiodic luminosity variations, while in the disk-instability (DI) model, variability is caused by instabilities in the accretion disk around a supermassive black hole. We calculate fluctuating light curves and structure functions, V (τ ), by simple Monte-Carlo simulations on the basis of the two models. Each resultant V (τ ) possesses a power-law portion, [V (τ )] 1/2 ∝ τ β , at short time lags (τ ). The two models can be distinguished by the logarithmic slope, β; β ∼ 0.74-0.90 in the SB model and β ∼ 0.41-0.49 in the DI model, while the observed light curves exhibit β ∼ 0.35. Therefore,we conclude that the DI model is favored over the SB model to explain the slopes of the observational structure function, in the case of 0957+561, though this object is a radio-loud object and thus not really a fair test for the SB model. In addition, we examine the time-asymmetry of the light curves by calculating V (τ ) separately for brightening and decaying phases. The two models exhibit opposite trends of time-asymmetry to some extent, although the present observation is not long enough to test this prediction.

Narrow-line Seyfert 1 galaxies (NLS1s) exhibit extreme soft X-ray excess and large variability. We argue that both features can be basically accounted for by the slim disk model. We assume that a central black-hole mass in NLS1 is relatively small, M ∼ 10 5−7 M ⊙ , and that a disk shines nearly at the Eddington luminosity, L E . Then, the disk becomes a slim disk and exhibits the following distinctive signatures: (1) The disk luminosity (particularly of X-rays) is insensitive to mass-flow rates,Ṁ , since the generated energy is partly carried away to the black hole by trapped photons in accretion flow. (2) The spectra are multi-color blackbody. The maximum blackbody temperature is T bb ≃ 0.2(M/10 5 M ⊙ ) −1/4 keV, and the size of the blackbody emitting region is small, r bb < ∼ 3r S (with r S being Schwarzschild radius) even for a Schwarzschild black hole. (3) All the ASCA observation data of NLS1s fall onto the region ofṀ /(L E /c 2 ) > 10 (with L E being the Eddington luminosity) on the (r bb , T bb ) plane, supporting our view that a slim disk emits soft X-rays at ∼ L E in NLS1s. (4) Magnetic energy can be amplified, at most, up to the equipartition value with the trapped radiation energy which greatly exceeds radiation energy emitted from the disk. Hence, energy release by consecutive magnetic reconnection will give rise to substantial variability in soft X-ray emission.