Toshihiro Kawaguchi

Black-Hole Accretion Disks

We overview the theory of black hole accretion disks. In the first half, we introduce basic accretion disk models with emphasis on optically thin, advection-dominated accretion flows (ADAFs). We then discuss a potentially useful test of the disk model, which uses gravitational microlens events. The second part will be on time variability and time-dependent models. We will summarize observed complex variability and discuss its implications from the theoretical point of view. Finally, we will touch on the most important issue in future research; that is, magnetic field activity in accretion disks.

Optical Variability in Active Galactic Nuclei: Starbursts or Disk Instabilities?

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 AGNs, 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 produces 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 ∝ τβ, 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 in explaining the slopes of the observational structure function in the case of 0957+561, though this object is a radio-loud object and thus is 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 the brightening and the 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.

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 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.

Slim-Disk Model for Soft X-Ray Excess and Variability of Narrow-Line Seyfert 1 Galaxies

Narrow-line Seyfert 1 galaxies (NLSls) exhibit an 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 ~ 105~7A^, and that a disk shines nearly at the Eddington luminosity, L^- Then, the disk becomes a slim disk and exhibits the following distinctive signatures: (1) The disk luminosity (particularly of X-rays) is insensitive to the mass-flow rates, M, 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 Ti,b — 0.2(M/10 5M®)~1 ^4 keV, and the size of the blackbody emitting region is small, rt>b % 3rg (with rs being Schwarzschild radius), even for a Schwarzschild black hole. (3) All of the ASCA observation data of NLSls fall onto the region of M/(LE/c2) > 10 (with LE being the Eddington luminosity) on the (rbt>,7bb) plane, supporting our view that a slim disk emits soft X-rays at ~ Lg in NLSls. (4) Magnetic energy can be amplified, at most, up to the equipartition value with the trapped radiation energy, which greatly exceeds the radiation energy emitted from the disk. Hence, energy release by consecutive magnetic reconnection will give rise to substantial variability in soft X-ray emission.

Broadband Spectral Energy Distributions of Active Galactic Nuclei from an Accretion Disk with Advective Coronal Flow

Recent multiwaveband observations of Seyfert nuclei and QSOs established significant deviations in the spectral shape of the big blue bump from a blackbody spectral shape; soft X-ray excess has a spectral index α (Fν ∝ ν-α) of 1.6 and hard X-ray tail with α of ~0.7. We construct a disk-corona model which accounts for such broadband spectral properties. We study the emission spectrum emerging from a vertical disk-corona structure composed of two-temperature plasma by solving hydrostatic equilibrium and radiative transfer self-consistently. A fraction f of viscous heating due to mass accretion is assumed to be dissipated in a corona with a Thomson optical depth of τc, where advective cooling is also included, and a remaining fraction, 1 - f, dissipates within a main body of the disk. Our model can nicely reproduce the soft X-ray excess with a power-law shape and the hard tail extending to ~50 keV. The different spectral slopes (α ~ 1.5 below 2 keV and ~0.5 above) are the results of different emission mechanisms and different sites; the former slope is due to unsaturated Comptonization from the innermost zone, and the latter is due to a combination of the Comptonization, bremsstrahlung, and a reflection of the coronal radiation at the disk-corona boundary from the inner to surrounding zone (≤300 Schwarzschild radii). The emergent optical spectrum is redder (α ~ 0.3) than that of the standard disk (α ~ -0.3), being consistent with observations, due to the different efficiencies of spectral hardening of disk emission at different radii. Further, we find that the cutoff frequency of the hard X-ray (~coronal electron temperature) and broadband spectral shape are insensitive to the black hole mass, while the peak frequency of the big blue bump is sensitive to the mass as the peak frequency ∝M.

Two-temperature coronal flow above a thin disk

We extended the disk corona model to the inner region of galactic nuclei by including different temperatures in ions and electrons as well as Compton cooling. We found that the mass evaporation rate, and hence the fraction of accretion energy released in the corona, depend strongly on the rate of incoming mass flow from the outer edge of the disk, a larger rate leading to more Compton cooling, less efficient evaporation, and a weaker corona. We also found a strong dependence on the viscosity, with higher viscosity leading to an enhanced mass flow in the corona and therefore more evaporation of gas from the disk below. If we take accretion rates in units of the Eddington rate, our results become independent of the mass of the central black hole. The model predicts weaker contributions to the hard X-rays for objects with higher accretion rate like narrow-line Seyfert 1 galaxies, in agreement with observations. For luminous active galactic nuclei, strong Compton cooling in the innermost corona is so efficient that a large amount of additional heating is required to maintain the corona above the thin disk.

Subaru high-z exploration of low-luminosity quasars (SHELLQs). I. Discovery of 15 quasars and bright galaxies at 5.7 < z < 6.9

We report the discovery of 15 quasars and bright galaxies at 5.7 6 galaxies, compared with that of quasars, at magnitudes fainter than M1450 ~ -22 mag or zAB ~24 mag. Follow-up studies of the discovered objects as well as further survey observations are ongoing.

A Bar Fuels a Supermassive Black Hole?: Host Galaxies of Narrow-Line Seyfert 1 Galaxies*

We present optical images of nearby 50 narrow-line Seyfert 1 galaxies (NLS1s) that cover all the NLS1s at z < 0.0666 and δ ≥ -25° known in 2001. Among the 50 NLS1s, 40 images are newly obtained by our observations and 10 images are taken from archive data. Motivated by the hypothesis that NLS1s are in an early phase of a supermassive black hole (BH) evolution, we present a study of NLS1 host galaxy morphology to examine trigger mechanism(s) of active galactic nuclei (AGNs) by seeing the early phase of AGN. With these images, we made morphological classification by visual inspection and by quantitative method, and found a high bar frequency of the NLS1s in the optical band; the bar frequency is 85% ± 7% among disk galaxies (64%-71% in total sample) which is more frequent than that (40%-70%) of broad-line Seyfert 1 galaxies (BLS1s) and normal disk galaxies, although the significance is marginal. Our results confirm the claim by Crenshaw and coworkers with a similar analysis for 19 NLS1s. The frequency is comparable to that of H II/starburst galaxies. We also examined the bar frequency against width of the broad Hβ emission line, Eddington ratio, and BH mass, but no clear trend is seen. Possible implications, such as an evolutionary sequence from NLS1s to BLS1s, are discussed briefly.

The largest blueshifts of the [O III] emission line in two narrow-line quasars

We have obtained optical intermediate-resolution spectra (R = 3000) of the narrow-line quasars DMS 0059-0055 and PG 1543+489. The [O III] emission line in DMS 0059-0055 is blueshifted by 880 km s-1 relative to Hβ. We also confirm that the [O III] emission line in PG 1543+489 has a relative blueshift of 1150 km s-1. These two narrow-line quasars show the largest [O III] blueshifts known to date among type 1 active galactic nuclei (AGNs). The [O III] emission lines in both objects are broad (1000-2000 km s-1), and those in DMS 0059-0055 show strong blue asymmetry. We interpret the large blueshift and the profile of the [O III] lines as the result of an outflow interacting with circumnuclear gas. Among type 1 AGNs with large blueshifted [O III], there is no correlation between the Eddington ratios and the amount of [O III] blueshift. Combining our new data with published results, we confirm that the Eddington ratios of such AGNs are the highest among AGNs with the same black hole mass. These facts suggest that the Eddington ratio is a necessary condition for [O III] blueshifts or that the [O III] blueshifts weakly depend on the Eddington ratio. Our new sample suggests that there are possibly other necessary conditions to produce an outflow than a high Eddington ratio: a large black hole mass (>107 M☉), a high mass accretion rate (>2 M☉ yr-1), or a large luminosity [λLλ(5100 A) > 1044.6 ergs s-1].

Growth of massive black holes by super-Eddington accretion

Narrow-Line Seyfert 1 galaxies (NLS1s) and Narrow-Line quasars (NLQs) seem to amount to ∼ 10–30% of active galactic nuclei (AGNs) in the local universe. Together with their average accretion rate, we argue that a black hole (BH) growth by factor of 8–800 happens in these super-Eddington accretion phase of AGNs. Moreover, there is a possible, systematic underestimation of accretion rates (in the Eddington unit) due to an overestimation of BH mass by massive accretion discs for super-Eddington objects. If it is true, the factor of BH growth above may be larger by order(s) of magnitude. In contrast, the growth factor expected in sub-Eddington phase is only ∼ 2. Therefore, the cosmic BH growth by accretion is likely dominated by super-Eddington phase, rather than sub-Eddington phase which is the majority among AGNs.
This analysis is based on the fraction and the average accretion rate of NLS1s and NLQs obtained for