Shin Mineshige

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.

Supercritical Accretion Flows around Black Holes: Two-dimensional, Radiation Pressure-dominated Disks with Photon Trapping

The quasi-steady structure of supercritical accretion flows around a black hole is studied based on two-dimensional radiation-hydrodynamic (2D-RHD) simulations. The supercritical flow is composed of two parts: the disk region and the outflow regions above and below the disk. Within the disk region the circular motion and the patchy density structure are observed, which is caused by Kelvin-Helmholtz instability and probably by convection. The mass accretion rate decreases inward, roughly in proportion to the radius, and the remaining part of the disk material leaves the disk to form the outflow because of the strong radiation pressure force. We confirm that photon trapping plays an important role within the disk. Thus, matter can fall onto the black hole at a rate exceeding the Eddington rate. The emission is highly anisotropic and moderately collimated so that the apparent luminosity can exceed the Eddington luminosity by a factor of a few in the face-on view. The mass accretion rate onto the black hole increases with the absorption opacity (metallicity) of the accreting matter. This implies that the black hole tends to grow faster in metal-rich regions, such as in starburst galaxies or star-forming regions.

GLOBAL STRUCTURE OF THREE DISTINCT ACCRETION FLOWS AND OUTFLOWS AROUND BLACK HOLES FROM TWO-DIMENSIONAL RADIATION-MAGNETOHYDRODYNAMIC SIMULATIONS

We present the detailed global structure of black hole accretion flows and outflows through newly performed two-dimensional radiation-magnetohydrodynamic simulations. By starting from a torus threaded with weak toroidal magnetic fields and by controlling the central density of the initial torus, ρ0, we can reproduce three distinct modes of accretion flow. In model A, which has the highest central density, an optically and geometrically thick supercritical accretion disk is created. The radiation force greatly exceeds the gravity above the disk surface, thereby driving a strong outflow (or jet). Because of mild beaming, the apparent (isotropic) photon luminosity is ~22L E (where L E is the Eddington luminosity) in the face-on view. Even higher apparent luminosity is feasible if we increase the flow density. In model B, which has moderate density, radiative cooling of the accretion flow is so efficient that a standard-type, cold, and geometrically thin disk is formed at radii greater than ~7 R S (where R S is the Schwarzschild radius), while the flow is radiatively inefficient otherwise. The magnetic-pressure-driven disk wind appears in this model. In model C, the density is too low for the flow to be radiatively efficient. The flow thus becomes radiatively inefficient accretion flow, which is geometrically thick and optically thin. The magnetic-pressure force, together with the gas-pressure force, drives outflows from the disk surface, and the flow releases its energy via jets rather than via radiation. Observational implications are briefly discussed.

Spectrum of Optically Thin Advection-dominated Accretion Flow around a Black Hole: Application to Sagittarius A*

The global structure of optically thin advection-dominated accretion flows composed of two-temperature plasma around black holes is calculated. We adopt the full set of basic equations, including the advective energy transport in the energy equation for the electrons. The spectra emitted by the optically thin accretion flows are also investigated. The radiation mechanisms that are taken into account are bremsstrahlung, synchrotron emission, and Comptonization. The calculation of the spectra and that of the structure of the accretion flows are made to be completely consistent by calculating the radiative cooling rate at each radius. As a result of the advection domination for the ions, the heat transport from the ions to the electrons becomes practically zero, and the radiative cooling balances with the advective heating in the energy equation of the electrons. Following up on the successful work of Narayan et al., we applied our model to the spectrum of Sgr A*. We find that the spectrum of Sgr A* is explained by the optically thin advection-dominated accretion flow around a black hole of mass MBH = 106 M☉. The parameter dependence of the spectrum and the structure of the accretion flows is also discussed.

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.

Can Neutrino-cooled Accretion Disks Be an Origin of Gamma-Ray Bursts?

It is often proposed that a massive torus with approximately solar mass surrounding a stellar-mass black hole could be a central engine of gamma-ray bursts. We study the properties of such massive accretion tori (or disks) based on the α viscosity model. For surface density exceeding about 1020 g cm-2, which is realized when ~1 M☉ of material is contained within a disk of size ~5 × 106 cm, we find that (1) the luminosity of photons is practically zero because of significant photon trapping, (2) neutrino cooling dominates over advective cooling, (3) the pressure of degenerate electrons dominates over the pressure of gas and photons, and (4) the magnetic field strength exceeds the critical value of about 4 × 1013 G, even if we take 0.1% of the equipartition value. The possible observable quantum electrodynamical (QED) effects arising from supercritical fields are discussed. Most interestingly, photon splitting may occur, producing a significant number of photons of energies below ~511 keV, thereby possibly suppressing e± pair creation.

Slim-Disk Model for Ultraluminous X-Ray Sources

The ultraluminous X-ray sources (ULXs) are unique in exhibiting moderately bright X-ray luminosities, LX ~ 1038-1040 ergs s-1, and relatively high blackbody temperatures, Tin ~ 1.0-2.0 keV. From the constraint that LX cannot exceed the Eddington luminosity LE, we require relatively high black hole masses, M ~ 10-100 M☉; however, for such large masses the standard disk theory predicts lower blackbody temperatures, Tin < 1.0 keV. To understand a cause of this puzzling fact, we carefully calculate the accretion flow structure shining at ~LE, fully taking into account the advective energy transport in the optically thick regime and the transonic nature of the flow. Our calculations show that at high accretion rate ( 30LE/c2) an apparently compact region with a size of Rin (1-3)rg (with rg being the Schwarzschild radius) is shining with a blackbody temperature of Tin 1.8(M/10 M☉)-1/4 keV even for the case of a nonrotating black hole. Furthermore, Rin decreases as increases, contrary to the canonical belief that the inner edge of the disk is fixed at the radius of the marginally stable last circular orbit. Accordingly, the loci of a constant black hole mass on the H-R diagram (representing the relation between LX and Tin both on the logarithmic scales) are not straight but bent toward the lower M-direction in the frame of the standard disk relation. We also plot the ASCA data of some ULXs on the same H-R diagram, finding that they all fall on the regions with relatively high masses, M ~ 10-30 M☉, and high accretion rates, 10LE/c2. Interestingly, IC 342 source 1, in particular, was observed to move along the constant M line (not constant Rin line) in our simulations. This provides firm evidence that at least some ULXs are shining at LE and contain black holes with M 10-100 M☉.

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.

The Hot Disk Corona and Magnetic Turbulence in Radio-quiet Active Galactic Nuclei: Observational Constraints

We compile a sample consisting of 56 radio-quiet active galactic nuclei so as to investigate the statistical properties of the hot corona in accretion disks from ASCA observations. The black hole masses in the sample are estimated via several popular methods, and the bolometric luminosities from the multiwavelength continuum. This allows us to estimate the Eddington ratio (E = L-bol/L-Edd) so that the undergoing physical processes can be tested via hard X-ray data. We use the least- squares method of multivariate regression and find a strong correlation between F-X = L2-10 keV/L-bol and E as F-X proportional to E-064. This indicates that the release of gravitational energy in the hot corona is controlled by the Eddington ratio. On the other hand, the correlation between the hard X- ray spectral index (Gamma) and E depends critically on the types of objects: Gamma is nearly constant (Gamma proportional to E-0) in broad- line Seyfert 1 galaxies, whereas Gamma proportional to log E-0.18 in narrow- line Seyfert 1 galaxies, although this is not very significant. We can set constraints on the forms of the magnetic stress tensor on the condition that F-X is proportional to the fraction f of gravitational energy dissipated in the hot corona and that f is proportional to the magnetic energy density in the disk. We find that the shear stress tensor is favored by the correlation in the present sample, t. P where P-gas is the gas pressure.

Disk-instability model for soft-X-ray transients containing black holes

Consideration is given to the time-dependent behavior of the accretion disks in low-mass X-ray binaries as a model for soft X-ray transients. The thermal stability and vertical structure properties of the disk are studied in the range between 10 to the 7th and 10 to the 8th cm. An Eulerian time-dependent thin disk model is used to show that, for relevant accretion rates, the disk suffers a thermal instability leading to intermittent accretion onto the central compact object. The disk instability model reproduces the basic features of outbursts of soft X-ray transients. Also, the physical state of quiescence of the binary A0620-00 is examined, focusing on the possible existence of a black hole. It is suggested that the modulation of the accretion flow in the inner disk may be responsible for the bimodal behavior observed in the spectra of black hole candidates such as Cyg X-1, GX 339-4, and A0620-00. 72 refs.