Tadahiro Manmoto

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.

Advection-dominated Accretion Flow around a Kerr Black Hole

The effects of the spin of central black holes on the structure and the spectrum of optically thin, advection-dominated accretion flows (ADAFs) around rotating supermassive black holes are investigated. The global two-temperature structure of ADAFs in the Kerr metric is obtained numerically by solving the full relativistic hydrodynamical equations including the energy equations for the ions and for the electrons. The advected fraction of the dissipated energy is not treated as a parameter and the detailed radiation processes are calculated self-consistently. We find that the two-temperature structure of ADAFs is accurately calculated by setting the advected fraction of the dissipated energy to be unity. We find that the particles are hotter when a is positive than when a = 0, while the particles are cooler when a is negative than when a = 0. The changes in a have less effect on the electron temperature than on the ion temperature. The spectra of the emitted photons are also calculated by solving the equations of the general relativistic optics. The entire part of the spectra is enhanced when a is positive, while the entire part of the spectra is reduced when a is negative, in comparison with the case of a = 0. The spectrum of the synchrotron photons and the Comptonized synchrotron photons are modified more largely by the black hole spin and the inclination angle than the spectrum of the bremsstrahlung photons. The effect of the inclination angle on the spectra increases as the value of a increases. In the case of a = -0.95, the inclination has little effect on the shape of the spectrum. The spectrum of Sgr A* from the radio band to the X-ray band is nicely reproduced with the model of an ADAF around a high-spin black hole. The existence of a high-spin black hole at the Galactic center is not ruled out by the ADAF model.

X-Ray Fluctuations from Locally Unstable Advection-dominated Disks

The response of advection-dominated accretion disks to local disturbances is examined by one-dimensional numerical simulations. It is generally believed that advection-dominated disks are thermally stable. We, however, find that any disturbance added onto accretion flow at large radii does not decay so rapidly that it can move inward with roughly the free-fall velocity. Although disturbances continue to be present, the global disk structure will not be modified greatly. This can account for persistent hard X-ray emission with substantial variations observed in active galactic nuclei and stellar black hole candidates during the hard state. Moreover, when the disturbance reaches the innermost parts, an acoustic wave emerges, propagating outward as a shock wave. The resultant light variation is roughly (time) symmetric and is quite reminiscent of the observed X-ray shots of Cygnus X-1.

Emission from Isolated Black Holes and MACHOs Accreting from the Interstellar Medium

Adopting the advection-dominated accretion flow (ADAF) model, we have reexamined the radiation emitted by isolated stellar-mass black holes accreting from the interstellar medium of our Galaxy. Two distinct types of black holes are given consideration: (1) remnants of conventional massive star evolution (remnant black holes [RBHs], mass ~10 M☉) and (2) low-mass objects that possibly constitute the dark halo and cause the observed microlensing events (MACHOBHs, mass ~0.5 M☉). In contrast to previous studies incorporating spherical accretion models, the characteristic hard X-ray emission from ADAFs serves as an important observational signature. Standard estimates for the present density of RBHs suggest that a fair number of objects, either in nearby molecular clouds (such as Orion) or in the general Galactic disk, may be detected by near-future X-ray instruments. Their observation may also be feasible at lower frequencies, particularly in the infrared and optical bands. On the other hand, the smaller mass and larger velocity on average of MACHOBHs compared with RBHs make them unlikely to be observable in the foreseeable future.

Trapped Low-Frequency Oscillations in the Transition Region between Advection-dominated Accretion Flows and Standard Disks

We consider narrow transition regions in accretion disks where an outer standard disk (Shakura-Sunyaev-type disk; SSD) changes to an inner advection-dominated accretion flow (ADAF). By using the WKB method, we show that if the transition occurs in a relatively narrow region, trapped low-frequency oscillations, or slowly growing localized motions, are possible in the transition region. Their characteristic frequencies are much lower than the Keplerian frequency in this region. This type of trapped oscillation may be a possible cause of the 1-15 Hz quasi-periodic oscillations observed in GRS 1915+105.

Advection-dominated accretion flows: Low-luminosity objects can contain supermassive black holes!

Abstract A model of optically thin, advection-dominated accretion flows (ADAFs) has made rapid progress recently and attracted much attention now. It can nicely reproduce the observed spectra of Sgr A ∗ and other low-luminosity AGNs. We here review the unique properties of ADAFs, focusing on their emission properties and time-dependent behavior. The issues involved with ADAFs are also discussed.

An X-Ray Microlensing Test of Au-Scale Quasar Disk Structure

Abstract The innermostregion of quasars can be resolved by microlensing on scales down to a few AU. For this purpose, X-rays originating from the innermost regions can be selectively amplified by microlensing resulting from the “caustic crossing” events, and observations at X-ray bands are most preferable. If the observation has been done, lower limits of black hole mass will be determined, because information about gravitational potential of the black hole which is surrounded by accretion disk will be obtained.