Recent Research Activities

Black Hole Accretion Flow

It is almost established that optically thin advection-dominated accretion flow (ADAF) is responsible for hard-X - gamma-ray emission from black-hole objects. The ADAF is characterized by low density and high temperature, thereby emissivity being low. Thus, librated energy of accreting gas is carried by accreting gas without being radiated. We have calculated a two-temperature disk and its spectrum.

Hard-Soft Spectral Transition of Black-Hole Candidates

Stellar-mass black-hole candidates are known to oscillate between the hard (low) and soft (high) spectral states. The former type of the disk can well be described by the standard disk model, whereas the latter is represented by the optically thin ADAF. We have succeeded in calculating such a state transition by time-dependent disk evolutionary simulations.

Fluctuations from Black-Hole Objects

Radiation from black-hole objects (close binary systems and galactic nuclei) is known to exhibit apparently random (1/f) fluctuations. Postulating their origin being related to magnetic-field dissipation via magnetic reconnection, we could reproduce the basic fluctuation properties by cellular-automaton simulations based on the notion of self-organized criticality (SOC) and also by three-dimensional MHD disk simulations.

Revealing Quasar Central Structure by Microlensing

Multiple images of Q2237+0305(Einstein Cross) are results of gravitational lensing of a distant quasar by a nearby galaxy. If stars in the lensing galaxy passes just in front of an image of the quasar center, radiation from the vicinity of a black hole can be selectively amplified. Observations are scheduled with ground-based telescopes, HST, AXAF, which will resolve the quasar central structure on length scales down to several AUs.