Fig. 2: (a) 3D MHD numerical modeling of black hole accretion flows. (b) Power spectrum of mass accretion rate toward the black hole. Resonant oscillations that are discovered by our numerical experiments are shown in a red arrow. Numerical computations were carried out by using Fujitsu VPP5000. IDL was used for analysis. AVS/Express on SuSE Linux was used for 3D visualization.
With the help of Dr. Makoto Miyoshi, a radio astronomer at the National Astronomical Observatory of Japan (NAOJ), I discovered multiple QPOs in millimeter radio emissions from the Galactic Center, Sagittarius A*. Based on my model of multiple QPOs, the observed frequencies can be interpreted as overtones of the resonant oscillations of the accretion disk around a supermassive black hole with a moderate spin parameter. This result contradict with the standard model because such a supermassive black hole could accumulate huge amount of angular momentum via accretion process and therefore it should have an extreme spin parameter. Our result suggested that the standard model must be revised so that the effect of Blandford-Znajek mechanism (Blandford & Znajek 1977) takes into account in order to extract the angular momentum of black hole during its growth phase.
In the meantime, I had decided to expand my knowledge and experiences through attending KITP workshops “Physics Outflows and Accretion Disks” in 2005. During the workshop, I discussed with Prof. Henk Spruit and Prof. Omar Blae about the issues on radiation MHD in accretion disks. Prof. Blae was the person who encouraged to investigate radiation processes of accretion disk when I participated the EURO summer school 2002 at Les Houches in France. Since then solving radiation transfer problems on MHD modeling have always been under my consideration because it is inevitable not only to change the structure of accretion disks but also to drive outflows and jets which expel surplus angular momentum.
When I had been a postdoctoral fellow at University of Tsukuba since 2005, I developed radiative transfer code including inverse-Compton scattering process in order to elucidate MHD models by comparing the observations over a broad-frequency range (see Fig. 3). At the same time, I contributed for developing a radiation MHD (RMHD) code which was aimed for evaluating the structure change of accretion disk and the acceleration of jets/outflows as a result of radiation force in a super-critical accretion flow around a black hole. Our RMHD paper was awarded as the PASJ excellent paper in 2012.