Masato TSUBOI

Space VLBI (very long baseline interferometry) mission, ASTRO-G, was authorized in 2006 by JAXA, aiming at the launch in 2012. ASTRO-G is a follow-on mission of HALCA (VSOP) mission in 1990s, which is the world first space VLBI mission, ASTRO-G will consists of a huge effective aperture with diameter of 30,000Km together with radio antennas in the ground. They will achieve the world highest angular resolution to observe the proximity of black holes.

We have performed monitoring observations of the flux density toward the<br /> Galactic center compact radio source, Sagittarius A* (Sgr A*), which is a<br /> supermassive black hole, from 1996 to 2005 using the Nobeyama Millimeter Array<br /> of the Nobeyama Radio Observatory, Japan. These monitoring observations of Sgr<br /> A* were carried out in the 3- and 2-mm (100 and 140 GHz) bands, and we have<br /> detected several flares of Sgr A*. We found intraday variation of Sgr A* in the<br /> 2000 March flare. The twofold increase timescale is estimated to be about 1.5<br /> hr at 140 GHz. This intraday variability suggests that the physical size of the<br /> flare-emitting region is compact on a scale at or below about 12 AU (~150 Rs;<br /> Schwarzschild radius). On the other hand, clear evidence of long-term periodic<br /> variability was not found from a periodicity analysis of our current millimeter<br /> data set.