Black Hole Binary
Some of the bright X-ray sources lying along the Galactic Plane are binary systems consisting of a black hole and an ordinary star (Fig. 2). Cygnus X-1, the most famous black hole binary, always shines brilliantly in X-ray, but its radiation spectrum switches between two extreme states. In most cases, Cygnus X-1 shows a power-law spectrum called the "hard state" where X-ray photons are distributed up to the high-energy level. Sometimes, however, it makes transition to a thermal spectrum called the "soft state" which is dominated by low-energy photons. It is believed that the former responds to the "coronal" state where gas around the black hole becomes hot and spread thinly. Meanwhile, the latter responds to the state where a thin accretion disk similar to a record is formed by high-density gas. The radius of the innermost edge of the accretion disk (the central hole in a record) cannot be less than three times the Schwarzschild radius Ethe distance of the gravitational sphere of black hole, and in proportion to black hole's mass Ebecause of the effect of general relativity. Thus, we can get a clue to estimate the mass of a black hole by examining the X-ray intensity and radiation spectrum in the soft state. As a result of analysis of the motion of the companion star rotating the center of gravity of the binary system, we found that Cygnus X-1 is a remarkably smaller object than normal stars, with an X-ray source mass about 10 times the solar mass but which emits hardly any visible light. If applying star theory, such an object must be a black hole.
At present, it is estimated that about 20 sources other than Cygnus X-1 must be black hole binaries. Unlike Cygnus X-1, mass is not yet determined accurately for all of them. We judge them to be black hole binaries because of the behavior of gas falling into black holes. They show common characteristics such as the fluctuation between extremely different spectrum states. In the following discussion, I consider such candidates as black hole binaries as well.
Most black hole binaries are "X-ray nova" that emits X-rays only for a few months, and not for most of their life. There are various kinds of nova. Some become active every few years while others showed only one activity over the 40-year X-ray observation history. In the past 20 years, a new candidate for a black hole binary emerges about once a year on average. Since MAXI is the most sensitive all-sky monitor to date, it can detect promptly the emergence of new X-ray nova and notify the world, eventually leading to detailed observation from the beginning. MAXI has another feature in that it can monitor changes in X-ray intensity and spectrum by tracking X-ray nova outbursts lasting several months from onset to extinction.