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Scientists from several countries have contributed to the research of cosmic X-rays. As Dr. Giacconi mentioned earlier, I believe Japan has been playing an important role in this. Japan was banned from conducting any aircraft research at the end of the Second World War. Once the ban was lifted, Dr. Hideo Itokawa started from scratch, and in the 1950s developed the so-called pencil rocket at the University of Tokyo. In 1970, the gradual development of related technologies led to the launch of Japan's first artificial satellite, OHSUMI, from Uchinoura Town in Ohsumi peninsula, Kagoshima Prefecture.
Japan has launched four satellites for the purpose of cosmic X-ray observation: HAKUCHO in 1979, TENMA in 1983, GINGA in 1987, and ASCA in 1993. We are proud of the fact that the sensitivity of these satellites increased rapidly with each generation: TENMA is ten times more sensitive than HAKUCHO, GINGA 200 times, and ASCA as much as 5000 times.
The development of X-ray observation has led to the discovery of 20 to 30 black-hole binary stars similar to Cygnus X-1 in our Galaxy. Many of these are so-called X-ray novas, which normally emit no X-rays but suddenly brighten with X-rays from time to time, when they accumulate gas from their partner star. Even so, there is no denying that Cygnus X-1 is the king of black holes. In fact, some X-ray novas have a neutron star instead of a black hole. We can distinguish these from ones with a black hole by comparing their X-ray properties with those of Cygnus X-1.
Of course, it is likely that quite a few black holes exist on their own and do not form binary stars with other celestial bodies. Generally speaking, though, if they don't emit X-rays while absorbing gas, they remain unknown to us. It is only in a binary pairing that a black hole absorbs enough gas to emit strong X-rays and thus lets its presence be known.
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