Hajime Inoue
Professor,
Institute of Space and Astronautical Sciences (ISAS)
One of the most important targets of modern astrophysics is gblack holesh. Observational study of black hole began in late 1960fs by the discovery of the first black hole candidate, Cygnus X-1, which was later found to be in a close binary. The mass of the X-ray emitting object was estimated, through an analysis of the binary motion, to be about 10 solar mass, which exceeds the mass upper limit of neutron stars. A bright binary X-ray source is generally considered to be due to a gravitational energy release of matter accreted by a collapsed star, such as a neutron star, or a black hole. So far, more than ten X-ray sources have been found to be black holes by way of the mass estimation. The X-ray spectra and time variability of black hole binaries have been investigated deeply. As a result, it has become possible to observe matter flow onto a black hole even at a distance of three or four times the radius of the event horizon of a black hole. Black holes have been found also at central nuclei of galaxies. Some of the nuclei are very bright and variable in various wavebands, and thus called active galactic nuclei (AGN). Although the mass of the black holes are as large as 10 - 100 million solar mass in usual AGN, there are several similarities in spectral and temporal behaviors between stellar size black holes in close binaries and AGN. One of the spectacular behaviors of the black hole sources is mass ejection with a velocity close to the speed of light, which are called, gjetsh. Recent observations have gradually been resolving in a situation in which mass ejection takes place. Finally, I would present recent discoveries of black holes with 100 and 1000 solar mass, which might be in an intermediate state from a stellar size black hole in an interstellar space of a galaxy to a massive black hole at the galactic nucleus.
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