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K. Makishima, A. Kubota, M. Sugiho, J. Kotoku (Dept. Physics, Univ. of Tokyo, Japan), T. Mizuno (Dept. Physical Science, Hiroshima Univ., Japan)
We review ASCA results on Ultra-Luminous Comact X-ray sources (ULXs), knwon for a long time in arm regions of nearby spiral galaxies. Although their high X-ray luminosity (typically 1039-40 erg/s) and variability suggest accreting ~100 Mo black holes, their spectral signature remained poorly known. With ASCA, we have found that ULXs exhibit X-ray spectra very similar to those of Galactic/Magellanic black-hole binaries, represented by either a multi-color disk blackbody model or a single power-law model (Makishima et al., ApJ 535, 362). Two ULXs in IC342 have shown remarkable transitions between these two characteristic states (Kubota et al., ApJ 547,119L). Furthermore, one of them may have an X-ray periodicity at ~30 hours, which is appropriate for a binary period of a semi-detached binary composed of a ~100 Mo black hole and ~100 Mo main-sequence star. All these ASCA results significantly reinforce the interpretation of ULXs in terms of accreting ~100 Mo black holes. However, the accretion disk temperatures of ULXs are systematically higher than those of the ordinary black-hole binaries. This problem may be solved by assuming a significant black-hole rotation and/or an optically-thick ADAF scenario.