Verifying the Black Hole Interpretation of Ultra Luminous Cosmic X-ray Sources

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Over two decades, cosmic X-ray investigators have observed a number of mysterious luminous X-ray sources in arm regions of nearby spiral galaxies. Through observations with the 4th Japanese X-ray astronomy satellite ASCA (1993-2001), we have arrived at a novel hypothesis that these "ultra-luminous compact X-ray sources" (ULXs) are mass-accreting black holes (BHs), possibly with relatively large masses. The present research aims at strengthening this novel view.We have analyzed archival ASCA, Chandra, and XMM-Newton data of a large number of ULXs, and obtained further pieces of evidence that they are actually intermediate mass BHs. Using the Subaru telescope, we detected an optical counterpart to one ULX, and obtained a hint of variability which suggests that the companion (the X-ray emitter) is fairly massive. Using the HETE-2 spacecraft, we have detected an intense gamma-ray burst GRB030329, leading to the world-wide discovery that it is associated with a hyper-nova that must have produced a BH. We also succeeded in deducing a unified picture on BHs with various masses under very high accretion rates.All these results are to be further developed by the 5th Japanese X-ray mission Suzaku, which has been launched successfully on 2005 July 10. We have contributed to the development of the Hard X-ray Detector (HXD) onboard Suzaku.To further promote the ULX investigation beyond the Suzaku HXD, we need hard X-ray detectors with imaging capability, Accordingly, we have developed Fourier-synthesis imagers using modulation collimators and CeTe strip detectors, and verified their performance in laboratory experiments. Our achievements further include the successful development of stacked thin-plate scintillators in place of CeTe, and the evaluation of new poly-crystalline scintillators.