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K. Ebisawa (NASA/GSFC)
The Ultra-luminous Compact X-ray Sources (ULXs) in nearby spiral galaxies have significantly hard X-ray energy spectra compared to Galactic black hole candiates in the soft states. Typical ULX accretion disk temperature is too high to be explained in the framework of the standard accretion disk model in the Schwarzschild metric. We examine several possibilities to solve this ``too-hot'' disk problem of ULXs. In particular, we have calculated an extreme Kerr disk model to fit the observed spectra. We found that the Kerr disk spectra will become significantly harder compared to the Schwarzschild disk only when the disk is highly inclined, inspite that the inner disk temperature becomes much higher than in the case of the Schwarzschild black hole. We will have to consider other causes to make the ULX disk spectra hard.
The author(s) of this abstract have provided an email address for comments about the abstract: ebisawa@subaru.gsfc.nasa.gov