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R. Miyawaki (University of Tokyo), M. Sugiho (NEC Toshiba Space System), A. Kubota (Institute of Space and Astronomical Science), K. Makishima (University of Tokyo), M. Namiki (Osaka University), T. Tanaka (Institute of Space and Astronomical Science), N. Tsunoda (RIKEN)
Observations with ASCA have for the first time revealed that ultra-luminous compact X-ray sources (ULXs) in nearby spiral galaxies often exhibit multi-color disk (MCD) type spectra (Makishima et al. 2000), and sometimes show spectral transitions between the MCD and power-law (PL) type states. These suggest their nature as intermediate-mass black holes under high accretion rates. In order to reinforce the ASCA view, we analyzed the archival Chandra and XMM-Newton data of about 65 nearby galaxies. We detected more than 1000 point sources with luminosities exceeding 2e38 erg/s. We derived their luminosity functions, and found them to depend significantly on the morphologycal type of the host galaxy. We also analyzed the spectra of about 50 objects with enough counts. About a third of them showed the MCD type convex continua, another third favored the PL modeling, while the remaining third too poor to definitely classify. Combining the ASCA results, 5 objects showed spectral transitions between the above two states. One of our sample sources, NGC 253 X-1 with a relatively low luminosity, showed time variability which is consistent with the presence of a standard accretion disk.
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Bulletin of the American Astronomical Society, 36 #2
© YEAR. The American Astronomical Soceity.