The nature of ultraluminous compact X-ray sources in nearby spiral galaxies

Studies were made of ASCA spectra of seven ultraluminous compact X-ray sources in nearby spiral galaxies: M33 X-8, M81 X-6, IC 342 source 1, Dwingeloo 1 X-1, NGC 1313 source B, and two sources in NGC 4565. With the 0.5-10 keV luminosities in the range 10³⁹-10⁴⁰ ergs s^-1, they are thought to represent a class of enigmatic X-ray sources often found in spiral galaxies. For some of them, the ASCA data are newly processed or the published spectra are reanalyzed. For others, the published results are quoted. The ASCA spectra of all seven sources have been described successfully with so-called multicolor disk blackbody emission arising from optically thick standard accretion disks around black holes. Except for the case of M33 X-8, the spectra do not exhibit hard tails. For the source luminosities not to exceed the Eddington limits, the black holes are inferred to have rather high masses, up to ∼ 100 M_⊙. However, the observed innermost disk temperatures of these objects, T_in = 1.1-1.8 keV, are too high to be compatible with the required high black hole masses, as long as the standard accretion disks around Schwarzschild black holes are assumed. Similarly high disk temperatures are also observed from two Galactic transients with superluminal motions, GRO 1655-40 and GRS 1915+105. The issue of unusually high disk temperature may be explained by the black hole rotation, which makes the disk get closer to the black hole and hence hotter.