Spectral variability of ultraluminous compact X-ray sources in nearby spiral galaxies

Using the X-ray data taken with ASCA, a detailed analysis was made of intensity and spectral variations of three ultraluminous extragalactic compact X-ray sources (ULXs): IC 342 source 1, M81 X-6, and NGC 1313 source B, all exhibiting X-ray luminosity in the range from 2 × 10³⁹ to 1.5 × 10⁴⁰ ergs s^-1. As already reported, IC 342 source 1 showed short-term X-ray intensity variability by a factor of 2.0 on a typical timescale of 10 ks. M81 X-6 varied by a factor of 1.6 across seven observations spanning 3 yr, while NGC 1313 source B varied by a factor of 2.5 between two observations conducted in 1993 July and 1995 November. The ASCA spectra of these sources acquired on these occasions were all described successfully as optically thick emission from standard accretion disks around black holes. This confirms previous ASCA works that explained ULXs as mass-accreting massive black hole binaries. In all three sources, the disk color temperature was uncomfortably high at T_in = 1.0-2.0 keV and was found to vary in proportion to the square root of the source flux. The apparent accretion disk radius is hence inferred to change as inversely proportional to T_in. This suggests a significant effect of advection in the accretion disk. However, even taking this effect fully into account, the too high values of T_in of ULXs cannot be explained. Further invoking the rapid black hole rotation may give a solution to this issue.