PROPERTIES OF M33 X-8, THE NUCLEAR SOURCE IN THE NEARBY SPIRAL GALAXY

The luminous nuclear source in the nearby spiral galaxy M33 was observ ed with ASCA. The energy spectrum of the source is found to be represe nted either by a power-law function multiplied by an exponential cutof f or by the multicolor disk model with an additional power-law tail (a prototype model for galactic black hole candidates). The e-folding en ergy of the cutoff in the former model is restricted to the range 2.0 +/- 0.3 keV. The spectrum is thus significantly softer than typical ac tive galactic nucleus spectra. With the latter model, the luminosity a nd the energy spectrum can be consistently explained by assuming a clo se binary system containing a black hole of about 10 M.. However, the probability that such a close binary would be located near the galacti c nucleus is low.