Properties of M33 X-8, the nuclear source in the nearby spiral galaxy

Abstract

The luminous nuclear source in the nearby spiral galaxy M33 was observed with ASCA. The energy spectrum of the source is found to be represented either by a power-law function multiplied by an exponential cutoff or by the multicolor disk model with an additional power-law tail (a prototype model for galactic black hole candidates). The e-folding energy 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 active galactic nucleus spectra. With the latter model, the luminosity and the energy spectrum can be consistently explained by assuming a close binary system containing a black hole of about 10 M_sun_. However, the probability that such a close binary would be located near the galactic nucleus is low.