2002ApJ...569..204L

We have used the Hubble Space Telescope to obtain moderate resolution 1150-5700 Å spectroscopy of the nucleus of M33 and a blue star ∼1" north-northwest of the nucleus in an attempt to find the optical counterpart of the nuclear X-ray source and to characterize stellar populations in the nuclear region. The Space Telescope Imaging Spectrograph spectra of the nucleus can be modeled in terms of two starbursts: one with a mass of about 9000 M_☉ at 40 Myr and the other with a mass of about 76,000 M_☉ at 1 Gyr. The blue star is a late-type O giant with no obvious spectral anomalies to indicate that it is associated with the luminous X-ray source. The nuclear region is not heavily reddened; 2200 Å absorption features in the spectra of both the nucleus and the star are weak. The data and the star formation history support the hypothesis that the M33 nuclear source, the brightest persistent source in the Local Group, is an ∼10 M_☉ black hole binary.