The Compact UV Nucleus of M33

Abstract

The most luminous X-ray source in the Local Group is associated with the nucleus of M33. This source, M33 X-8, appears modulated by ~20% over a ~106 day period, making it unlikely that the combined emission from unresolved sources could explain the otherwise persistent ~10³⁹ ergs s^-1 X-ray flux. We present here high-resolution UV imaging of the nucleus with the Planetary Camera of the Hubble Space Telescope undertaken in order to search for the counterpart to X-8. The nucleus is bluer and more compact than at longer wavelength images, but it is still extended, with half of its 3×10³⁸ ergs s^-1 UV luminosity coming from the inner 0.14". We cannot distinguish between a concentrated blue population and emission from a single object.