An ultraluminous supersoft X-ray source in M81: An intermediate-mass black hole?

Ultraluminous supersoft X-ray sources (ULSSSs) exhibit supersoft spectra with blackbody temperatures of 50 100 eV and bolometric luminosities above 10(39) ergs s(-1) and are possibly intermediate-mass black holes (IMBHs) of >= 103 or massive white dwarfs that are progenitors of Type Ia supernovae. In this Letter we report our M, optical studies of such a source in M81, M81-ULS1, with HST archive observations. M81-ULS1 is identified with a pointlike object, the spectral energy distribution of which reveals a blue component in addition to the companion of an asymptotic giant branch star. The blue component is consistent with the power law as expected from the geometrically thin accretion disk around an IMBH accretor but inconsistent with the power law as expected from the X-ray-irradiated flared accretion disk around a white dwarf accretor. This result is strong evidence that M81-ULS1 is an IMBH instead of a white dwarf.