  Ultraluminous supersoft X-ray sources (ULSSS) exhibit supersoft spectra with
blackbody temperatures of 50-100 eV and bolometric luminosities above $10^{39}$
erg s$^{-1}$, and are possibly intermediate mass black holes (IMBHs) of
$\ge10^3 M_\odot$ or massive white dwarfs that are progenitors of type Ia
supernovae. In this letter we report our optical studies of such a source in
M81, M81-ULS1, with HST archive observations. M81-ULS1 is identified with a
point-like object, the spectral energy distribution of which reveals a blue
component in addition to the companion of an AGB 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.
