Spectroscopic Studies of an Ultraluminous Supersoft X-Ray Source in M81
  Ultraluminous supersoft X-ray sources (ULSs) exhibit supersoft X-ray spectra
with blackbody temperatures below 0.1 keV and bolometric luminosities above
10$^{39}$ ergs s$^{-1}$. In this Letter, we report the first optical
spectroscopic observations of a ULS in M81 using the LRIS spectrograph on the
Keck I telescope. The detected Balmer emission lines show a mean intrinsic
velocity dispersion of 400$\pm$80 km s$^{-1}$, which is consistent with that
from an accretion disk. The spectral index of the continuum on the blue side is
also consistent with the multi-color disk model. The H$_{\alpha}$ emission line
exhibits a velocity of $\sim$180 km s$^{-1}$ relative to the local stellar
environment, suggesting that this ULS may be a halo system in M81 belonging to
an old population. No significant shift is found for the H$_{\alpha}$ emission
line between two observations separated by four nights.
