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 +/- 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 similar to 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.