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{sup 39} ergs s{sup −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{sup −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{sub α} emission line exhibits a velocity of ∼180 km s{sup −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{sub α} emission line between two observations separated by four nights.