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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 1039 ergs s−1 and are possibly intermediate-mass black holes (IMBHs) of ≥103 M☉ or massive white dwarfs that are progenitors of Type Ia supernovae. In this Letter we report our optical studies …