An Ultraluminous Supersoft X-ray Source in M81: An Intermediate-Mass Black Hole?

Abstract

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 $\ge 10^3{M}_{\odot }$ 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.

12 pages, 1 table, 3 figures

References in corpus (1)

• Bridging the gap between stellar-mass black holes and ultraluminous X-ray sources

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