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

TL;DR

This study investigates an ultraluminous supersoft X-ray source in M81, providing optical evidence that supports it being an intermediate-mass black hole rather than a white dwarf, based on spectral energy distribution analysis.

Contribution

The paper presents optical observations and spectral analysis that strongly suggest the source is an intermediate-mass black hole, offering new evidence for the nature of ultraluminous supersoft X-ray sources.

Findings

Spectral energy distribution indicates a blue component consistent with an accretion disk around an IMBH.

The blue component is inconsistent with a white dwarf accretor.

Results support the IMBH hypothesis over white dwarf models.

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 $\ge10^3 M_\odot$ 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.