Ultraluminous X-ray Source 1E 0953.8+6918 (M81 X-9): An Intermediate Mass Black Hole Candidate and its Environs

We present a ROSAT and ASCA study of the Einstein source X-9 and its relation to a shock-heated shell-like optical nebula in a tidal arm of the M81 group of interacting galaxies. Our ASCA observation of the source shows a flat and featureless X-ray spectrum well described by a multi-color disk blackbody model. The source most likely represents an optically thick accretion disk around an intermediate mass black hole in its high/soft state, similar to other variable ultraluminous X-ray sources observed in nearby disk galaxies. Using constraints derived from both the innermost stable orbit around a black hole and the Eddington luminosity, we find that the black hole is fast-rotating and that its mass is between $\sim 20/({\rm cos} i) {\rm M}_{\odot} - 110/({\rm cos} i)^{1/2} {\rm M}_{\odot}$, where $i$ is the inclination angle of the disk. The inferred bolometric luminosity of the accretion disk is $\sim (8 \times 10^{39} {\rm ergs s^{-1}})/({\rm cos} i)^{1/2}$. Furthermore, we find that the optical nebula is very energetic and may contain large amounts of hot gas, accounting for a soft X-ray component as indicated by archival ROSAT PSPC data. The nebula is apparently associated with X-9; the latter may be powering the former and/or they could be formed in the same event (e.g., a hypernova). Such a connection, if confirmed, could have strong implications for understanding both the birth of intermediate mass black holes and the formation of energetic interstellar structures.