Wang, QD2024-04-262024-04-262002-01-01https://doi.org/10.1046/j.1365-8711.2002.05317.xhttps://hdl.handle.net/20.500.14394/2697<p>This is the pre-published version harvested from ArXiv. The published version is located at <a href="http://onlinelibrary.wiley.com/doi/10.1046/j.1365-8711.2002.05317.x/abstract">http://onlinelibrary.wiley.com/doi/10.1046/j.1365-8711.2002.05317.x/abstract</a></p>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 multicolour disc blackbody model. The source most likely represents an optically thick accretion disc around an intermediate-mass black hole (M∼102 M⊙) in its high/soft state, similar to other variable ultraluminous X-ray sources observed in nearby disc 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 ∼80 M⊙–1.5×102 M⊙. The inferred bolometric luminosity of the accretion disc is ∼(1.1×1040 erg s−1)/(cos i). 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.* binaries: general; * ISM: bubbles; * galaxies: individual: M81; * X-rays: galaxiesAstrophysics and Astronomyarticle