Ultraluminous X-ray sources (ULXs)

What powers X-ray sources that are not associated with active galactic nuclei but exceed the Eddington limit of a neutron star or stellar black hole? Are they due to intermediate-mass black holes? Some ULXs are periodic, suggesting non-isotropic emission from a neutron star. Does this apply to all ULXs? How could such a system form and remain stable?

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Source: List of unsolved problems in physics (CC BY-SA 4.0)