2022ApJ...937....5S

Roche lobe overflow from a donor star onto a black hole or neutron star binary companion can evolve to a phase of unstable runaway mass transfer, lasting as short as hundreds of orbits (≲10² yr for a giant donor) and eventually culminating in a common-envelope event. The highly super-Eddington accretion rates achieved during this brief phase ($\dot{M}\gtrsim {10}^{5}{\dot{M}}_{\mathrm{Edd}})$ are accompanied by intense mass loss in disk winds, analogous to but even more extreme than ultraluminous X-ray (ULX) sources in the nearby universe. Also in analogy with the observed ULX, this expanding outflow will inflate an energetic “bubble” of plasma into the circumbinary medium. Embedded within this bubble is a nebula of relativistic electrons heated at the termination shock of the faster v ≳ 0.1c wind/jet from the inner accretion flow. We present a time-dependent, one-zone model for the synchrotron radio emission and other observable properties of such ULX “hypernebulae.” If ULX jets are sources of repeating fast radio bursts (FRB), as recently proposed, such hypernebulae could generate persistent radio emission and contribute large and time-variable rotation measure to the bursts, consistent with those seen from FRB 20121102 and FRB 20190520B. ULX hypernebulae can be discovered independently of an FRB association in radio surveys, such as VLASS, as off-nuclear point sources whose fluxes can evolve significantly on timescales as short as years, possibly presaging energetic transients from common-envelope mergers.