Radio Nebul\ae\ from Hyper-Accreting X-ray Binaries as Common Envelope Precursors and Persistent Counterparts of Fast Radio Bursts

TL;DR

This paper models hyper-nebulae formed by super-Eddington accretion in X-ray binaries, proposing they as persistent radio sources and potential FRB counterparts, with observable signatures in radio surveys.

Contribution

It introduces a time-dependent model for ULX hyper-nebulae's radio emission, linking them to FRB phenomena and predicting observable signatures independent of FRB detection.

Findings

Hyper-nebulae can produce persistent radio emission with variable flux.

They may contribute to the rotation measure observed in some FRBs.

Radio surveys can identify these nebulae as off-nuclear sources.

Abstract

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 ($\lesssim 10^{2}$ yr for a giant donor), and eventually culminating in a common envelope event. The highly super-Eddington accretion rates achieved during this brief phase ($\dot{M} \gtrsim 10^{5}\dot{M}_{\rm Edd})$ are accompanied by intense mass-loss in disk winds, analogous but even more extreme than ultra-luminous X-ray (ULX) sources in the nearby universe. Also in analogy with 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 $v \gtrsim 0.1 c$ 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 `hyper-nebulae'. If ULX jets are sources of repeating fast radio bursts (FRB), as recently proposed, such hyper-nebulae 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 190520B. ULX hyper-nebulae can be discovered independent 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.