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

Navin Sridhar, Brian D. Metzger|arXiv (Cornell University)|Jun 21, 2022

Astrophysical Phenomena and ObservationsPhysics and Astronomy168 references51 citations

TL;DR

This paper proposes that hyper-accreting X-ray binaries undergoing super-Eddington mass transfer produce short-lived, energetic radio nebulae—'ULX hyper-nebulæ'—that can serve as precursors to common envelope events and as persistent radio counterparts to repeating fast radio bursts (FRBs). The model shows these nebulae produce time-variable rotation measures and synchrotron radio emission matching FRB 20121102 and FRB 20190520B, with detectable radio transients on year-to-decade timescales in surveys like VLASS.

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 ($\dot{M} \gtrsim 10^{5}\dot{M}_{ m Edd})$ 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.

Motivation & Objective

• To model the electromagnetic emission from hyper-accreting X-ray binaries during unstable mass transfer preceding common envelope events.
• To explore whether such systems can produce persistent radio sources and time-variable rotation measures matching observed FRB counterparts.
• To assess the detectability of these hyper-nebulae in radio surveys like VLASS and their potential as precursors to luminous transients.
• To test the hypothesis that repeating FRBs are powered by relativistic jets in ULX-like systems with precessing outflows.
• To provide a unified framework linking FRB persistent radio sources, variable RM, and transient radio nebulae from short-lived hyper-accretion phases.

Proposed method

• Develops a time-dependent, one-zone model for synchrotron radio emission from relativistic electrons heated at jet termination shocks in expanding nebulae.
• Self-consistently evolves nebula size, magnetic field, electron energy distribution, and radiative/adiabatic losses over time.
• Incorporates disk winds (slow, equatorial) and relativistic jets (fast, bipolar) as drivers of nebula inflation and electron injection.
• Evaluates observable properties: radio luminosity, rotation measure (RM), dispersion measure, and spectral energy distribution as functions of accretion rate and jet parameters.
• Uses fiducial parameters (e.g., ˙M/ ˙MEdd ∈[10, 10⁷], vj ≳0.1c) to simulate evolution across free expansion, deceleration, and post-active phases.
• Compares model predictions to observed FRB 20121102 and FRB 20190520B radio counterparts, including spectral shape and RM variability.

Experimental results

Research questions

• RQ1 Can hyper-accreting X-ray binaries produce radio nebulae with time-variable rotation measures matching those observed in FRB 20121102 and FRB 20190520B?
• RQ2 Do ULX hyper-nebulæ from short-lived, super-Eddington accretion phases produce detectable radio transients on timescales of years to decades?
• RQ3 Can the persistent radio emission and spectral properties of FRB 20121102 and FRB 20190520B be reproduced by a model of a young, relativistically shocked nebula?
• RQ4 What role do jet velocity variations or electron temperature spreads play in producing broader-than-thermal radio spectra in FRB counterparts?
• RQ5 How many such hyper-nebulae might be detectable in current and future radio surveys like VLASS?

Key findings

• ULX hyper-nebulæ can reproduce the radio spectrum and time-variable rotation measure (|RM|max ≈ 10⁴ rad m⁻²) of FRB 20121102 and FRB 20190520B under close-to-fiducial assumptions.
• The broader observed radio spectra require a modest spread in electron temperatures, corresponding to ≲2–3 variations in jet velocity or injection times.
• The model explains the secular trend and fluctuation timescales of RM variations in FRB 20121102, 20190520B, and 20180916B due to turbulence or line-of-sight variations through the nebula.
• Up to ∼10³–10⁶ hyper-nebulae may exist within the VLASS survey volume, with O(10) showing detectable flux evolution on year-to-decade timescales.
• Radio transients from these systems may presage future common envelope events, such as luminous red novae or fast blue optical transients.
• X-ray and optical counterparts are faint and challenging to detect beyond ∼100 Mpc, but high-resolution radio follow-up could constrain the nebula aspect ratio and jet vs. wind dynamics.