Radiation pressure instability: from heart-beat states in black hole binary systems to Quasars and Changing-Look AGN

TL;DR

This work analyzes Radiation Pressure Instability (RPI) as a unifying mechanism for deterministic variability in accretion disks across mass scales. It couples 1D time-dependent viscous-diffusion and energy-balance models (including advection and irradiation) to produce limit-cycle oscillations that resemble heartbeat states in X-ray binaries and to explain duty cycles and state changes in quasars and Changing-Look AGN. The study connects microphysical disk processes (stress scaling with pressure, viscous heating, and radiative cooling) to macro-scale observables, and demonstrates how irradiation and boundary-layer effects modulate cycle properties. The findings provide a framework linking stellar-mass systems to AGN phenomenology and guide future radiation‑MHD simulations and time-domain observations to test RPI-driven variability.

Abstract

Radiation-pressure instability was identified soon after the seminal classical accretion disk models of Shakura-Sunyaev and Novikov-Thorne, yet its full implications remain an active area of investigation. These models form the backbone of our understanding of accretion onto compact objects and successfully describe the phenomenology of black hole and neutron star X-ray binaries, as well as luminous active galactic nuclei (AGN), in the regime of high mass accretion rates. At luminosities approaching a significant fraction of the Eddington limit (L/LEdd > 0.1), standard thin disks are predicted to become thermally unstable due to the dominance of radiation pressure. This prediction has found empirical support in several Galactic stellar-mass black hole systems, where the instability manifests as quasi-periodic, large-amplitude luminosity oscillations, so-called "heartbeat states", and has been proposed as a driver of observed signatures of deterministic chaos in accretion-driven light curves. The scope of radiation-pressure-induced variability extends beyond stellar-mass black holes: both black holes across mass scales and accreting neutron stars can exhibit related behavior, though the presence of a boundary layer in neutron stars adds complexity and offers a unique laboratory for testing the interplay between accretion dynamics and the central object. On extragalactic scales, the instability has been invoked to explain the duty cycles and apparent short lifetimes of radio-loud AGN, as well as the dramatic spectral-state transitions seen in Changing-Look AGN. (...)

Figures (8)

• Figure 1: Schematic timeline showing milestones in the accretion theory
• Figure 2: Regions of accretion disks dominated by different physical processes affecting thermal balance: pressure components, opacity sources. The disk outer radius must fit into the Roche lobe size, depending on the typical mass ratio of the binary. The inner radius can extend only to the ISCO orbit for BH, but cannot be smaller than WD radius for Cataclysmic Variables. The red solid line marks Eddington accretion rate limit. The blue dashed line marks region with temperature $10^{4} K$ of Hydrogen recombination, where the other opacity sources are important.
• Figure 3: Symbolic representation of the thermal equilibria (S-curves) of the accretion disk. Since the curve apparently shifts up with radius, at high $\dot M$ the instability might be driven by more distant, rather than inner, parts. However, the shape of the curve also varies with the radius, and at the largest radius showed above ($10^{5} R_{S}$) the instability disappears, so the outermost part of the flow is always stable against RPI instability. Presented plots are based on the recent computations by liu.
• Figure 4: Attractor point on the stable branch of disk solutions (left) and limit cycle around repelling fixed point in the unstable branch of disk solutions (right).
• Figure 5: Solutions of the NS boundary layer structure, with a thin Keplerian accretion disk. Quantities plotted are: surface density, central temperature, geometrical thickness, angular velocity, density, and radial velocity. Parameters of the simulation: neutron star mass $M_{\rm NS} = 1.4 M_{\odot}$, neutron star radius $R_{\rm NS}$ = 2.41 $R_{\rm Schw}$, angular momentum coupling parameter $j=1.064$.