Universal Accretion Mechanisms in Astrophysics

• Universal accretion mechanisms are physical processes that consistently govern how matter, from gas to dark matter, is assimilated by various astrophysical objects.
• They explain diverse phenomena including clumpy wind accretion in X-ray binaries, planetary formation scaling laws, and universal dark matter halo and black hole growth patterns.
• Analytic and numerical models reveal robust, scale-invariant dynamics while accounting for deviations due to feedback, instabilities, and environmental complexities.

Universal accretion mechanisms refer to families of physical processes governing how matter—be it gas, dust, stars, or dark matter—is assimilated by compact objects, stars, planets, or halos, with the remarkable property that their essential statistical, dynamical, or scaling features recur across a diverse range of astrophysical environments. These mechanisms are characterized by their robustness to initial conditions, wide applicability across different system masses and scales, and their emergence in settings from planetary formation to galaxy evolution, star cluster assembly, and the rapid growth of black holes. Universality is manifest in scaling relations, self-similar profiles, and accretion histories, which can be predicted from first principles or simple physical ingredients and often confirmed by both analytic models and numerical simulations.

1. Universal Accretion in Clumpy Winds and X-ray Binaries

A canonical instance is the clumpy wind accretion mechanism in high-mass X-ray binaries, particularly in Supergiant Fast X-ray Transients (SFXTs) (Ducci et al., 2010). Here, the donor OB star emits a wind comprised of dense clumps, statistically described by a power-law mass distribution, $p(M_{\rm cl})\propto M_{\rm cl}^{-\zeta}$ between minimum and maximum clump masses. A compact object (typically a neutron star) traveling through this wind episodically captures clumps, with each capture producing X-ray flares. The wind velocity follows a $\beta$-law: $v_w(r) = v_\infty(1-R_p/r)^\beta,$ where $v_\infty$ is the terminal velocity and $R_p$ the stellar radius. The stochastic nature of clump accretion yields a broad dynamic luminosity range, with outburst and quiescent states both accounted for by the same physical process, largely insensitive to microphysical details.

The universality extends further when considering X-ray photoionization: high luminosity from the neutron star ionizes the local wind, suppressing the line-driving force and reducing $v_{\rm rel}$, thereby enhancing the gravitational capture radius: $R_a = \frac{2GM_x}{v_{\rm rel}^2 + c_s^2}.$ Slower wind material can accumulate sufficient angular momentum to form transient accretion disks, naturally producing episodic, quasi-periodic flaring even in otherwise spherically-fed systems.

Alternatively, a regime dominated by Rayleigh-Taylor (RT) instability at the magnetospheric boundary can explain flares of particular temporal shape, with the accretion gating by the magnetosphere controlled by the X-ray luminosity $L_x$ surpassing a critical threshold set by the neutron star magnetic field, mass, and temperature.

2. Universality in Planetary Accretion and Scaling Laws

In planetary system assembly, simplified accretion models incorporating total angular momentum conservation, mass- and velocity-dependent accretion probabilities, and collisional energy dissipation reveal emergent universal features (Hernández-Mena et al., 2010). The combined effect of “runaway” (mass-selective) and “velocity-selection” accretion channels ensures that, despite variable system parameters (disc mass $M_{\rm disc}$, central mass $M_0$, or angular momentum $L_{\rm tot}$), the statistical outcomes—specifically the relation between final planet mass and semi-major axis—are invariant when recast with appropriately scaled variables: $$R_{\rm scale} = \frac{(L_{\rm tot}/M_{\rm disc})^2}{GM_0},$$

$$a' = \frac{a}{R_{\rm scale}},\quad m' = \frac{m}{M_{\rm disc}}.$$

The resulting distribution of planetary orbits forms robust "clustering" at characteristic locations, reflecting the underlying self-similar gravitational dynamics rather than the specifics of the initial disc. Such universality offers a rational explanation for observed patterns like Titius–Bode spacings and supports the view that apparently diverse planetary architectures are governed by fundamental, scale-invariant accretion physics.

However, in some contexts—such as the assembly of hot super-Earth systems—empirical studies demonstrate that no single universal accretion profile matches the diversity of minimum-mass nebular disk slopes inferred from exoplanet observations. Rather, a broad range ($\Sigma \propto r^\gamma$ with $\gamma$ ranging from $-3.2$ to $0.5$) is required, implicating distribution-preserving processes such as embryo migration and late-time dynamical assembly as necessary additions or alternatives to pure in-situ universal models (Raymond et al., 2014).

3. Universal Mass Growth and Halo Assembly

Hierarchical structure formation provides another paradigm, where the universal nature of dark matter halo accretion is analytically derived from extended Press–Schechter (EPS) theory (Correa et al., 2014). The accretion history, $M(z)$, obeys a scaled form: $M(z) = M_0 (1 + z)^{a f(M_0)} e^{-f(M_0)z},$ with $M_0$ the $z=0$ mass, $f(M_0)$ a function of the initial variance of the density perturbation, and $a$ a parameter set by the cosmology via the linear growth factor $D(z)$. This formulation unifies the rapid, exponential mass accumulation at high redshift (matter-dominated era) and the slower, power-law growth in the dark energy-dominated regime. Importantly, the universality of accretion histories transcends the detailed initial conditions and is dictated by the stochastic nature of cosmological fluctuations and the time-dependent growth of structure, offering a predictive link to halo concentrations and assembly bias.

The angular pattern of subhalo accretion is also universal, traceable to the eigenframe of the local velocity shear tensor (Libeskind et al., 2014, Kubik et al., 2017): $$\Sigma_{ij} = -\frac{1}{2H(z)}\left(\frac{\partial v_i}{\partial r_j} + \frac{\partial v_j}{\partial r_i}\right),$$ where the direction of weakest collapse (${\bf e}_3$) derived from the eigenvalues of $\Sigma_{ij}$ robustly aligns with the principal direction of subhalo infall. This “beaming” pattern persists across halo and subhalo masses, redshifts, and even in both cold and warm dark matter cosmologies, although the precise strength of alignment can be modulated by the mass function and collapse histories.

4. Black Hole Growth: Universal Accretion and Regimes

Black holes—from stellar remnants to supermassive objects—exhibit a further suite of universal accretion behaviors. Novel analytic and simulation-based models demonstrate that accretion rates measured at radial distance $r$ from a black hole typically obey power-law scalings,

$\dot{M}_{\rm in}(r) \propto r^s,$

with $s \approx 0.66$ for the “magnetically arrested disk” (MAD) state and $s \approx 0.87$ for the “rocking accretion disk” (RAD) state (Lalakos et al., 29 May 2025). This invariance is seen irrespective of the details of galactic scale inflow, magnetic field topology, or dynamical regime, persisting over several orders of magnitude between the Bondi radius and the event horizon.

The MAD state is regulated by accumulation of poloidal magnetic flux, leading to jet-driven outflows and periodic disruption of coherent inflow, while the RAD state features chaotically reorienting disks with weaker, intermittent jets. The MAD–RAD cycle’s universality and characteristic timescale (scaling with $t_{\rm B}\propto r_{\rm B}^{3/2}$) potentially explains the duty cycles of jetted AGN outbursts, such as those inferred in M87*.

Direct relevance extends to black hole seed growth, with mechanisms such as supra-exponential Bondi accretion (enabled by photon trapping and suppressed disk formation due to low angular momentum capture (Alexander et al., 2014)), biconical-dominated accretion flows in the hyperaccretion regime (Park et al., 2020), and “choked” hydrodynamical flows around event horizons when polar–equatorial density contrasts force excess inflow into bipolar outflows (Tejeda et al., 2019). Each exhibits self-similar scaling in growth, insensitivity to boundary conditions beyond critical thresholds, and universality in morphological inflow patterns.

A distinct but related avenue involves “dark-to-black” accretion, where ultralight scalar dark matter around black holes is efficiently accreted after a critical boson–black hole mass product $\mu M_{\rm BH}$ is attained (Sanchis-Gual et al., 1 Oct 2025). The scalar quasi-bound states decay with rate $|\Gamma|=16\mu^6 M_{\rm BH}^5$ in the relativistic regime, producing exponential black hole growth once resonance is achieved, allowing seed black holes to attain supermassive status within 10⁸ years even in the absence of conventional baryonic inflow.

5. Universal Accretion in Star Cluster and Compact Object Evolution

Universal accretion processes are also pivotal in star cluster formation and compact object growth. Star clusters assemble from both dense filaments and the ambient medium, as delineated by (Karam et al., 2023): $$\dot{M}_{\rm fil} \approx \rho_{\rm fil}\, v_{\rm fil}\, A_{\rm fil}, \qquad \dot{M}_{\rm bg} \approx \frac{4\pi G^2 M^2 \rho_{\rm bg}}{(c_s^2 + v_{\rm rel}^2)^{3/2}},$$ revealing how cluster mass buildup results from a sum of highly anisotropic episodic (filamentary) and isotropic background accretion, largely independent of fine-tuning. Feedback and dynamical mass loss modestly temper growth but do not erase the underlying statistical stability of mass assembly pathways.

In gas-rich globular clusters, a universal accretion scenario for compact objects emerges wherein oxygen–neon white dwarfs (ONe WDs) undergo accretion-induced collapse (AIC) into neutron stars by capturing ambient second-generation (2P) cluster gas (Perets, 2022). The Bondi–Hoyle–Lyttleton rate is parameterized as

$$\frac{dM_{\rm BHL}}{dt}=6.8\times10^{-8}\,M_\odot\,{\rm yr}^{-1}\left(\frac{m}{M_\odot}\right)^2,$$

and after accounting for retention efficiencies, enables the production of low-velocity neutron stars resolved in globular clusters, regardless of initial cluster escape speed or supernova kick statistics. This universal gas-driven growth enables retention fractions consistent with observed pulsar populations, and the same physics offers pathways for type-Ia supernovae, further AIC events, and even black hole formation, suggesting a shared set of accretion-induced phenomena across dense stellar environments.

6. Limitations, Constraints, and Diversity of Accretion Profiles

Not all accretion-driven phenomena are governed by fully universal mechanisms. As shown for hot super-Earth systems (Raymond et al., 2014), empirical data reveal a broad spread of minimum-mass disk slopes that cannot be accommodated by a single in situ accretion scenario with a universal gas disk profile. Instead, complex dynamical flows—such as type-I migration of planetary embryos and post-assembly mergers—decouple solid accumulation from gas distribution, breaking universality at the level of surface density profiles and requiring models that allow for a diversity of accretion outcomes shaped by migration traps and merger histories.

While in many regimes feedback processes, magnetic fields, or external perturbations can modulate inflow, the statistical signatures and scaling relations of universal accretion mechanisms—robustness to system mass, self-similar time evolution, and insensitivity to initial angular momentum or density fields—persist. This indicates that, even where universality is not exact, the deviations can be traced to specific physical or dynamical processes layered atop a fundamentally robust gravitational, hydrodynamical, or magnetohydrodynamical scaffold.

7. Broader Implications and Applications

Universal accretion mechanisms enable predictive frameworks across astrophysical disciplines:

• For galaxy formation and cosmology, the universality of halo mass accretion histories informs halo models, empirical fitting functions, and links to observable properties such as concentration and assembly bias (Correa et al., 2014).
• In star and cluster formation, isotropic and anisotropic accretion channels explain the emergence of hierarchical clusters, mass functions, and early feedback.
• Black hole growth, jet launching, and AGN feedback are fundamentally shaped by the universality of inflow–outflow structure, as exemplified by universal scaling in MAD/RAD states (Lalakos et al., 29 May 2025).
• In compact object evolution and retention, accretion-induced collapse mediated by gas-rich environments provides a generic framework connecting white dwarf, neutron star, and black hole populations (Perets, 2022).

By highlighting the parameter-invariant, scale-free, and self-similar nature of accretion in these domains, research into universal accretion mechanisms offers a unifying thread through which the complex and varied phenomena of cosmic structure and compact object evolution may be better interpreted and connected.