Compact stellar systems hosting an intermediate mass black hole: magnetohydrodynamic study of inflow-outflow dynamics

TL;DR

The paper investigates wind-fed accretion onto an IMBH embedded in a compact WR star cluster, motivated by IRS 13E. Using 3D (magneto)hydrodynamic simulations with moving wind sources and a metallicity-dependent cooling function, the study shows that only about $10^{-5}$ of the injected wind mass is captured, with turbulence from wind–wind collisions driving outflows that dominate the dynamics. Enhanced cooling at high metallicity forms dense clumps that rarely reach the IMBH, and the integrated X-ray emission is dominated by colliding winds rather than near-horizon accretion, yielding a faint, highly variable near-horizon signal. These results explain the low detectability of IMBHs in dense clusters and provide a framework for planning future multi-wavelength observational strategies with next-generation facilities like Lynx, AXIS, and JWST. Overall, wind-fed accretion in such environments appears inefficient for rapid IMBH growth, unless supplemented by external gas inflow or tidal events.

Abstract

Intermediate-mass black holes (IMBHs) are a missing link in black hole demographics, with only tentative observational evidence to date. Dense stellar clusters such as IRS 13E near the Galactic Center are promising IMBH hosts, where accretion is likely driven by winds from nearby Wolf-Rayet (WR) stars. Yet, the dynamics of such wind-fed systems remain largely unexplored. We investigate how high-velocity stellar winds, magnetic fields, and metallicity-dependent radiative cooling influence gas dynamics and black hole accretion in compact WR clusters. Using three-dimensional (magneto)hydrodynamic simulations, we model each WR star as a source of mass, momentum, energy, and magnetic flux, and include a cooling function that depends on chemical abundance. We compare isotropic versus disk-like stellar distributions to explore the impact of cluster geometry. Across all models, we find that the accretion rate onto the IMBH is suppressed by up to five orders of magnitude relative to the total stellar mass-loss rate. Turbulent, shock-heated outflows driven by wind-wind collisions dominate the flow, expelling most injected gas. While enhanced cooling in high-metallicity runs promotes the formation of dense clumps, these structures are typically unable to reach the black hole. The system's integrated X-ray luminosity is dominated by colliding WR winds, masking the IMBH's radiative signature. Accretion occurs in short-lived, quasi-periodic episodes triggered by close stellar passages, but even these flares remain difficult to detect against the luminous wind background. Our results naturally explain the low detectability of IMBHs in compact WR clusters and provide theoretical predictions to guide future X-ray and infrared observational strategies.

Figures (17)

• Figure 1: 3D volume rendering of the density of the simulated stellar cluster. Left: Colliding stellar winds 620 years into the Model II spherical simulation. Right: Outbursts of cool clumps 815 years into the Model III spherical simulation, driven by the gas’s higher metal content, which enhances cooling efficiency and amplifies the development of the thin-shell instability.
• Figure 2: Plane-parallel cuts ($y=0$) of fluid quantities for different models. Left and middle column: Model II without ($\beta_\mathrm{w} = \infty$) and with magnetized winds ($\beta_\mathrm{w} =100$) 620 years into the simulation. Right column: Model III simulation 815 years into the simulation. Top row: electron number density $n_{\rm e}$; middle row: accretion rate $\dot{M}$; bottom row: temperature.
• Figure 3: Time and angle-averaged quantities in our simulations as a function of distance from the black hole, $r$. Different lines denote different configurations of the simulation. a) density in the units of electron number density. b) accretion rate $\dot{M}$. c) radial velocity of the gas $v_\mathrm{r}$. d) Temperature of the gas. For the density and temperature radial profiles, a line representing $\propto r^{-1}$ is also plotted for reference, highlighting the approximate power law in both quantities in the inner regions of the simulations.
• Figure 4: Time and angle-averaged absolute value of the specific angular momentum $\langle|l|\rangle$ (dashed lines) as well as the absolute value of the average $|\langle l\rangle|$ (solid lines) normalized by the Keplerian value, $l_\mathrm{Kep}$, as a function of distance from the black hole, $r$.
• Figure 5: Time and angle-averaged value of plasma $\beta$ as a function of distance from the black hole, $r$, for the Model II simulation with magnetized winds using $\beta_{\rm w} = 100$. Different lines denote average $\langle \beta \rangle$ (solid line), density-weighted average $\langle \beta \rangle_\rho$ (dashed line), and $\langle \tilde{\beta} \rangle_\rho = 2\langle P +\rho v^2\rangle_\rho/\langle B^2\rangle_\rho$ (dotted line).