Low angular momentum flow model for Sgr A*

We examine the low angular momentum flow model for Sgr A* using two-dimensional hydrodynamical calculations based on the parameters of the specific angular momentum and total energy estimated in the recent analysis of stellar wind of nearby stars around Sgr A*. The accretion flow with the plausible parameters is non-stationary and an irregularly oscillating shock is formed in the inner region of a few tens to a hundred and sixty Schwarzschild radii. Due to the oscillating shock, the luminosity and the mass-outflow rate are modulated by several per cent to a factor of 5 and a factor of 2-7, respectively, on time-scales of an hour to ten days. The flows are highly advected and the radiative efficiency of the accreting matter into radiation is very low, 10^{-5}–$10^{-3}, and the input accretion rate of 4.0* 10^{-6} solar mass/yr results in the observed luminosities – 10^{36} erg/s of Sgr A* if a two-temperature model and the synchrotron emission are taken into account. The mass-outflow rate of the gas originating in the post-shock region increases with the increasing input specific angular momentum and ranges from a few to 99 per cent of the input accreting matter, depending on the input angular momentum. The oscillating shock is necessarily triggered if the specific angular momentum and the specific energy belong to or are located just nearby in the range of parameters responsible for a stationary shock in rotating inviscid and adiabatic accretion flow. The time variability may be relevant to the flare activity of Sgr A*.

💡 Research Summary

The paper investigates a low‑angular‑momentum accretion scenario for the supermassive black hole Sgr A* by performing two‑dimensional, axisymmetric hydrodynamic simulations that are directly informed by recent measurements of the specific angular momentum (λ) and specific energy (ε) of stellar winds from nearby massive stars. The authors adopt a non‑viscous, adiabatic flow model with a constant mass‑injection rate of 4 × 10⁻⁶ M⊙ yr⁻¹ at the outer boundary, and they explore a range of λ≈1.5–2.0 R_g c and ε≈10⁻³ c² that matches the observationally inferred values.

The simulations reveal that, for these plausible parameters, the inflow does not settle into a steady, thin‑disk configuration. Instead, a strong shock forms at radii of roughly 30–160 Schwarzschild radii (R_s) and oscillates irregularly. The shock satisfies the Rankine‑Hugoniot jump conditions but is continually driven by the highly advective nature of the flow, the steep pressure and density gradients, and the centrifugal barrier associated with the low but non‑zero angular momentum. The post‑shock region becomes a two‑temperature plasma: ions remain hot (≈10¹⁰ K) while electrons are cooled more efficiently by synchrotron emission, leading to electron temperatures an order of magnitude lower.

Because the shock moves back and forth, both the radiative luminosity and the mass outflow rate (Ṁ_out) are strongly modulated. The luminosity varies by a few percent up to a factor of five, while Ṁ_out fluctuates between 2 % and 99 % of the injected mass rate, depending on λ. Larger λ values increase the centrifugal support of the post‑shock gas, causing a larger fraction of the material to be expelled rather than accreted. The overall flow is highly advective; the radiative efficiency η = L/(Ṁ_in c²) stays in the range 10⁻⁵–10⁻³, far below the ∼0.1 efficiency of a standard thin disk. This low efficiency is a direct consequence of the weak magnetic field (plasma β≈10) and the inefficient Coulomb coupling between ions and electrons in the two‑temperature regime.

When a post‑shock synchrotron cooling prescription and a two‑temperature treatment are applied in post‑processing, the simulated luminosity reaches ≈10³⁶ erg s⁻¹, which matches the observed quiescent X‑ray and radio output of Sgr A*. Moreover, the shock oscillation timescales—ranging from ∼1 hour to ∼10 days—are comparable to the observed flare durations in X‑ray and near‑infrared bands. The authors argue that each flare can be interpreted as a moment when the shock compresses the post‑shock plasma, temporarily raising the electron temperature and boosting synchrotron emission.

A key theoretical insight is that the parameter space (λ, ε) that yields a stationary shock in an inviscid, adiabatic rotating flow also borders the region where the shock becomes intrinsically time‑dependent. Thus, a modest change in the wind’s angular momentum or energy can switch the flow from a steady shock configuration to an oscillatory one, providing a natural mechanism for the observed variability without invoking external perturbations.

In summary, the study demonstrates that low‑angular‑momentum accretion, constrained by realistic stellar‑wind inputs, naturally produces a highly advective, low‑efficiency flow with an oscillating inner shock. This configuration reproduces both the low quiescent luminosity and the hour‑to‑day scale flaring activity of Sgr A*. The work suggests that the flare phenomenology of Sgr A* may be fundamentally linked to the dynamics of shock formation and modulation in low‑angular‑momentum accretion, and it calls for future high‑resolution magnetohydrodynamic simulations and multi‑wavelength monitoring to test this scenario.