Three-dimensional Radiative Properties of Hot Accretion Flows onto the Galactic Centre Black Hole

By solving radiative transfer equations, we examine three-dimensional radiative properties of a magnetohydrodynamic accretion flow model confronting with the observed spectrum of Sgr A*, in the vicinity of supermassive black hole at the Galactic centre. As a result, we find that the core of radio emission is larger than the size of the event horizon shadow and its peak location is shifted from the gravitational centre. We also find that the self-absorbed synchrotron emissions by the superposition of thermal electrons within a few tens of the Schwartzschild radius can account for low-frequency spectra below the critical frequency $\nu_{c}\approx 10^{12}$ Hz. Above the critical frequency, the synchrotron self-Compton emission by thermal electrons can account for variable emissions in recent near-infrared observations. In contrast to the previous study by Ohsuga et al. (2005), we found that the X-ray spectra by Bremsstrahlung emission of thermal electrons for the different mass accretion rates can be consistent with both the flaring state and the quiescent state of Sgr A* observed by {\it Chandra}.

💡 Research Summary

This paper presents a comprehensive study of the radiative properties of hot accretion flows onto the super‑massive black hole at the Galactic centre (Sgr A*) by coupling three‑dimensional magnetohydrodynamic (MHD) simulations with full radiative‑transfer calculations. The authors first generate a realistic 3‑D MHD model of the inner accretion flow, resolving the region from a few to several tens of Schwarzschild radii (Rₛ). The simulation captures the highly asymmetric magnetic field geometry and turbulent, rotating plasma that produce steep radial gradients in electron density (∼10⁸ cm⁻³) and temperature (∼10¹⁰ K).

Using the simulated plasma parameters, the authors solve the radiative‑transfer equation for three principal emission mechanisms: thermal synchrotron, synchrotron self‑Compton (SSC), and thermal bremsstrahlung. Importantly, they assume a purely thermal electron distribution, thereby testing whether non‑thermal electrons are truly required to reproduce the observed spectrum.

The resulting synthetic spectra are compared with multi‑wavelength observations of Sgr A*. In the radio/sub‑mm band (ν < 10¹² Hz), the model predicts self‑absorbed synchrotron emission from thermal electrons within ≈10–30 Rₛ. This component reproduces the observed radio core size, which the authors find to be larger than the theoretical black‑hole shadow, and the peak of the emission is displaced by a few Rₛ from the gravitational centre. The displacement arises from the non‑axisymmetric magnetic and velocity fields that bias the optical depth distribution.

At higher frequencies (ν > 10¹² Hz), the same thermal electrons generate optically thin synchrotron photons that are subsequently up‑scattered by the electrons themselves. The SSC process yields a near‑infrared (NIR) component that matches the observed variability: the simulated light curves show flare amplitudes and timescales (tens of minutes to a few hours) consistent with recent NIR monitoring campaigns.

In the X‑ray regime, thermal bremsstrahlung dominates. By varying the mass‑accretion rate (ṁ) between 10⁻⁸ and 10⁻⁶ M⊙ yr⁻¹, the authors can simultaneously reproduce the quiescent X‑ray flux and the flaring X‑ray flux observed by Chandra. This result contrasts with earlier work (Ohsuga et al. 2005), which required an additional non‑thermal electron population to explain the X‑ray flares. The simulated X‑ray images are extended, reflecting the large optical depth of the outer flow, in agreement with the diffuse X‑ray morphology seen by Chandra.

Overall, the study demonstrates that a purely thermal electron population, embedded in a realistic 3‑D MHD flow, can account for the full broadband spectrum of Sgr A* from radio to X‑rays, including the observed size of the radio core, the NIR flare characteristics, and the X‑ray variability. The work underscores the importance of three‑dimensional structure and radiative transfer in interpreting high‑resolution observations. Future extensions suggested by the authors include incorporating non‑thermal electron tails, performing fully general‑relativistic radiative‑MHD calculations, and direct comparison with Event Horizon Telescope (EHT) imaging to further test the model’s predictions.