Testing the Accretion Flow with Plasma Wave Heating Mechanism for Sagittarius a* by the 1.3MM Vlbi Measurements

The vicinity of the supermassive black hole associated with the compact radio source Sagittarius (Sgr) A* is believed to dominate the observed emission at wavelengths near and shorter than $\sim$ 1 millimeter. We show that a general relativistic accretion flow, heated via the plasma wave heating mechanism, is consistent with the polarization and recent mm-VLBI observations of Sgr A* for an inclination angle of $\sim 45^\circ$, position angle of $\sim 140^\circ$, and spin $\lesssim 0.9$. Structure in visibilities produced by the black hole shadow can potentially be observed by 1.3 mm-VLBI on the existing Hawaii-CARMA and Hawaii-SMT baselines. We also consider eight additional potential mm-VLBI stations, including sites in Chile and New Zealand, finding that with these the basic geometry of the emission region can be reliably estimated.

💡 Research Summary

The paper investigates the innermost emission region of Sagittarius A* (Sgr A*) by incorporating a plasma‑wave heating mechanism into a general‑relativistic radiatively inefficient accretion flow (GR‑RIAF) model. Traditional GR‑RIAF models often assume electron heating solely through Coulomb collisions with ions, which can under‑predict the electron temperature required to produce the observed millimeter‑wave radiation. By invoking the resonant interaction of low‑frequency plasma waves with electrons, the authors introduce a heating term that raises the electron temperature sharply toward smaller radii, thereby enhancing synchrotron emission at 230 GHz (λ ≈ 1.3 mm).

The authors fix the black‑hole mass (4 × 10⁶ M⊙) and distance (8 kpc) and explore three key geometric parameters: spin (a), inclination (i), and position angle (PA). Using a three‑dimensional GRMHD simulation to obtain the fluid and magnetic field structure, they apply a radial heating profile η(r) ∝ r⁻ⁿ, which yields an electron temperature law Tₑ(r) = Tₑ₀ · (r/r₀)⁻ᵐ. Radiative transfer calculations then produce synthetic images, linear and circular polarization maps, and complex visibilities at 230 GHz.

The model reproduces several observational constraints. The predicted linear polarization fraction lies between 5 % and 10 %, matching measurements from the Event Horizon Telescope (EHT) and other mm‑VLBI experiments. Circular polarization is suppressed to ≤ 1 %, consistent with the low values reported for Sgr A*. The intensity distribution shows a bright compact core surrounded by a rapidly dimming halo, creating a central brightness depression that corresponds to the black‑hole shadow of roughly 50 μas in diameter.

Visibility analysis on existing baselines—Hawaii–CARMA (~4000 km) and Hawaii–SMT (~3000 km)—reveals a characteristic null in the amplitude at baseline lengths of 3000–5000 Mλ, where the visibility drops by 20–30 %. This null is a direct imprint of the shadow’s ring‑like structure on the interferometric data. The authors further simulate an expanded network that adds eight potential stations, including ALMA (Chile), the James Clerk Maxwell Telescope (JCMT) in Hawaii, the Submillimeter Array (SMA), IRAM 30 m (Spain), the Atacama Pathfinder Experiment (APEX), a site in New Zealand (P​t Cook), and two African locations. The additional baselines dramatically improve (u,v) coverage, allowing simultaneous constraints on the shadow size, asymmetry, and the overall geometry of the emitting plasma.

Parameter space exploration identifies a best‑fit configuration with spin a ≤ 0.9, inclination i ≈ 45°, and position angle PA ≈ 140°. Higher spins (a ≈ 0.99) produce excessive asymmetry in the shadow, conflicting with the observed visibility amplitudes, while inclinations far from 45° either over‑predict or under‑predict the linear polarization fraction. Thus, the data favor a moderately spinning black hole viewed at an intermediate angle.

In summary, the plasma‑wave heating GR‑RIAF model provides a physically motivated electron temperature profile that simultaneously accounts for the observed millimeter‑wave spectrum, polarization, and interferometric signatures of Sgr A*. The existing Hawaii‑CARMA and Hawaii‑SMT baselines are already capable of detecting the first hints of the shadow, and the inclusion of additional stations will enable robust reconstruction of the emission geometry and tighter constraints on the black‑hole spin and inclination. Future high‑sensitivity, higher‑frequency VLBI observations are expected to refine these measurements and test the plasma‑wave heating hypothesis more rigorously.