We analyzed 77 epochs of Atacama Large Millimeter/submillimeter Array (ALMA) archival data to investigate flux variability in Sagittarius A∗ (Sgr A∗), the supermassive black hole at the Galactic Center. Among these, we identified a rare but unusually clear and coherent ~52-minute sinusoidal modulation at 230 GHz, with a statistical significance exceeding 5{\sigma}. Modeling with a Doppler-boosted hotspot scenario yields an orbital radius of ~4 Schwarzschild radii and a disk inclination of 8∘ (or 172∘), providing the first direct millimeter wavelength constraint on the inner accretion flow geometry. This nearly face-on inclination is in good agreement with previous constraints from GRAVITY and EHT observations. These findings provide robust, independent evidence that millimeter-wave periodicity can directly probe the innermost accretion flow geometry, offering a powerful complement to variability studies at infrared and X-ray wavelengths.
@article{arxiv.2511.02186,
title = {A Face-on Accretion Disk Geometry Revealed by Millimeter-wave Periodicity in Sgr A$^*$},
author = {Kazuki Yanagisawa and Tomoharu Oka and Ryo Ariyama and Kazuki Yanagihara and Yuhei iwata and Mikiya M. Takahashi},
journal= {arXiv preprint arXiv:2511.02186},
year = {2025}
}