A Face-on Accretion Disk Geometry Revealed by Millimeter-wave Periodicity in Sgr A$^*$

TL;DR

This study reports the detection of a ~52-minute periodic signal in millimeter-wave flux from Sgr A$^*$, revealing a nearly face-on accretion disk geometry near the black hole, consistent with previous observations.

Contribution

It provides the first direct millimeter wavelength constraint on the inner accretion flow geometry of Sgr A$^*$, demonstrating millimeter periodicity as a tool for probing black hole accretion structures.

Findings

Detected a coherent 52-minute sinusoidal modulation at 230 GHz.

Inferred a disk inclination of approximately 8°, nearly face-on.

Confirmed consistency with previous IR and EHT constraints.

Abstract

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$^\circ$ (or 172$^\circ$), 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.