Evidence for Multiple Orbiting Hotspots in the 340 GHz Variability of Sgr A*

TL;DR

This study analyzes ALMA data of Sgr A* at 340 GHz, finding evidence consistent with multiple orbiting hotspots affecting its millimeter variability, though not statistically significant as periodic signals.

Contribution

It introduces a model of multiple decaying hotspots to explain Sgr A*'s millimeter variability, unifying periodic and non-periodic behaviors.

Findings

Detected short-timescale components (~30 min and ~50 min) in the light curve.

Significant peaks under white noise assumption, but not under red noise, indicating no definitive periodicity.

Variability consistent with multiple orbiting hotspots with decaying emission.

Abstract

We analyzed 11 epochs of archival Atacama Large Millimeter/submillimeter Array (ALMA) data to investigate flux density variability of Sgr A* at 340 GHz. In one epoch, the light curve exhibits two short-timescale components with characteristic periods of ~30 min and ~50 min. While the corresponding peaks in the periodogram are highly significant under a white-noise assumption, their significance decreases below 3 \sigma when red-noise variability is taken into account, and we therefore do not regard them as statistically significant periodic detections. Nevertheless, the observed timescales are comparable to the orbital period near the innermost stable circular orbit of Sgr A*, and the light curve shows phase-dependent structure and amplitude evolution consistent with orbital modulation. We find that the variability is well described by a model involving multiple orbiting hotspots with decaying emission. This interpretation suggests that both periodic and non-periodic variability in Sgr A* may arise from a common physical origin in orbiting structures within the accretion flow, providing a unified framework for its millimeter variability.