Short timescale variation in the submillimeter flux of Sagittarius A*

TL;DR

This study uses high-cadence ALMA observations to analyze short-timescale flux variability of Sagittarius A*, revealing a flat, white-noise regime below a few minutes and red-noise behavior at longer timescales, with no dominant periodicity.

Contribution

First high-cadence, high-SNR monitoring of Sgr A* at 340 GHz, identifying a characteristic transition timescale in flux variability.

Findings

No dominant narrow periodicity detected.

Identified a flat, white-noise regime below 2.3–6.3 minutes.

Observed red-noise-like variability at longer timescales.

Abstract

We study short-timescale 340 GHz flux-density variability of Sgr A* using ALMA Cycle 3 observations. Careful self-calibration enabled 10 s snapshot imaging with very high effective image-domain SNR, allowing high-cadence monitoring of Galactic Center sources. To reduce atmospheric and instrumental effects, we measured Sgr A* relative to multiple non-variable sources in the same field and corrected apparent variability caused by time-dependent u-v coverage and PSF changes using simulations with a static input model. We then searched for characteristic timescales over 20 s < tau < Tobs/3 using structure functions, the Lomb--Scargle method, and state-space-model autoregressive spectral analysis. No dominant narrow periodicity is found. Instead, the data show a short-timescale flat, white-noise-like regime at tau below about 2.3--6.3 min, followed by red-noise-like behavior at longer timescales. This flat regime appears in both active and quiescent phases, suggesting statistically independent fluctuations on these timescales. We interpret its upper boundary as an empirical transition timescale between decorrelated short-timescale fluctuations and longer-timescale correlated variability. The physical origin of this flat component remains uncertain, since previous theoretical and numerical studies more commonly report red-noise-like or broken-power-law variability.