Short-timescale variation in the submillimeter flux of Sagittarius A*

Miyoshi M.; Kato Y.; Asaki Y.; Tsuboi M.; Uehara K.; Oka T.; Takahashi M.; Ishitsuka J.K.; Tsutsumi T.; Miyazaki A.; Matsumoto R. · Publications of the Astronomical Society of Japan (2026) · DOI: 10.1093/pasj/psag058

Resumen

We present a study of short-timescale (Formula presented) GHz flux-density variability of Sagittarius A(Formula presented) (Sgr A(Formula presented) ) using ALMA Cycle 3 observations. Careful self-calibration enabled snapshot imaging with (Formula presented) s integrations, achieving an effective image-domain SNR exceeding (Formula presented) and allowing high-cadence monitoring of major Galactic Center sources. To suppress common-mode atmospheric and instrumental fluctuations, we measured the flux of Sgr A(Formula presented) relative to multiple non-variable sources within the same field of view. We further quantified and corrected apparent variability induced by time-dependent u–v coverage and the associated PSF changes by performing imaging simulations with a static input model. These procedures isolate the intrinsic intensity variations of Sgr A(Formula presented) with substantially reduced non-source contamination. We searched for characteristic variability timescales over (Formula presented) using structure-function analysis, the Lomb–Scargle method, and state-space-model-based autoregressive spectral analysis. No dominant narrow periodic component is detected. Instead, we identify a distinct short-timescale, flat (white-noise-like) regime at (Formula presented) –(Formula presented), followed by red-noise-like behavior at longer timescales. The short-timescale white-noise regime appears in both active and quiescent phases, indicating statistically independent fluctuations on these timescales. We interpret the upper boundary of the white-noise regime as an empirical transition timescale, below which fluctuations remain effectively decorrelated and above which temporally correlated variability emerges. Existing theoretical and numerical studies of black-hole accretion flows have not, to our knowledge, explicitly predicted a physically white power spectrum at the shortest timescales, although they do not necessarily exclude such behavior. Reported results have more commonly shown red-noise-like or broken-power-law variability, and the physical origin of the flat short-timescale component identified here therefore remains uncertain. © The Author(s) 2026. Published by Oxford University Press on behalf of the Astronomical Society of Japan. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site-for further information please contact journals.permissions@oup.com. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/pages/standard-publication-reuse-rights)

Instituciones: Universidad Nacional del Centro del Perú