Nonstop Variability of Sgr A* Using JWST at 2.1 and 4.8 <i>μ</i>m Wavelengths: Evidence for Distinct Populations of Faint and Bright Variable Emission

Abstract We present the first results of JWST Cycle 1 and 2 observations of Sgr A* using NIRCam taken simultaneously at 2.1 and 4.8 μ m for a total of ∼48 hr over seven different epochs in 2023 and 2024. We find correlated variability at 2.1 and 4.8 μ m in all epochs, continual short-timescale (a few seconds) variability, and epoch-to-epoch variable emission implying long-term (∼days to months) variability of Sgr A*. A highlight of this analysis is the evidence for subminute, horizon-scale time variability of Sgr A*, probing inner accretion disk size scales. The power spectra of the light curves in each observing epoch also indicate long-term variable emission. With continuous observations, JWST data suggest that the flux of Sgr A* is fluctuating constantly. The flux density correlation exhibits a distinct break in the slope at ∼3 mJy at 2.1 μ m. The analysis indicates two different processes contributing to the variability of Sgr A*. Brighter emission trends toward shallower spectral indices than the fainter emission. Cross-correlation of the light curves indicates for the first time a time delay of 3–40 s in the 4.8 μ m variability with respect to 2.1 μ m. This phase shift leads to loops in plots of flux density versus spectral index as the emission rises and falls. Modeling suggests that the synchrotron emission from the evolving, age-stratified electron population reproduces the shape of the observed light curves with a direct estimate of the magnetic field strengths in the range between 40 and 90 G and an upper cutoff energy, E c , between 420 and 720 MeV.