Non-stop Variability of Sgr A* using JWST at 2.1 and 4.8 micron Wavelengths: Evidence for Distinct Populations of Faint and Bright Variable Emission

Abstract

We present first results of JWST Cycle 1 and 2 observations of Sgr A* using NIRCam taken simultaneously at 2.1 and 4.8 micron for a total of ~48 hours over seven different epochs in 2023 and 2024. We find correlated variability at 2.1 and 4.8 micron in all epochs, continual short-time scale (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 sub-minute, 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 micron. The analysis indicates two different processes contributing to the variability of Sgr A*. Brighter emission trends towards shallower spectral indices than the fainter emission. Cross correlation of the light curves indicates for the first time, a time delay of 3 - 40 sec in the 4.8 micron variability with respect to 2.1 micron. This phase shift leads to loops in plots of flux density vs 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 40-90 G, and upper cutoff energy, E_c, between 420 and 720 MeV.