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Abstract Sgr A* is the variable electromagnetic source associated with accretion onto the Galactic center supermassive black hole. While the near-infrared (NIR) variability of Sgr A* was shown to be consistent over two decades, unprecedented activity in 2019 challenges existing statistical models. We investigate the origin of this activity by recalibrating and reanalyzing all of our Keck Observatory Sgr A* imaging observations from 2005–2022. We present light curves from 69 observation epochs using the NIRC2 imager at 2.12μm with laser-guide star adaptive optics. These observations reveal that the mean luminosity of Sgr A* increased by a factor of ∼3 in 2019, and the 2019 light curves had higher variance than in all time periods we examined. We find that the 2020–2022 flux distribution is statistically consistent with the historical sample and model predictions, but with fewer bright measurements above 0.6 mJy at the ∼2σlevel. Since 2019, we have observed a maximumKs(2.2μm) flux of 0.9 mJy, compared to the highest pre-2019 flux of 2.0 mJy and highest 2019 flux of 5.6 mJy. Our results suggest that the 2019 activity was caused by a temporary accretion increase onto Sgr A*, possibly due to delayed accretion of tidally stripped gas from the gaseous object G2 in 2014. We also examine faint Sgr A* fluxes over a long time baseline to search for a quasi-steady quiescent state. We find that Sgr A* displays flux variations over a factor of ∼500, with no evidence for a quiescent state in the NIR.
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This content will become publicly available on February 18, 2026
Nonstop Variability of Sgr A* Using JWST at 2.1 and 4.8 μ 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 4.8μm for a total of ∼48 hr over seven different epochs in 2023 and 2024. We find correlated variability at 2.1 and 4.8μm 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 2.1μm. 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 4.8μm variability with respect to 2.1μm. 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,Ec, between 420 and 720 MeV.
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- PAR ID:
- 10591434
- Publisher / Repository:
- The Astrophysical Journal Letters
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 980
- Issue:
- 2
- ISSN:
- 2041-8205
- Page Range / eLocation ID:
- L35
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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