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Abstract We present the first estimate of the intrinsic binary fraction of young stars across the central ≈0.4 pc surrounding the supermassive black hole (SMBH) at the Milky Way Galactic center (GC). This experiment searched for photometric variability in 102 spectroscopically confirmed young stars, using 119 nights of 10″ wide adaptive optics imaging observations taken at W. M. Keck Observatory over 16 yr in the $K\prime$-[2.1μm] andH-[1.6μm] bands. We photometrically detected three binary stars, all of which are situated more than 1″ (0.04 pc) from the SMBH and one of which, S2-36, is newly reported here with spectroscopic confirmation. All are contact binaries or have photometric variability originating from stellar irradiation. To convert the observed binary fraction into an estimate of the underlying binary fraction, we determined the experimental sensitivity through detailed light-curve simulations, incorporating photometric effects of eclipses, irradiation, and tidal distortion in binaries. The simulations assumed a population of young binaries, with stellar ages (4 Myr) and masses matched to the most probable values measured for the GC young star population, and underlying binary system parameters (periods, mass ratios, and eccentricities) similar to those of local massive stars. As might be expected, our experimental sensitivity decreases for eclipses narrower in phase. The detections and simulations imply that the young, massive stars in the GC have a stellar binary fraction ≥71% (68% confidence), or ≥42% (95% confidence). This inferred GC young star binary fraction is consistent with that typically seen in young stellar populations in the solar neighborhood. Furthermore, our measured binary fraction is significantly higher than that recently reported by Chu et al. based on radial velocity measurements for stars ≲1″ of the SMBH. Constrained with these two studies, the probability that the same underlying young star binary fraction extends across the entire region is <1.4%. This tension provides support for a radial dependence of the binary star fraction, and therefore, for the dynamical predictions of binary merger and evaporation events close to the SMBH. 

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 maximumK_s(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.