2024ApJ...977..228P

In this work, we measure the spectral index of Sagittarius A* (Sgr A*) between the H (1.6 μm) and $K^{\prime} $ (2.2 μm) broadband filters in the near-infrared (NIR), sampling over a factor ∼40 in brightness, the largest range probed to date by a factor ∼3. Sgr A*-NIR is highly variable, and studying the spectral index α (with F_ν ∝ ν^α^) is essential to determine the underlying emission mechanism. For example, variations in α with flux may arise from shifts in the synchrotron cutoff frequency, changes in the distribution of electrons, or multiple concurrent emission mechanisms. We investigate potential variations of ${\alpha }_{H-K^{\prime} }$ with flux by analyzing seven epochs (2005-2022) of Keck Observatory imaging observations from the Galactic Center Orbits Initiative. We remove the flux contribution of known sources confused with SgrA*-NIR, which can significantly impact color at faint flux levels. We interpolate between the interleaved H and $K^{\prime} $ observations using multi-output Gaussian processes. We introduce a flexible empirical model to quantify α variations and probe different scenarios. The observations are best fit by an ${\alpha }_{H-K^{\prime} }=-0.50\pm {0.08}_{\mathrm{stat}}\pm {0.17}_{\mathrm{sys}}$ that is constant from ∼1 mJy to ∼40 mJy (dereddened 2 μm flux). We find no evidence for a flux dependence of Sgr A*'s intrinsic spectral index. In particular, we rule out a model explaining NIR variability purely by shifts in the synchrotron cutoff frequency. We also constrain the presence of redder, quiescent emission from the black hole, concluding that the dereddened 2 μm flux contribution must be ≤0.3 mJy at 95% confidence level.