New Evidence for a Flux-independent Spectral Index of Sgr A in the Near-infrared

In this work, we measure the spectral index of Sagittarius A* (Sgr A*) between the H (1.6 μm) and K′ (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 Fν ∝ να) 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 αH−K′ 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′ 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 αH−K′=−0.50±0.08stat±0.17sys 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.