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

TL;DR

This study measures the near-infrared spectral index of Sgr A* across a wide brightness range, finding it remains constant and independent of flux, thus constraining emission models and the contribution of quiescent emission.

Contribution

It provides the first extensive analysis showing the spectral index of Sgr A* is flux-independent over a large brightness range, challenging models based on synchrotron cutoff shifts.

Findings

Spectral index α ≈ -0.50 remains constant from 1 to 40 mJy.

No evidence of flux-dependent spectral index variations.

Limits on quiescent emission contribution at 2 μm are ≤ 0.3 mJy.

Abstract

In this work, we measure the spectral index of Sagittarius A* (Sgr A*) between the $H$ (1.6 $\mu$m) and $K^\prime$ (2.2 $\mu$m) broadband filters in the near-infrared (NIR), sampling over a factor $\sim 40$ in brightness, the largest range probed to date by a factor $\sim 3$. Sgr A*-NIR is highly variable, and studying the spectral index $\alpha$ (with $F_\nu \propto \nu^{\alpha}$) is essential to determine the underlying emission mechanism. For example, variations in $\alpha$ 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 7 epochs (2005 to 2022) of Keck Observatory imaging observations from the Galactic Center Orbits Initiative (GCOI). We remove the flux contribution of known sources confused with Sgr A*-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 $\alpha$ variations and probe different scenarios. The observations are best fit by an $\alpha_{H-K^\prime} = - 0.50 \pm 0.08 _{\rm stat} \pm 0.17_{\rm sys}$ that is constant from $\sim 1$ mJy to $\sim 40$ mJy (dereddened 2 $\mu$m 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 $\mu$m flux contribution must be $\leq 0.3$ mJy at 95% confidence level.