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

Abstract In this work, we measure the spectral index of Sagittarius A* (Sgr A*) between the H (1.6 μ m) and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>K</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:math> (2.2 μ m) 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>H</mml:mi> <mml:mo>−</mml:mo> <mml:mi>K</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:mrow> </mml:msub> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>K</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>H</mml:mi> <mml:mo>−</mml:mo> <mml:mi>K</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>0.50</mml:mn> <mml:mo>±</mml:mo> <mml:msub> <mml:mrow> <mml:mn>0.08</mml:mn> </mml:mrow> <mml:mrow> <mml:mi>stat</mml:mi> </mml:mrow> </mml:msub> <mml:mo>±</mml:mo> <mml:msub> <mml:mrow> <mml:mn>0.17</mml:mn> </mml:mrow> <mml:mrow> <mml:mi>sys</mml:mi> </mml:mrow> </mml:msub> </mml:math> that is constant from ∼1 mJy to ∼40 mJy (dereddened 2 μ 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 μ m flux contribution must be ≤0.3 mJy at 95% confidence level.