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Sagittarius A* (Sgr A*) is the variable radio, near-infrared (NIR), and X-raysource associated with accretion onto the Galactic center black hole. Wepresent an analysis of the most comprehensive NIR variability dataset of Sgr A*to date: eight 24-hour epochs of continuous monitoring of Sgr A* at 4.5 $\mu$mwith the IRAC instrument on the Spitzer Space Telescope, 93 epochs of 2.18$\mu$m data from Naos Conica at the Very Large Telescope, and 30 epochs of 2.12$\mu$m data from the NIRC2 camera at the Keck Observatory, in total 94,929measurements. A new approximate Bayesian computation method for fitting thefirst-order structure function extracts information beyond current Fast FourierTransformation (FFT) methods of power spectral density (PSD) estimation. With acombined fit of the data of all three observatories, the characteristiccoherence timescale of Sgr A* is $\tau_{b} = 243^{+82}_{-57}$ minutes ($90\%$credible interval). The PSD has no detectable features on timescales down to8.5 minutes ($95\%$ credible level), which is the ISCO orbital frequency for adimensionless spin parameter $a = 0.92$. One light curve measuredsimultaneously at 2.12 and 4.5 $\mu$m during a low flux-density phase gave aspectral index $\alpha_s = 1.6 \pm 0.1$ ($F_\nu \propto \nu^{-\alpha_s}$). Thisvalue implies that the Sgr A* NIR color becomes bluer during higherflux-density phases. The probability densities of flux densities of thecombined datasets are best fit by log-normal distributions. Based on thesedistributions, the Sgr A* spectral energy distribution is consistent withsynchrotron radiation from a non-thermal electron population from below 20 GHzthrough the NIR.

astrophysical journal/the astrophysical journal

Variability Timescale and Spectral Index of Sgr A* in the Near Infrared:  Approximate Bayesian Computation Analysis of the Variability of the Closest  Supermassive Black Hole