@article{57ac6fa01ef44600a5ba33caee4af04a,
title = "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",
abstract = "Sagittarius A∗ (Sgr A∗) is the variable radio, near-infrared (NIR), and X-ray source associated with accretion onto the Galactic center black hole. We present an analysis of the most comprehensive NIR variability data set of Sgr A∗ to date: eight 24 hr epochs of continuous monitoring of Sgr A∗ at 4.5 μm with the IRAC instrument on the Spitzer Space Telescope, 93 epochs of 2.18 μm data from Naos Conica at the Very Large Telescope, and 30 epochs of 2.12 μm data from the NIRC2 camera at the Keck Observatory, in total 94,929 measurements. A new approximate Bayesian computation method for fitting the first-order structure function extracts information beyond current fast Fourier transformation (FFT) methods of power spectral density (PSD) estimation. With a combined fit of the data of all three observatories, the characteristic coherence timescale of Sgr A∗ is minutes (90% credible interval). The PSD has no detectable features on timescales down to 8.5 minutes (95% credible level), which is the ISCO orbital frequency for a dimensionless spin parameter a = 0.92. One light curve measured simultaneously at 2.12 and 4.5 μm during a low flux-density phase gave a spectral index α s = 1.6 ± 0.1 (). This value implies that the Sgr A∗ NIR color becomes bluer during higher flux-density phases. The probability densities of flux densities of the combined data sets are best fit by log-normal distributions. Based on these distributions, the Sgr A∗ spectral energy distribution is consistent with synchrotron radiation from a non-thermal electron population from below 20 GHz through the NIR.",
keywords = "Galaxy: center, accretion, accretion disks, black hole physics, methods: statistical, radiation mechanisms: non-thermal",
author = "G. Witzel and G. Martinez and J. Hora and Willner, {S. P.} and Morris, {M. R.} and C. Gammie and Becklin, {E. E.} and Ashby, {M. L.N.} and F. Baganoff and S. Carey and T. Do and Fazio, {G. G.} and A. Ghez and Glaccum, {W. J.} and D. Haggard and R. Herrero-Illana and J. Ingalls and R. Narayan and Smith, {H. A.}",
note = "Funding Information: We thank the anonymous referee for helpful comments. We thank Arno Witzel, Rainer Sch{\"o}del, Andreas Eckart, Dan Marrone, Stefan Gillessen, Matthew Malkan, Aurelien Hees, Zhiyuan Li, Leo Meyer, and Silke Britzen for fruitful discussions. We thank Jean Turner for giving us access to her UNIX server for debugging our C++ code. We thank Nick Robertson for his excellent IT support. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. We thank the staff of the Spitzer Science Center for their help in planning and executing these demanding observations. The W. M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The authors wish to recognize that the summit of Mauna Kea has always held a very significant cultural role for the indigenous Hawaiian community. We are most fortunate to have the opportunity to observe from this mountain. The observatory was made possible by the generous financial support of the W. M. Keck Foundation. Support for this work was provided by NSF grants AST-0909218, AST-1412615. The Keck observations were conducted in the framework of the UCLA Galactic Center Orbits Initiative. R.N. was supported by the NSF grant AST-1312651. C.F.G. is supported by NSF grant AST-1333612 and AST-1716327. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1548562. The XSEDE allocation IDs are TG-AST170006 and TG-AST080026N. The computations were executed on the clusters Stampede, Comet, Bridges, and SuperMIC. This work used the UCLA Hoffman2 cluster. Publisher Copyright: {\textcopyright} 2018. The American Astronomical Society. All rights reserved.",
year = "2018",
month = aug,
day = "10",
doi = "10.3847/1538-4357/aace62",
language = "English (US)",
volume = "863",
journal = "Astrophysical Journal",
issn = "0004-637X",
publisher = "IOP Publishing Ltd.",
number = "1",
}