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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

Witzel, G. and Martinez, G. and Hora, J. and Willner, S. P. and Morris, M. R. and Gammie, C. and Becklin, E. E. and Ashby, M. L. N. and Baganoff, F. and Carey, S. and Do, T. and Fazio, G. G. and Ghez, A. and Glaccum, W. J. and Haggard, D. and Herrero-Illana, R. and Ingalls, J. and Narayan, R. and Smith, H. A. (2018) 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. Astrophysical Journal, 863 (1). Art. No. 15. ISSN 1538-4357.

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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 τ_b = 243(+82)_(-57) 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 (F_ν ∝ ν^(αs)) (). 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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Witzel, G.0000-0003-2618-797X
Martinez, G.0000-0002-7476-2521
Hora, J.0000-0002-5599-4650
Willner, S. P.0000-0002-9895-5758
Morris, M. R.0000-0002-6753-2066
Gammie, C.0000-0001-7451-8935
Ashby, M. L. N.0000-0002-3993-0745
Carey, S.0000-0002-0221-6871
Do, T.0000-0001-9554-6062
Fazio, G. G.0000-0002-0670-0708
Ghez, A.0000-0003-3230-5055
Haggard, D.0000-0001-6803-2138
Ingalls, J.0000-0003-4714-1364
Narayan, R.0000-0002-1919-2730
Additional Information:© 2018 The American Astronomical Society. Received 2018 March 9; revised 2018 May 17; accepted 2018 May 30; published 2018 August 7. We thank the anonymous referee for helpful comments. We thank Arno Witzel, Rainer Schödel, 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. Software: Astropy (Astropym Collaboration:Robitaille et al.2013), Matplotlib (Hunter 2007), AIROPA (Witzel et al. 2016), StarFinder (Diolaiti et al. 2000), FFTW (Frigo & Johnson 2005), NumPy (Jones et al. 2001), Stan (Carpenter et al. 2017), (Foreman-Mackey 2016). Facilities: Spitzer/IRAC - , Keck/NIRC2 - , VLT/NaCo. -
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
W. M. Keck FoundationUNSPECIFIED
Subject Keywords:accretion, accretion disks – black hole physics – Galaxy: center – methods: statistical – radiation mechanisms: non-thermal
Issue or Number:1
Record Number:CaltechAUTHORS:20180809-102954902
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Official Citation:G. Witzel et al 2018 ApJ 863 15
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:88689
Deposited By: Tony Diaz
Deposited On:09 Aug 2018 17:55
Last Modified:04 Nov 2019 19:17

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