Journal
ASTROPHYSICAL JOURNAL
Volume 863, Issue 1, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.3847/1538-4357/aace62
Keywords
accretion; accretion disks; black hole physics; Galaxy: center; methods: statistical; radiation mechanisms: non-thermal
Categories
Funding
- W. M. Keck Foundation
- NSF [AST-0909218, AST-1412615, AST-1312651, AST-1333612, AST-1716327]
- National Science Foundation [ACI-1548562]
- Division Of Astronomical Sciences
- Direct For Mathematical & Physical Scien [1412615] Funding Source: National Science Foundation
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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 mu m with 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 94,929 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 tau(b) = 243(-57)(+82) 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 mu m during a low flux-density phase gave a spectral index alpha(s) = 1.6 +/- 0.1 (F-nu proportional to nu-alpha(s)). 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 nonthermal electron population from below 20 GHz through the NIR.
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