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, nearinfrared (NIR), and Xray 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 firstorder 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}_{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 μm during a low fluxdensity phase gave a spectral index α _{ s } = 1.6 ± 0.1 ({F}_{ν }\propto {ν }^{{α }_{s}}). This value implies that the Sgr A* NIR color becomes bluer during higher fluxdensity phases. The probability densities of flux densities of the combined data sets are best fit by lognormal 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.
 Publication:

The Astrophysical Journal
 Pub Date:
 August 2018
 DOI:
 10.3847/15384357/aace62
 arXiv:
 arXiv:1806.00479
 Bibcode:
 2018ApJ...863...15W
 Keywords:

 accretion;
 accretion disks;
 black hole physics;
 Galaxy: center;
 methods: statistical;
 radiation mechanisms: nonthermal;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 Accepted for publication in ApJ on May 30, 2018. A machine readable version of the light curve data is included in the journal's online publication. Version 2 includes proof corrections