A Near-Infrared Variability Study of the Galactic Black Hole: A Red Noise Source With No Detected Periodicity

We present the results of near-infrared (NIR; 2 and 3 μm) monitoring of Sgr A*-IR with 1 minute time sampling using the natural and laser guide star adaptive optics system at the Keck II telescope.Sgr A*-IR was observed continuously for up to 3 hr on each of seven nights, between 2005 July and 2007 August.Sgr A*-IR is detected at all times and is continuously variable, with a median observed 2 μm flux density of 0.192 mJy, corresponding to 16.3 mag at K .These observations allow us to investigate Nyquist sampled periods ranging from about 2 minutes to 1 hr.Using Monte Carlo simulations, we find that the variability of Sgr A* in this data set is consistent with models based on correlated noise with power spectra having frequency-dependent power-law slopes between 2.0 and 3.0, consistent with those reported for active galactic nucleus light curves.Of particular interest are periods of ∼ 20 minutes, corresponding to a quasiperiodic signal claimed based upon previous NIR observations and interpreted as the orbit of a "hot spot" at or near the last stable orbit of a spinning black hole.We find no significant periodicity at any timescale probed in these new observations for periodic signals.This study is sensitive to periodic signals with amplitudes greater than 20% of the maximum amplitude of the underlying red noise component for light curves with a duration greater than ∼ 2 hr at a 98% confidence limit.