A near-IR variability study of the Galactic black hole: a red noise source with no detected periodicity

TL;DR

This study monitored Sgr A*-IR in the near-infrared over multiple nights, finding its variability consistent with red noise and detecting no significant periodicity, challenging previous claims of quasi-periodic signals.

Contribution

The paper provides the first extensive near-infrared variability analysis of Sgr A*-IR with high time resolution, showing no evidence of periodicity and characterizing its red noise behavior.

Findings

Sgr A*-IR exhibits continuous variability consistent with red noise.

No significant periodicity detected at any timescale in the data.

Variability amplitude sensitivity threshold is about 20% for >2-hour light curves.

Abstract

We present the results of near-infrared (2 and 3 microns) monitoring of Sgr A*-IR with 1 min time sampling using the natural and laser guide star adaptive optics (LGS AO) system at the Keck II telescope. Sgr A*-IR was observed continuously for up to three hours 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 micron flux density of 0.192 mJy, corresponding to 16.3 magnitude at K'. These observations allow us to investigate Nyquist sampled periods ranging from about 2 minutes to an hour. 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 to 3.0, consistent with those reported for AGN light curves. Of particular interest are periods of ~20 min, corresponding to a quasi-periodic signal claimed based upon previous near-infrared 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 time scale 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 duration greater than ~2 hours at a 98% confidence limit.