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

Creators: Do, T.; Ghez, A. M.; Morris, M. R.; Yelda, S.; Meyer, L.; Lu, J. R.; Hornstein, S. D.; Matthews, K.

Abstract

We present the results of near-infrared (NIR; 2 and 3 μ) 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 μ 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.

Additional Information

© 2009. The American Astronomical Society. Received 2008 May 31; accepted 2008 October 2; published 2009 January 28. Print publication: Issue 2 (2009 February 1). The authors thank the staff of the Keck observatory, especially Randy Campbell, David LeMignant, Jim Lyke, Steven McGee, and Hien Tran for all their help in obtaining the new observations and Guillaume Bélanger and Matt Malkan for helpful discussions. Support for this work was provided by NSF grant AST-0406816 and the NSF Science & Technology Center for AO, managed by UCSC (AST-9876783), and the Levine-Leichtman Family Foundation. The IR data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors recognize and acknowledge the very significant cultural role that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facilities: Keck:II (NIRC2).

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