Monitoring the Variability of the Supermassive Black Hole at the Galactic Center

The variability of the supermassive black hole at the center of the Galaxy, Sgr A*, has been widely studied over the years in a variety of wavelengths. However, near-infrared studies of the variability of Sgr A* only began in 2003 with the then new technique Adaptive Optics (AO) as speckle shift-and-add data did not reach sufficient depth to detect Sgr A* (K < 16). We apply our new speckle holography approach to the analysis of data obtained between 1995 and 2005 with the speckle imaging technique (reaching K < 17) to re-examine the variability of Sgr A* in an effort to explore the Sgr A* accretion flow over a time baseline of 20 years. We find that the average magnitude of Sgr A* from 1995 to 2005 (K = 16.49 +/- 0.086) agrees very well with the average AO magnitude from 2005-2007 (Kp = 16.3). Our detections of Sgr A* are the first reported prior to 2002. In particular, a significant increase of power in the PSD between the main correlation timescale of ~300 min and 20 years can be excluded. This renders 300 min the dominant timescale and setting the variability state of Sgr A* in the time since 1995 apart from states discussed in the context of the X-ray echoes in the surrounding molecular clouds (for which extended bright periods of several years are required). Finally, we note that the 2001 periapse passage of the extended, dusty object G1, a source similar to G2, had no apparent effect on the emissivity of the accretion flow onto Sgr A*.