Radio Variability of Sagittarius A*—a 106 Day Cycle

We report the presence of a 106 day cycle in the radio variability of Sagittarius A* based on an analysis of data observed with the Very Large Array over the past 20 years. The pulsed signal is most clearly seen at 1.3 cm with a ratio of cycle frequency to frequency width . The periodic signal is also clearly observed at f/Df p 2.2 5 0.3 2 cm. At 3.6 cm the detection of a periodic signal is marginal. No significant periodicity is detected at both 6 and 20 cm. Since the sampling function is irregular, we performed a number of tests to ensure that the observed periodicity is not the result of noise. Similar results were found for a maximum entropy method and a periodogram with a CLEAN method. The probability of false detection for several different noise distributions is less than 5% based on Monte Carlo tests. The radio properties of the pulsed component at 1.3 cm are a spectral index a » (for ), an amplitude , and a characteristic timescale days. a 1.0 5 0.1 S / nD S p 0.42 5 0.04 Jy Dt … 25 5 5 FWHM The lack of a VLBI detection of a secondary component suggests that the variability occurs within Sgr A* on a scale of »5 AU, suggesting an instability of the accretion disk. Subject headings: accretion, accretion disks — black hole physics — galaxies: active — Galaxy: center — radio continuum: galaxies On-line material: color figures