2001ApJ...547L..29Z

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 f/Δf=2.2±0.3. The periodic signal is also clearly observed at 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 α∼1.0±0.1 (for S∝ν^α), an amplitude ΔS=0.42±0.04 Jy, and a characteristic timescale Δt_FWHM~25±5 days. 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.