Radio Variability of Sagittarius A * - A 106 Day Cycle

We report the presence of a 106-day cycle in the radio variability of Sgr A* based on an analysis of data observed with the Very Large Array (VLA) 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 /∆f = 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 insure that the observed periodicity is not the result of noise. Similar results were found for a maximum entropy method and periodogram with 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 spectral index α ∼ 1.0 ± 0.1 (for S ∝ ν α), amplitude ∆S = 0.42 ± 0.04 Jy and characteristic time scale ∆t F W HM ≈ 25 ± 5 days. The lack of 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.