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 f/Deltaf = 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 alpha similar to 1.0 +/- 0.1 (for S proportional to nu (alpha)), an amplitude DeltaS = 0.42 +/- 0.04 Jy, and a characteristic timescale Deltat(FWHM) approximate to 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 similar to5 AU, suggesting an instability of the accretion disk.