Soft Gamma Repeater 1806-20: Weak Evidence for a Periodicity following the Bursts

The study of soft gamma repeaters began with the detection of the soft gamma-ray burst of 1979 March 5 that was followed by long-lasting 8 s pulsations. Later weaker bursts were detected from the same source, although they did not exhibit detectable pulsations. Here we analyze data from the UCB/Los Alamos experiment on the International Cometary Explorer (ICE) of a different soft gamma repeater, SGR 1806-20. A search for periodicities in data following the bursts showed a signal at 2.8 s. The probability that a periodic signal of this size would occur by chance somewhere in the power spectrum is ∼1.3%. The detected amplitude is ∼25 times weaker than what would result if the SGR 1806-20 bursts were simply scaled-down replicas of the March 5th event.