Possible Periodic Windowed Behavior in SGR1935+2154 Bursts

Abstract

It has been proposed that repeating fast radio burst (FRB) sources, FRB 20180916B and FRB 20121102A display periodic windowed behavior (PWB) in the times of FRB detections. In PWB, events occur only within a periodic activity window spanning a fixed fraction of the period, but no events might be detected during many periods. During UT 2020 April 28, two peaks of a soft gamma burst (detected with Insight-HXMT) were identified with two FRB peaks from soft gamma repeater (SGR) 1935+2154. As SGRs are a type of magnetar, a neutron star with an extremely high magnetic field, ∼1014 G, these observations establish a link between at least some FRBs and magnetars. The analysis herein shows possible PWB in SGR 1935+2154's soft gamma-ray burst times. For 161 bursts from 2014 through 2020 from IPN (Interplanetary Network) instruments, a resolved minimum in active window fraction versus period occurs at a 231 ± 9.3 day period, at a 55% active window fraction. The data cover only 6 bursting episodes, however, the periodicity result appears relatively robust: First, the IPN has excellent time coverage and is unlikely to miss bright bursts outside of active windows. Second, for various data subsets (with significantly less bursts), either the best or second best measured period was consistent with 231 days, even in the sub-sample with the richest bursting episode removed. In addition, simulations show that even small numbers of uniform random bursts do not show PWB, and that the results are not due to limited sampling. The appearance of PWB could have alternative explanations, including a non-uniform distribution of burst times; a firm conclusion of PWB would be evident, however, after the observation of more episodes consistent (within a small phase margin) with the given windows. If the periodicity of this SGR’s bursts can be verified, and if similar behavior were observed in other repeating FRB sources, it might suggest a linkage between FRB and SGR burst mechanisms or emission conditions.