Statistical properties of the repeating fast radio burst source FRB 121102

Currently, FRB 121102 is the only fast radio burstsource that was observed to give out burstsrepeatedly. It shows a high repeating rate, with more than one hundred burstsbeing spotted, but with no obvious periodicity in the activities. Thanks to its repetition, the source was well localized with a subarcsecond accuracy, leading to a redshift measurement of about 0.2. FRB 121102 is a unique source that can help us understand the enigmatic nature of fast radio bursts. In this study, we analyze the characteristics of the waiting times between burstsfrom FRB 121102. It is found that there is a clear bimodal distribution for the waiting times. While most waiting times cluster at several hundred seconds, a small portion of the waiting times are strikingly in the range of 2--40 millisecond. More interestingly, it is found that the waiting time does not correlate with the burstintensity, either for the preceding burstor for the subsequent burst. It strongly indicates that the repeating burstsshould be generated by some external mechanisms, but not internal mechanisms. As a result, the models involving collisions between small bodies and neutron stars could be competitive mechanisms for this interesting source.