Erratic profiles of type II bursts of very low fluence from the rapid burster.

Abstract

Results are presented of approximately 17 hr of Exosat observations of the Rapid Burster in September 1985 during which about 541 type II bursts lasting less than 30 s were observed. Although these bursts are known to have an approximately time-invariant profile showing successive peaks in the burst's decay portion, most of the 20-s bursts at the extreme low end of the fluence distribution have different profiles. In these low-fluence bursts the valleys between the peaks are deeper than in bursts of higher fluence, although the location of the peaks, in comparison with the time-invariant profile, remains approximately the same. For the bursts with the lowest fluences, the profile can become erratic, showing a series of flares. The fluence distribution of the bursts and the fluence levels where the burst profile begins to become noticeably erratic are somewhat different for different data sets.