Recurrent population 2 X-ray transients - similarities to SU UMa cataclysmic variables

Quasi-periodic outburst activity is not uncommon among Population 2 X-ray binaries. This paper reports observations of such activity in several sources, made by the Vela 5B X-ray monitor. Typical periods are 1/2 to 2 years with an rms scatter in interval time of approx. 10%. This activity is reminiscent of the superoutburst cycles of SU UMa CV's with respect to mean recurrence times, the variation of the recurrence times about their mean, and the total mass transferred during outburst. However, the outbursts in the X-ray sources have substantially longer duration, 50 to 100 days instead of approx. 10 days. The author suggests that SU UMa and X-ray transient outbursts may be caused by similar mass-transfer instabilities.