Peculiar Outbursts of an Ultra-luminous Source: Likely Signs of an Aperiodic Disk-wind

The metal-rich globular cluster RZ 2109 in the massive Virgo elliptical galaxy NGC 4472 (M49) harbors the ultra-luminous X-ray source XMMU 122939.7+075333. Previous studies showed that this source varies between bright and faint phases on timescales of just a few hours. Here, we report the discovery of two peculiar X-ray bursting events that last for about 8 and 3.5 hr separated by about 3 days. Such a recurring X-ray burst-like behavior has never been observed before. We argue that type-I X-ray bursts or super bursts as well as outburst scenarios requiring a young stellar object are highly unlikely explanations for the observed light curve, leaving an aperiodic disk-wind scenario driven by hyper-Eddington accretion as a viable explanation for this new type of X-ray flaring activity.