2011ApJ...740L..27L

Recently, a hard X-ray transient event, Sw J1644+57, was discovered by the Swift satellite. It likely marks the onset of a relativistic jet from a supermassive black hole (BH), possibly triggered by a tidal disruption event (TDE). Another candidate in the same category, Sw J2058+05, was also reported. The low event rate suggests that only a small fraction of TDEs launch relativistic jets. A common speculation is that these rare events are related to rapidly spinning BHs. We attribute jet launching to the Blandford-Znajek mechanism and use the available data to constrain the BH spin parameter for the two events. It is found that the two BHs indeed carry a moderate to high spin, suggesting that BH spin is likely the crucial factor behind the Sw J1644+57-like events.