Black hole Spin in Sw J1644+57 and Sw J2058+05

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, possibly triggered by a tidal disruption event. 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 black holes. We attribute jet launching to the Blandford-Znajek mechanism, and use the available data to constrain the black hole spin parameter for the two events. It is found that the two black holes indeed carry a moderate to high spin, suggesting that black hole spin is likely the crucial factor behind the Sw J1644+57 - like events.