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Sw 1644+57/GRB 110328A: the physical origin and the composition of the relativistic outflow

Sw 1644+57/GRB 110328A is a remarkable cosmological X-ray outburst detected by the {\it Swift} satellite. Its early-time ($t\lesssim 0.1$ days since the trigger) X-ray emission resembles some gamma-ray bursts (GRBs), e.g., GRB 090417B. But the late-time flaring X-ray plateau lasting $> 40$ days renders it unique. We examine the possibilities that the outburst is a super-long GRB powered either by the fallback accretion onto a nascent black hole or by a millisecond pulsar, and find out that these two scenarios can address some but not all of the main observational features. We then focus on the model of tidal disruption of a (giant) star by a massive black hole. The mass of the tidal-disrupted star is estimated to be $\gtrsim$ a few solar masses. A simple/straightforward argument for a magnetic origin of the relativistic outflow is presented.