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.
