  The unusual transient Swift J1644+57 likely resulted from a collimated
relativistic jet powered by accretion onto a massive black hole (BH) following
the tidal disruption (TD) of a star. Several mysteries cloud the interpretation
of this event: (1) extreme flaring and `plateau' shape of the X-ray/gamma-ray
light curve during the first 10 days after the gamma-ray trigger; (2)
unexpected rebrightening of the forward shock radio emission months after
trigger; (3) no obvious evidence for jet precession, despite misalignment
typically expected between the angular momentum of the accretion disk and BH;
(4) recent abrupt shut-off in jet X-ray emission after 1.5 years. Here we show
that all of these seemingly disparate mysteries are naturally resolved by one
assumption: the presence of strong magnetic flux Phi threading the BH.
Initially, Phi is weak relative to high fall-back mass accretion rate, Mdot,
and the disk and jets precess about the BH axis = our line of sight. As Mdot
drops, Phi becomes dynamically important and leads to a magnetically-arrested
disk (MAD). MAD naturally aligns disk and jet axis along the BH spin axis, but
only after a violent rearrangement phase (jet wobbling). This explains the
erratic light curve at early times and the lack of precession at later times.
We use our model for Swift J1644+57 to constrain BH and disrupted star
properties, finding that a solar-mass main sequence star disrupted by a
relatively low mass, M~10^5-10^6 Msun, BH is consistent with the data, while a
WD disruption (though still possible) is disfavored. The magnetic flux required
to power Swift J1644+57 is too large to be supplied by the star itself, but it
could be collected from a quiescent `fossil' accretion disk present in the
galactic nucleus prior to the TD. The presence (lack of) of such a fossil disk
could be a deciding factor in what TD events are accompanied by powerful
jets.[abridged]
