The unusual transient Swift J1644+57 likely resulted from a collimated relativistic jet , powered by the sudden onset of accretion onto a massive black hole ( BH ) following the tidal disruption ( TD ) of a star .
However , several mysteries cloud the interpretation of this event , including ( 1 ) the extreme flaring and ‘ plateau ’ shape of the X-ray/ \gamma -ray light curve during the first t - t _ { trig } \sim 10 days after the \gamma - ray trigger ; ( 2 ) unexpected rebrightening of the forward shock radio emission at t - t _ { trig } \sim months ; ( 3 ) lack of obvious evidence for jet precession , despite the misalignment typically expected between the angular momentum of the accretion disk and BH ; ( 4 ) recent abrupt shut-off in the jet X-ray emission at t - t _ { trig } \sim 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 _ { { \bullet } } threading the BH .
Just after the TD event , \Phi _ { { \bullet } } is dynamically weak relative to the high rate of fall-back accretion \dot { M } , such that the accretion disk ( jet ) freely precesses about the BH axis = our line of site .
As \dot { M } decreases , however , \Phi _ { { \bullet } } becomes dynamically important , leading to a state of ‘ magnetically-arrested ’ accretion ( MAD ) .
MAD naturally aligns the jet with the BH spin , but only after an extended phase of violent rearrangement ( jet wobbling ) , which in Swift J1644+57 starts a few days before the \gamma -ray trigger and explains the erratic early light curve .
Indeed , the entire X-ray light curve can be fit to the predicted power-law decay \dot { M } \propto t ^ { - \alpha } ( \alpha \simeq 5 / 3 - 2.2 ) if the TD occurred a few weeks prior to the \gamma -ray trigger .
Jet energy directed away from the line of site , either prior to the trigger or during the jet alignment process , eventually manifests as the observed radio rebrightening , similar to an off-axis ( orphan ) gamma-ray burst afterglow .
As suggested recently , the late X-ray shut-off occurs when the disk transitions to a geometrically-thin ( jet-less ) state once \dot { M } drops below \sim the Eddington rate .
We predict that , in several years , a transition to a low/hard state will mark a revival of the jet and its associated X-ray emission .
We use our model for Swift J1644+57 to constrain the properties of the BH and disrupted star , finding that a solar-mass main sequence star disrupted by a relatively low mass M _ { { \bullet } } \sim 10 ^ { 5 } -10 ^ { 6 } M _ { \odot } 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 much too large to be supplied by the star itself , but it could be collected from a quiescent ‘ fossil ’ accretion disk that was 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 .