SWIFT J164449.3+573451: A plunging event with a poynting-flux-dominated outflow

Swift J164449+573451 is a peculiar outburst which is most likely powered by the tidal disruption of a star by a massive black hole. Within the tidal disruption scenario, we show that the periastron distance is considerably smaller than the disruption radius and the outflow should be launched mainly via magnetic activities ( e. g., the Blandford-Znajek process), otherwise the observed long-lasting X-ray afterglow emission satisfying the relation L-X alpha (M) over dot cannot be reproduced, where L-X is the X-ray luminosity and (M) over dot is the accretion rate. We also suggest that L-X alpha (M) over dot may hold in the quick decline phase of gamma-ray bursts.