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The recently discovered high-energy transient Sw J1644+57 is thought to arisefrom the tidal disruption of a passing star by a dormant massive black hole.The long-term, bright radio emission of Sw J1644+57 is believed to result fromthe synchrotron emission of the blast wave produced by an outflow expandinginto the surrounding medium. Using the detailed multi-epoch radio spectraldata, we are able to determine the total number of radiating electrons in theoutflow at different times, and further the evolution of the cross section ofthe outflow with time. We find that the outflow gradually transits from aconical jet to a cylindrical one at later times. The transition may be due tocollimation of the outflow by the pressure of the shocked jet cocoon that formswhile the outflow is propagating in the ambient medium. Since cylindrical jetsusually exist in AGNs and extragalactic jets, this may provide independentevidence that Sw J1644+57 signals the onset of an AGN.

Probing the structure of the outflow in the tidal disruption flare Sw J1644+57 with long-term radio emission