Afterglow Model for the Radio Emission from the Jetted Tidal Disruption Candidate Swift J1644+57

Abstract: The recent transient event Swift J1644+57 has been interpreted as emission from a collimated relativistic jet, powered by the sudden onset of accretion onto a supermassive black hole following the tidal disruption of a star. Here we model the radio-microwave emission as synchrotron radiation produced by the shock interaction between the jet and the gaseous circumnuclear medium (CNM). At early times after the onset of the jet (t < 5-10 days) a reverse shock propagates through and decelerates the ejecta, while at later times the outflow approaches the Blandford-McKee self-similar evolution (possibly modified by additional late energy injection). The achromatic break in the radio light curve of J1644+57 is naturally explained as the transition between these phases. We show that the temporal indices of the pre- and post-break light curve are consistent with those predicted if the CNM has a wind-type radial density profile n ~ 1/r^2. The observed synchrotron frequencies and self-absorbed flux constrain the fraction of the post-shock thermal energy in relativistic electrons epsilon_e ~ 0.03-0.1; the CNM density at 1e18 cm ~ 1-10 1/cm^3; and the initial Lorentz factor Gamma_j ~ 10-20 and opening angle theta_j ~ (0.1-1)Gamma_j^-1 ~ 0.01-0.1 of the jet. Radio modeling thus provides robust independent evidence for a narrowly collimated outflow. Extending our model to the future evolution of J1644+57, we predict that the radio flux at low frequencies (<~ few GHz) will begin to brighten more rapidly once the characteristic frequency crosses below the self-absorption frequency on a timescale of months (indeed, such a transition may already have begun). Our results demonstrate that relativistic outflows from tidal disruption events provide a unique probe of the conditions in distant, previously inactive galactic nuclei, complementing studies of normal AGN.