Afterglow model for the radio emission from the jetted tidal disruption candidate SwiftJ1644+57

Abstract

The recent transient event SwiftJ1644+57 has been interpreted as emission from a collimated relativistic jet, powered by the sudden onset of accretion on to 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≲ 5-10d) 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 SwiftJ1644+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∝r -2. The observed synchrotron frequencies and self-absorbed flux constrain the fraction of the post-shock thermal energy in relativistic electrons ε e≈ 0.03-0.1, the CNM density at 10 18cm n 18≈ 1-10cm -3 and the initial Lorentz factor Γ j≈ 10-20 and opening angle of the jet. Radio modelling thus provides robust independent evidence for a narrowly collimated outflow. Extending our model to the future evolution of SwiftJ1644+57, we predict that the radio flux at low frequencies (ν≲ few GHz) will begin to brighten more rapidly once the characteristic frequency ν m crosses below the radio band after it decreases below the self-absorption frequency on a time-scale 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 active galactic nuclei. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.