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

TL;DR

This paper models the radio emission from the tidal disruption event Swift J1644+57 as synchrotron radiation from a relativistic jet interacting with the surrounding medium, revealing jet properties and circumnuclear environment characteristics.

Contribution

It introduces a detailed afterglow model for the radio emission, linking observed light curve features to jet dynamics and circumnuclear medium properties, providing new insights into tidal disruption events.

Findings

Radio light curve break explained by jet evolution transition.

Circumnuclear medium consistent with wind-like density profile.

Jet collimation and energy parameters constrained by radio data.

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.