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Shock within a shock: revisiting the radio flares of NS merger ejecta and GRB-supernovae

Fast ejecta expelled in binary neutron star (NS) mergers or energetic supernovae (SNe) should produce late-time synchrotron radio emission as the ejecta shocks into the surrounding ambient medium. Models for such radio flares typically assume the ejecta expands into an unperturbed interstellar medium (ISM). However, it is also well-known that binary NS mergers and broad-lined Ic SNe can harbor relativistic jetted outflows. In this work, we show that such jets shock the ambient ISM ahead of the ejecta, thus evacuating the medium into which the ejecta subsequently collides. Using an idealized spherically-symmetric model, we illustrate that this inhibits the ejecta radio flare at early times $t < t_{\rm col} \approx 12 \, {\rm yr} \, (E_{\rm j}/10^{49} \, {\rm erg})^{1/3} (n/1 \, {\rm cm}^{-3})^{-1/3} (v_{\rm ej}/0.1c)^{-5/3}$ where $E_{\rm j}$ is the jet energy, $n$ the ISM density, and $v_{\rm ej}$ the ejecta velocity. We also show that this can produce a sharply peaked enhancement in the light-curve at $t = t_{\rm col}$. This has implications for radio observations of GW170817 and future binary NS mergers, gamma-ray burst (GRB) SNe, decade-long radio transients such as FIRST J1419, and possibly other events where a relativistic outflow precedes a slower-moving ejecta. Future numerical work will extend these analytic estimates and treat the multi-dimensional nature of the problem.