Constraining the Ejecta for the Nonthermal Emission from GW 170817

Abstract

We consider a simple model for the nonthermal emission from GW 170817, in which a quasi-spherical ejecta is released in the merger event, with the kinetic energy distributed over the momentum as E (> γβ ) ∝ ( γβ ) − k . The ejecta drives a shock into the medium and gives rise to synchrotron radiation. Using multiband observations, we constrain k to be ≈6.7, the total kinetic energy to be (0.3–5) × 10 51 erg (assuming the density of the medium to be ∼10 −2 cm −3 and the postshock magnetic field to carry a fraction 10 −5 –10 −3 of the postshock internal energy), the slowest ejecta velocity to be ∼(0.7–0.8) c , and the fastest ejecta to have a Lorentz factor of ∼4–7. We conclude that the subrelativistic dynamical ejecta responsible for the kilonova cannot produce nonthermal emission. The coexistence of nonthermal and thermal kilonova emission implies that two simultaneous ejecta are ejected at different angles.