Anisotropic Ejecta from Binary Neutron Star Mergers: Self-Consistent Main Thermal and Late-Time Radio Emission of NS-Powered Kilonovae

Abstract

The interaction between the fast-moving ejecta and the interstellar medium can produce long-lasting radio signals after binary neutron star mergers. Searching for such radio signals is a way to test the central engine of kilonovae and short gamma-ray bursts. With a magnetar as the central engine, the spin-down energy powers the main thermal and late-time radio emissions of the kilonova. However, both the thermal and radio emissions are strongly affected by the ejecta distribution, e.g., the two-component ``blue" and ``red" emissions of AT 2017gfo corresponding to the GW 170817 event. In this study, we investigate the distribution of the merger ejecta, analyzing several possible anisotropic distributions and demonstrating their impacts on the emission properties, particularly the late-time radio light curves. Under a bipolar and equatorial ejecta configuration, corresponding to the wind and dynamical components of the merger ejecta, the late-time radio light curves reveal distinct two-peak features, which are consistent with the main thermal light curves. The anisotropic distribution of the ejecta intrinsically connects the main thermal and late-time radio emissions, forming a self-consistent evolutionary picture. A combined analysis of the main thermal and late-time radio emissions provides a way to constrain the geometry of the merger ejecta and to probe the properties of the central engine. Furthermore, using the fitting parameters from the main thermal emission of AT 2017gfo, we calculate the corresponding potential late-time radio light curves. The results show that, under typical parameters, the non-detection of radio signals in observations is consistent with the theoretical expectation.