Probing Binary Neutron Star Merger Ejecta and Remnants with Gravitational Wave and Radio Observations

We present a study aimed at quantifying the detectability of radio counterparts of binary neutron star (BNS) mergers with total masses $\lesssim 3$\,M$_{\odot}$, which may form neutron star (NS) remnants. We focus on mergers localized by gravitational-wave (GW) observations to sky areas $\lesssim 10$\,deg$^2$, a precision that greatly facilitates optical counterpart identification and enables radio discovery even without detections at other wavelengths. Widely separated GW detectors are essential for building samples of well-localized BNS mergers accessible to US-based radio telescopes, with minimum yearly detection rates (assuming the smallest values of the BNS local merger rate) ranging from a few with current GW detectors to hundreds with next-generation GW instruments. Current GW networks limit well-localized detections to $z\lesssim 0.2$, while next-generation GW detectors extend the reach to $z\lesssim 0.8$, encompassing the median redshift of short gamma-ray bursts (GRBs). With next-generation radio arrays operating at a several tens of GHz and providing an order of magnitude improvement in sensitivity compared to the most sensitive ones available today, short GRB-like jet afterglows can be detected for a large fraction of the considered BNS mergers. At lower radio frequencies, detections with current radio interferometric arrays are feasible, though subject to synchrotron self-absorption effects. The enhanced sensitivity and survey speed of future radio interferometers operating at a few GHz, combined with the higher detection rate of well-localized BNSs enabled by next-generation GW observatories, are key to probing disk-wind and dynamical ejecta afterglows, as well as remnant diversity.