Optimized Radio Follow Up of Binary Neutron-Star Mergers

TL;DR

This paper identifies optimal radio observation strategies for detecting and characterizing electromagnetic counterparts of binary neutron-star mergers, emphasizing the importance of timing and sensitivity for accurate property reconstruction.

Contribution

It presents a simulation-based framework to optimize radio follow-up observations of BNS mergers, considering various viewing angles and ISM densities, and discusses future improvements with next-generation radio arrays.

Findings

Radio observations are optimal for exploring fast ejecta in low-density ISM.

Proper timing of observations enables accurate reconstruction of merger properties.

Next-generation radio arrays will significantly enhance BNS merger detectability.

Abstract

Motivated by the recent discovery of the binary neutron-star (BNS) merger GW170817, we determine the optimal observational setup for detecting and characterizing radio counterparts of nearby ($d_L\sim40$\,Mpc) BNS mergers. We simulate GW170817-like radio transients, and radio afterglows generated by fast jets with isotropic energy $E_{\rm iso}\sim 10^{50}$\,erg, expanding in a low-density interstellar medium (ISM; $n_{\rm ISM}=10^{-4}-10^{-2}$\,cm$^{-3}$), observed from different viewing angles (from slightly off-axis to largely off-axis). We then determine the optimal timing of GHz radio observations following the precise localization of the BNS radio counterpart candidate, assuming a sensitivity comparable to that of the Karl G. Jansky Very Large Array. The optimization is done so as to ensure that properties such as viewing angle and circumstellar density can be correctly reconstructed with the minimum number of observations. We show that radio is the optimal band to explore the fastest ejecta from BNSs in low-density ISM, since the optical emission is likelyto be dominated by the so-called `kilonova' component, while X-rays from the jet are detectable only for a small subset of the BNS models considered here. Finally, we discuss how future radio arrays like the next generation VLA (ngVLA) would improve the detectability of BNS mergers with physical parameters similar to the ones here explored.