Detectability of electromagnetic counterparts from neutron star mergers: prompt emission versus afterglow

Abstract

ABSTRACT Electromagnetic observations of the first binary neutron star (BNS) merger detected in gravitational waves, GW170817, have established that relativistic jets can be successfully launched in BNS mergers. Typically, such jets produce emission in two phases: γ-ray prompt emission and multiwavelength afterglow. Because of relativistic beaming and the jet’s angular structure, the detectability of both these counterparts is dependent on the angle (θ${\rm v}$) between the observer’s line of sight and the jet axis. We compare the detectability of prompt and afterglow emission from off-axis jets, assuming standard detector thresholds such as that of Fermi Gamma-ray Burst Monitor (GBM), Chandra, and Jansky Very Large Array (VLA). We find that for top-hat jets, afterglow is a more likely counterpart than the prompt emission even with unfavourable afterglow parameters. For structured jets with a Gaussian profile, prompt emission is more promising than the afterglows at extreme viewing angles, under the assumption that the total energy emitted in the prompt phase equals the kinetic energy of the outflow. Assuming a Gaussian jet profile, we forecast the population of γ-ray detections and find that extreme viewing angle events like GRB 170817A will be rare. In our simulated sample, the observed isotropic equivalent energy in γ-rays is moderately correlated with the viewing angle, such that a low Eiso,γ is almost always associated with a high off-axis viewing angle.