Detectability of Electromagnetic counterparts from Neutron Star mergers: prompt emission vs afterglow

TL;DR

This paper compares the detectability of prompt gamma-ray emission and afterglow from neutron star merger jets, revealing that afterglows are generally more detectable for top-hat jets, while prompt emission can dominate at extreme viewing angles in structured jets.

Contribution

It provides a comparative analysis of prompt and afterglow detectability for different jet structures and forecasts the rarity of off-axis gamma-ray detections like GRB170817A.

Findings

Afterglow detection is more likely than prompt emission for top-hat jets.

Prompt emission can be more promising at extreme viewing angles in structured jets.

Off-axis gamma-ray events like GRB170817A are predicted to be rare.

Abstract

Electromagnetic observations of the first binary Neutron Star (BNS) merger GW170817 has established that relativistic jets can be successfully launched in BNS mergers. Typically, such jets produce emission in two phases: $\gamma$-ray prompt emission and multi-wavelength afterglow. Due to relativistic de-boosting, the detectability of both these counterparts is dependent on the angle ($\theta_v$) 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. We find that for top-hat jets, afterglow is a more potential counterpart than the prompt emission even with unfavorable 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 energy emitted in the prompt phase equals the kinetic energy of the outflow. Assuming a Gaussian jet profile, we forecast the population of $\gamma$-ray detections and find that extreme viewing angle events like GRB170817A will be rare. In our simulated sample, the observed isotropic equivalent energy in $\gamma$-rays is moderately correlated with the viewing angle, such that a low $E_{\rm iso, \gamma}$ is almost always associated with a high off-axis viewing angle.