Jet-driven and jet-less fireballs from compact binary mergers

TL;DR

This paper proposes a model where compact binary mergers produce isotropic fireballs that generate observable prompt and afterglow emissions without requiring jets, expanding detection possibilities for such cosmic events.

Contribution

It introduces a jet-less fireball scenario for neutron star mergers, predicting observable emissions independent of jet orientation, unlike traditional jet-based models.

Findings

Isotropic fireballs can produce detectable emissions within 90 Mpc.

Prompt emission can mimic off-axis jet signals, complicating interpretation.

Afterglow properties can distinguish between jet and jet-less scenarios.

Abstract

During a compact binary merger involving at least one neutron star, a small fraction of the gravitational energy could be liberated in such a way to accelerate a small fraction (~ 10^-6) of the neutron star mass in an isotropic or quasi-isotropic way. In presence of certain conditions, a pair-loaded fireball can form, which undergoes accelerated expansion reaching relativistic velocities. As in the standard fireball scenario, internal energy is partly transformed into kinetic energy. At the photospheric radius, the internal radiation can escape, giving rise to a pulse that lasts for a time equal to the delay time since the merger. The subsequent interaction with the interstellar medium can then convert part of the remaining kinetic energy back into radiation in a weak isotropic afterglow at all wavelengths. This scenario does not require the presence of a jet: the associated isotropic prompt and afterglow emission should be visible for all NS-NS and BH-NS mergers within 90 Mpc, independent of their inclination. The prompt emission is similar to that expected from an off-axis jet, either structured or much slower than usually assumed (Gamma ~ 10), or from the jet cocoon. The predicted afterglow emission properties can discriminate among these scenarios.