The Population of Helium-Merger Progenitors: Observational Predictions

Abstract: The helium-merger gamma-ray burst progenitor is produced by the rapid accretion onto a compact remnant (neutron star or black hole) when it undergoes a common envelope inspiral with its companion's helium core. This merger phase produces a very distinct environment around these outbursts and recent observations suggest that, in some cases, we are detecting the signatures of the past merger in the GRB afterglow. These observations allow us, for the first time, to study the specific features of the helium merger progenitor. In this paper, we couple population synthesis calculations to our current understanding of gamma-ray burst engines and common envelope evolution to make observational predictions for the helium-merger gamma-ray burst population. Many mergers do not produce GRB outbursts and we discuss the implications of these mergers with the broader population of astrophysical transients.