The properties of long gamma-ray bursts in massive compact binaries

TL;DR

This paper models how a black-hole companion in a binary system influences fallback accretion in long gamma-ray bursts, predicting observable features like a break in the light curve and late-time flares.

Contribution

It introduces a population synthesis model to study the impact of binary companions on fallback accretion in long gamma-ray bursts, linking binary parameters to observable signatures.

Findings

A break in the accretion curve occurs around 10,000 seconds.

A late-time energy flare of about 0.1 foe is predicted.

Break times correlate negatively with flare energy.

Abstract

We consider a popular model for long-duration gamma-ray bursts, in which the progenitor star, a stripped helium core, is spun up by tidal interactions with a black- hole companion in a compact binary. We perform population synthesis calculations to produce a representative sample of such binaries, and model the effect that the companion has on material that falls back on to the newly-formed black hole. Taking the results of hydrodynamic models of black-hole formation by fallback as our starting point, we show that the companion has two main effects on the fallback process. First, a break forms in the accretion curve at around 10 000 s. Secondly, subsequent to the break, we expect to see a flare of total energy around 0.1 foe. We predict that the break time is set largely by the semi-major axis of the binary at the time of explosion, and that this correlates negatively with the flare energy. Although comparison with observations is non-trivial, we show that our predicted break times are comparable to those found in the X-ray light curves of canonical long-duration gamma-ray bursts. Similarly, the flare properties that we predict are consistent with the late-time flares observed in a sub-sample of bursts.