Nucleosynthesis of elements in gamma ray bursts engines

TL;DR

This paper investigates how nucleosynthesis occurs in gamma-ray burst engines, focusing on the formation of heavy elements in the accretion disks around black holes, with implications for observed spectra.

Contribution

It provides a detailed hydrodynamic and nuclear reaction network model of nucleosynthesis in GRB accretion disks, highlighting conditions for heavy element formation.

Findings

Heavy nuclei up to germanium and gallium are synthesized in the disk.

High accretion rates (up to 1 solar mass/sec) facilitate heavy element production.

Nucleosynthesis conditions influence GRB jet composition and afterglow spectra.

Abstract

We consider the gamma ray burst (GRB) central engine that is powered by the collapse of a massive rotating star or compact binary merger. The engine is a hot and dense accretion disk, which is composed of free nucleons, electron-positron pairs, and helium, and cooled by neutrino emission. A significant number density of neutrons in the inner disk body provide conditions for neutron rich plasma in the GRB outflows or jets. Helium is synthesized in the inner disk if the accretion rate is large, and heavy nuclei are also formed in the outer disk at distances above 150-250 $r_{g}$ from the black hole. We study the process of nucleosynthesis in the GRB engine, depending on its physical properties. The GRB central engine is hydrodynamically modeled in the frame of a dense and hot disk which accretes with a high rate (up to 1 Solar mass per second) onto a maximally spinning, stellar mass black hole. The synthesis of heavy nuclei up to germanium and gallium is then followed by the nuclear reaction network. The accretion at high rate onto a Kerr black hole feeds the engine activity and establishes conditions for efficient synthesis of heavy nuclei in the disk. These processes may have important observational implications for the jet deceleration process and heavy elements observed in the spectra of GRB afterglows.