On the synthesis of heavy nuclei in protomagnetar outflows and implications for ultra-high energy cosmic rays

TL;DR

This paper investigates how neutrino-driven winds from protomagnetars can synthesize heavy nuclei, potentially reaching ultra-high energies, with implications for understanding the origins of ultra-high energy cosmic rays.

Contribution

It demonstrates that Poynting-flux dominated outflows from protomagnetars can efficiently produce heavy nuclei up to iron peak elements and accelerate them to ultra-high energies.

Findings

Heavy nuclei can be synthesized in magnetized protomagnetar winds.

Nuclei can reach energies >10^{20} eV within gamma-ray emission regions.

Production of elements heavier than lanthanides is limited by electron fraction.

Abstract

It has been suggested that strongly magnetised and rapidly rotating protoneutron stars (PNSs) may produce long duration gamma-ray bursts (GRBs) originating from stellar core collapse. We explore the steady-state properties and heavy element nucleosynthesis in neutrino-driven winds from such PNSs whose magnetic axis is generally misaligned with the axis of rotation. We consider a wide variety of central engine properties such as surface dipole field strength, initial rotation period and magnetic obliquity to show that heavy element nuclei can be synthesised in the radially expanding wind. This process is facilitated provided the outflow is Poynting-flux dominated such that its low entropy and fast expansion timescale enables heavy nuclei to form in a more efficient manner as compared to the equivalent thermal GRB outflows. We also examine the acceleration and survival of these heavy nuclei and show that they can reach sufficiently high energies $\gtrsim 10^{20}\ {\rm eV}$ within the same physical regions that are also responsible for powering gamma-ray emission, primarily through magnetic dissipation processes. Although these magnetised outflows generally fail to achieve the production of elements heavier than lanthanides for our explored electron fraction range 0.4-0.6, we show that they are more than capable of synthesizing nuclei near and beyond iron peak elements.