The Influence of Stellar Spin on Ignition of Thermonuclear Runaways

TL;DR

This study investigates how the spin rate of neutron stars influences the ignition of thermonuclear bursts, revealing that faster rotation stabilizes nuclear burning and reduces burst frequency at high accretion rates.

Contribution

It provides observational evidence linking neutron star spin to nuclear ignition stability, a novel insight into the role of stellar rotation in accretion-driven nuclear processes.

Findings

Faster-spinning neutron stars have lower burst rates at high accretion.

High spin rates (> 400 Hz) promote stabilization of nuclear burning.

The results suggest a dynamical dependence of ignition on stellar spin.

Abstract

Runaway thermonuclear burning of a layer of accumulated fuel on the surface of a compact star provides a brief but intense display of stellar nuclear processes. For neutron stars accreting from a binary companion, these events manifest as thermonuclear (type-I) X-ray bursts, and recur on typical timescales of hours to days. We measured the burst rate as a function of accretion rate, from seven neutron stars with known spin rates, using a burst sample accumulated over several decades. At the highest accretion rates, the burst rate is lower for faster spinning stars. The observations imply that fast (> 400 Hz) rotation encourages stabilization of nuclear burning, suggesting a dynamical dependence of nuclear ignition on the spin rate. This dependence is unexpected, because faster rotation entails less shear between the surrounding accretion disk and the star. Large-scale circulation in the fuel layer, leading to enhanced mixing of the burst ashes into the fuel layer, may explain this behavior; further numerical simulations are required to confirm.