Carbon production on accreting neutron stars in a new regime of stable nuclear burning

TL;DR

This paper presents a new steady-state burning regime on accreting neutron stars that produces sufficient carbon at realistic accretion rates, potentially explaining the origin of superbursts.

Contribution

It introduces a novel stable hydrogen and helium burning regime that generates pure carbon ashes at accretion rates near 10% of Eddington, aligning with superburst observations.

Findings

Stable burning produces enough carbon for superbursts.

The regime occurs at realistic accretion rates (~10% Eddington).

Increased base heating sustains steady-state burning.

Abstract

Accreting neutron stars exhibit Type I X-ray bursts from both frequent hydrogen/helium flashes as well as rare carbon flashes. The latter (superbursts) ignite in the ashes of the former. Hydrogen/helium bursts, however, are thought to produce insufficient carbon to power superbursts. Stable burning could create the required carbon, but this was predicted to only occur at much larger accretion rates than where superbursts are observed. We present models of a new steady-state regime of stable hydrogen and helium burning that produces pure carbon ashes. Hot CNO burning of hydrogen heats the neutron star envelope and causes helium to burn before the conditions of a helium flash are reached. This takes place when the mass accretion rate is around 10% of the Eddington limit: close to the rate where most superbursts occur. We find that increased heating at the base of the envelope sustains steady-state burning by steepening the temperature profile, which increases the amount of helium that burns before a runaway can ensue.