# Thermonuclear Bursts with Short Recurrence Times from Neutron Stars Explained by Opacity-Driven Convection

## TL;DR

This paper presents the first one-dimensional simulations explaining short recurrence time thermonuclear bursts on neutron stars through opacity-driven convection, matching observed burst patterns and properties.

## Contribution

It introduces a novel simulation approach that reproduces short recurrence time bursts by modeling convection and fuel mixing in neutron star envelopes.

## Key findings

- Simulations reproduce short recurrence time bursts with ~30% occurrence.
- Shorter recurrence bursts are less bright and shorter in duration.
- Convective mixing driven by opacity explains fuel ignition at shallow depths.

## Abstract

Thermonuclear flashes of hydrogen and helium accreted onto neutron stars produce the frequently observed Type I X-ray bursts. It is the current paradigm that almost all material burns in a burst, after which it takes hours to accumulate fresh fuel for the next burst. In rare cases, however, bursts are observed with recurrence times as short as minutes. We present the first one-dimensional multi-zone simulations that reproduce this phenomenon. Bursts that ignite in a relatively hot neutron star envelope leave a substantial fraction of the fuel unburned at shallow depths. In the wake of the burst, convective mixing events driven by opacity bring this fuel down to the ignition depth on the observed timescale of minutes. There, unburned hydrogen mixes with the metal-rich ashes, igniting to produce a subsequent burst. We find burst pairs and triplets, similar to the observed instances. Our simulations reproduce the observed fraction of bursts with short waiting times of ~30%, and demonstrate that short recurrence time bursts are typically less bright and of shorter duration.