Connecting the m-dots: accretion rates and thermonuclear burst recurrence times on neutron stars and white dwarfs

TL;DR

This paper compiles and models the recurrence times of thermonuclear bursts on neutron stars and white dwarfs, revealing a universal inverse relation with accretion rates and implications for ignition physics.

Contribution

It introduces a unified model linking burst recurrence times on neutron stars and white dwarfs, highlighting their dependence on accretion rates and comparing observational data with theoretical predictions.

Findings

Both neutron stars and white dwarfs follow a similar inverse linear relation between recurrence time and accretion rate.

Pure helium burst models match observations in ultra-compact X-ray binaries at low accretion rates.

The energy difference between bursts on white dwarfs and neutron stars aligns with their area ratio.

Abstract

We present a compilation of observed recurrence times ($t_{\rm rec}$) and infer the corresponding local mass-accretion rates ($\dot m$) for type I X-ray bursts, milliHertz quasi-periodic oscillating sources and recurrent novae eruptions. We construct models of the $t_{\rm rec}-\dot m$ relation for accreting white dwarfs and neutron stars and find that both are roughly consistent with a global inverse linear relation, connecting for the first time thermonuclear runaways on neutron stars and white dwarfs. We find that theoretical models of pure He bursts are in agreement with the best $t_{\rm rec}$ measurements in ultra-compact X-ray binaries at low $\dot m$ (4U~$0614+09$ and 2S~0918-549). We suggest that the transient Z source XTE~J1701-462 is a slow rotator, based on its mHz QPO properties. Finally, we discuss the implications for thermonuclear ignition and point out that the difference in eruption/burst energy ($E_{b_{WD}}/E_{b_{NS}}=2\times 10^4$) is consistent with the difference in area between neutron stars and white dwarfs $\left((R_{WD}/R_{NS})^2=4\times 10^4\right)$. We conclude that ignitions of thermonuclear shell flashes on neutron stars and white dwarfs depend primarily on the specific mass accretion rate and do not depend on the nature of the underlying compact object.