Rotational effects in thermonuclear Type I Bursts: equatorial crossing and directionality of flame spreading

TL;DR

This study investigates how rotation influences the propagation and structure of thermonuclear flames on neutron stars, especially across equatorial regions, revealing that flames can cross the equator and that propagation speed varies with direction.

Contribution

It extends previous work by analyzing meridional flame propagation and the effects of varying Coriolis force, including slow rotators like IGR J17480-2446.

Findings

Flame propagation occurs across the equator despite zero Coriolis force.

Propagation towards the pole is faster than towards the equator.

Slow rotation reduces Coriolis effects, affecting flame dynamics.

Abstract

In a previous study on thermonuclear (type I) nursts on accreting neutron stars we addressed and demonstrated the importance of the effects of rotation, through the Coriolis force, on the propagation of the burning flame. However, that study only analysed cases of longitudinal propagation, where the Coriolis force coefficient $2\Omega\cos\theta$ was constant. In this paper, we study the effects of rotation on propagation in the meridional (latitudinal) direction, where the Coriolis force changes from its maximum at the poles to zero at the equator. We find that the zero Coriolis force at the equator, while affecting the structure of the flame, does not prevent its propagation from one hemisphere to another. We also observe structural differences between the flame propagating towards the equator and that propagating towards the pole, the second being faster. In the light of the recent discovery of the low spin frequency of burster IGR~J17480-2446 rotating at 11 Hz (for which Coriolis effects should be negligible) we also extend our simulations to slow rotation.