Fast and slow magnetic deflagration fronts in type I X-ray bursts

Abstract

Type I X-ray bursts are produced by thermonuclear runaways that develop on accreting neutron stars. Once one location ignites, the flame propagates across the surface of the star. Flame propagation is fundamental in order to understand burst properties like rise time and burst oscillations. Previous work quantified the effects of rotation on the front, showing that the flame propagates as a deflagration and that the front strongly resembles a hurricane. However, the effect of magnetic fields was not investigated, despite the fact that magnetic fields strong enough to have an effect on the propagating flame are expected to be present on many bursters. In this paper, we show how the coupling between fluid layers introduced by an initially vertical magnetic field plays a decisive role in determining the character of the fronts that are responsible for the type I bursts. In particular, on a star spinning at 450 Hz (typical among the bursters), we test seed magnetic fields of 107-1010 G and find that for the medium fields the magnetic stresses that develop during the burst can speed up the velocity of the burning front, bringing the simulated burst rise time close to the observed values. By contrast, in a magnetic slow rotator like IGR J17480-2446, spinning at 11 Hz, a seed field ≥109 G is required to allow localized ignition and the magnetic field plays an integral role in generating the burst oscillations observed during the bursts.