Implications of burst oscillations from the slowly rotating accreting pulsar IGR 17480-2446 in the globular cluster Terzan 5

TL;DR

This paper reports the first detection of Type I X-ray burst oscillations in a slowly rotating neutron star, IGR J17480--2446, challenging existing models and suggesting magnetic confinement as the origin.

Contribution

It demonstrates that burst oscillations can occur in slowly rotating neutron stars and proposes a magnetic confinement mechanism over global modes or Coriolis effects.

Findings

Burst oscillations match the star's spin frequency.

Global modes and Coriolis force are unlikely causes.

Magnetic stresses likely confine the hot-spot.

Abstract

The recently-discovered accreting X-ray pulsar IGR J17480--2446 spins at a frequency of ~11 Hz. We show that Type I X-ray bursts from this source display oscillations at the same frequency as the stellar spin. IGR J17480--2446 is the first secure case of a slowly rotating neutron star which shows Type I burst oscillations, all other sources featuring such oscillations spin at hundreds of Hertz. This means that we can test burst oscillation models in a completely different regime. We explore the origin of Type I burst oscillations in IGR J17480--2446 and conclude that they are not caused by global modes in the neutron star ocean. We also show that the Coriolis force is not able to confine an oscillation-producing hot-spot on the stellar surface. The most likely scenario is that the burst oscillations are produced by a hot-spot confined by hydromagnetic stresses.