Lense-Thirring precession in ULXs as a possible means to constrain the neutron star equation-of-state

TL;DR

This paper proposes using Lense-Thirring precession in ultraluminous X-ray sources with neutron stars to constrain their equation-of-state by linking observed super-orbital periods and pulsar spin data.

Contribution

It introduces a novel method to infer neutron star properties and their equation-of-state from super-orbital periods and spin measurements without requiring system inclination or distance.

Findings

Stronger magnetic fields suggest stiffer equations of state.

Application to NGC 7793 P13 demonstrates the method's viability.

The approach offers a new way to probe neutron star structure.

Abstract

The presence of neutron stars in at least three ultraluminous X-ray sources is now firmly established and offers an unambiguous view of super-critical accretion. All three systems show long-timescale periods (60-80 days) in the X-rays and/or optical, two of which are known to be super-orbital in nature. Should the flow be classically super critical, i.e. the Eddington limit is reached locally in the disc (implying surface dipole fields that are sub-magnetar in strength), then the large scale-height flow can precess through the Lense-Thirring effect which could provide an explanation for the observed super-orbital periods. By connecting the details of the Lense-Thirring effect with the observed pulsar spin period, we are able to infer the moment-of-inertia and therefore equation-of-state of the neutron star without relying on the inclination of, or distance to the system. We apply our technique to the case of NGC 7793 P13 and demonstrate that stronger magnetic fields imply stiffer equations of state. We discuss the caveats and uncertainties, many of which can be addressed through forthcoming radiative magnetohydrodynamic (RMHD) simulations and their connection to observation.