Superfluid effects on gauging core temperatures of neutron stars in low-mass X-ray binaries

TL;DR

This paper investigates how superfluidity in neutron star cores affects the determination of their temperatures from X-ray burst observations, revealing a degeneracy that impacts understanding of neutron star thermal states.

Contribution

It introduces the idea that superfluid properties cause a degeneracy in core temperature estimates, linking superfluidity to burst recurrence times in neutron stars.

Findings

Core temperature estimates are degenerate due to superfluid effects.

Short burst sources tend to have higher core temperatures with normal neutrons.

Long burst sources likely have lower temperatures with superfluid neutrons.

Abstract

Neutron stars accreting matter from low-mass binary companions are observed to undergo bursts of X-rays due to the thermonuclear explosion of material on the neutron star surface. We use recent results on superfluid and superconducting properties to show that the core temperature in these neutron stars may not be uniquely determined for a range of observed accretion rates. The degeneracy in inferred core temperatures could contribute to explaining the difference between neutron stars which have very short recurrence times between multiple bursts and those which have long recurrence times between bursts: short bursting sources have higher temperatures and normal neutrons in the stellar core, while long bursting sources have lower temperatures and superfluid neutrons. If correct, measurements of the lowest luminosity from among the short bursting sources and highest luminosity from among the long bursting sources can be used to constrain the critical temperature for the onset of neutron superfluidity.