How bright can old magnetars be? Assessing the impact of magnetized envelopes and field topology on neutron star cooling

TL;DR

This paper investigates how magnetic field strength, topology, and envelope composition influence the cooling and surface temperature evolution of magnetars, revealing that older magnetars with core-threading fields are cooler than pulsars of similar age.

Contribution

It provides a comprehensive analysis of the effects of magnetic field topology and envelope properties on neutron star cooling, highlighting significant differences from previous models.

Findings

Older magnetars with core-threading fields are cooler than similar-aged pulsars.

Magnetic field topology significantly affects surface temperature predictions.

Envelope composition alters the relation between internal and surface temperatures.

Abstract

Neutron stars cool down during their lifetime through the combination of neutrino emission from the interior and photon cooling from the surface. Strongly magnetised neutron stars, called magnetars, are no exception, but the effect of their strong fields adds further complexities to the cooling theory. Besides other factors, modelling the outermost hundred meters (the envelope) plays a crucial role in predicting their surface temperatures. In this letter, we revisit the influence of envelopes on the cooling properties of neutron stars, with special focus on the critical effects of the magnetic field. We explore how our understanding of the relation between the internal and surface temperatures has evolved over the past two decades, and how different assumptions about the neutron star envelope and field topology lead to radically different conclusions on the surface temperature and its cooling with age. In particular, we find that relatively old magnetars with core-threading magnetic fields are actually much cooler than a rotation-powered pulsar of the same age. This is at variance with what is typically observed in crustal-confined models. Our results have important implications for the estimates of the X-ray luminosities of aged magnetars, and the subsequent population study of the different neutron star classes.