How Bright Can Old Magnetars Be?

This study (Dehman et al., MNRASL 520, L42–L47 (2023)) revisits how the outermost layers of neutron stars—their envelopes—affect the cooling and long-term luminosity of magnetars, neutron stars with extremely strong magnetic fields. Although envelopes are often treated as a simple boundary condition, we show that their composition (iron or light elements) and the strength and geometry of magnetic fields significantly alter the relation between internal and surface temperatures. By comparing a wide range of envelope models developed over the past two decades, we demonstrate that newer microphysical prescriptions systematically predict higher surface temperatures for the same interior conditions, especially in highly magnetized stars.

Using magnetothermal simulations, we investigate how different magnetic-field geometries shape the thermal evolution of magnetars. We focus on two extreme configurations that bracket the possible behavior of real neutron stars. In crust-confined models, most of the electric currents—and thus most magnetic energy—are located in the outer crust. Their dissipation deposits heat relatively close to the surface, where it can efficiently raise the X-ray luminosity. In core-threading models, by contrast, the bulk of the magnetic field penetrates the stellar core. The associated currents are extremely long-lived, so their dissipation is weak and occurs in regions where most of the heat is immediately lost through neutrino emission instead of reaching the surface.

These differences manifest clearly in the cooling history. During the early neutrino-cooling era, when photon emission is still negligible, the star’s luminosity is primarily determined by the envelope model: improved microphysical prescriptions or light-element compositions lead to systematically higher effective temperatures. However, once the star enters the photon-cooling era, this trend reverses. Models with hotter envelopes radiate energy more efficiently and therefore cool much faster. As a result, magnetars with core-threading magnetic fields experience a rapid luminosity drop—becoming surprisingly faint, even compared to rotation-powered pulsars of similar age. Conversely, crust-confined magnetars maintain elevated luminosities for far longer, sustained by continuous Joule heating in the outer crust, which counteracts the rapid photon losses.

These results show that the detectability of mature magnetars depends sensitively on both their magnetic structure and the physics of their envelopes. The distribution of electrical currents plays a decisive role in determining whether middle-aged magnetars remain bright or fade into faint, hardly detectable sources, with important consequences for population synthesis studies. Accurate modeling of magnetized envelopes is therefore essential, as it allows us to correctly interpret how the underlying distribution of electric currents—whether confined to the crust or threading the core—shapes the thermal emission and, in turn, the inferred magnetic-field evolution and population properties of neutron stars.