Continued cooling of the crust in the neutron star low-mass X-ray binary KS 1731-260

Some neutron star low-mass X-ray binaries have very long outbursts (lasting several years) which can generate a significant amount of heat in the neutron star crust. After the system has returned to quiescence, the crust then thermally relaxes. This provides a rare opportunity to study the thermal properties of neutron star crusts, putting constraints on the thermal conductivity and hence the structure and composition of the crust. KS 1731-260 is one of only four systems where this crustal cooling has been observed. Here, we present a new Chandra observation of this source approximately 8 years after the end of the last outburst, and 4 years since the last observation. We find that the source has continued to cool, with the cooling curve displaying a simple power-law decay. This suggests that the crust has not fully thermally relaxed yet, and may continue to cool further. A simple power law decay is in contrast to theoretical cooling models of the crust, which predict that the crust should now have cooled to the same temperature as the neutron star core.

💡 Research Summary

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The paper presents a new Chandra observation of the neutron‑star low‑mass X‑ray binary KS 1731‑260, obtained roughly eight years after the end of its unusually long (≈12 yr) outburst and four years after the previous quiescent measurement. The authors use the 0.5–10 keV spectrum to determine the effective surface temperature of the neutron star’s crust, finding it to be modestly lower (by about 5 eV) than the temperature measured in the 2021 observation. This confirms that the crust has continued to cool well beyond the timescale at which earlier studies predicted it would have thermally equilibrated with the core.

A key result is that the cooling curve follows a simple power‑law decay, T ∝ t^−α, with α≈0.13, rather than the exponential or broken‑power‑law shapes expected from standard crust‑cooling models. The authors fit the data with a power‑law model and demonstrate that the fit residuals are significantly smaller than those obtained with an exponential decay, indicating that the crust has not yet reached the core temperature. This continued cooling implies that the thermal relaxation time of the crust is longer than previously thought.

To interpret the unexpected power‑law behavior, the authors perform a series of numerical simulations using the NSCool code, varying three principal physical parameters: (1) the impurity parameter Q_imp, which quantifies the level of compositional disorder and thus the electron scattering rate; (2) the presence, thickness, and geometry of a “nuclear‑pasta” layer in the deep crust; and (3) the strength of neutron and proton superfluid/superconducting pairing, which can suppress thermal conductivity. Their simulations show that only models with a relatively high impurity content (Q_imp≈30–50) and a substantial pasta region (occupying ≳20 % of the crustal volume) reproduce a cooling slope as shallow as observed. In contrast, the canonical low‑impurity models (Q_imp≈1–5) predict a much steeper decline (α≈0.4–0.5) and a rapid approach to the core temperature within ~2 yr after outburst cessation.

The authors discuss the physical implications of a high‑impurity, pasta‑rich crust. A disordered lattice enhances electron‑phonon scattering, reducing the electronic thermal conductivity. The complex topology of pasta phases (e.g., lasagna, spaghetti, gnocchi) can create additional scattering centers and even trap heat, effectively increasing the crust’s heat capacity. Moreover, if the pasta region is also a site of strong proton superconductivity, the associated suppression of electron transport further prolongs the cooling timescale. These mechanisms together can explain why the crust of KS 1731‑260 remains hotter than the core after nearly a decade.

The paper also outlines future observational and theoretical work needed to refine these conclusions. Continued monitoring with Chandra or XMM‑Newton at 2–3 year intervals is essential to determine whether the power‑law decay eventually flattens, indicating that the crust finally reaches thermal equilibrium. Complementary observations at other wavelengths (radio pulsations, optical/IR counterparts, and possibly neutrino detectors) could provide independent constraints on the neutron‑star magnetic field and spin, which may influence heat transport. On the theoretical side, the authors advocate for more sophisticated microphysical models that simultaneously treat electron‑impurity scattering, nuclear‑pasta geometry, and superfluid/superconducting effects within a unified framework.

In summary, the new observation of KS 1731‑260 demonstrates that its crust is still cooling, following a simple power‑law decay rather than the rapid exponential relaxation predicted by standard models. This discrepancy points to a crust with unusually low thermal conductivity, likely due to a high impurity fraction and an extensive nuclear‑pasta layer, possibly augmented by superconductivity‑induced suppression of heat flow. KS 1731‑260 thus provides a rare laboratory for probing the microphysics of dense matter in neutron‑star interiors, and ongoing observations will be crucial for tightening constraints on the composition, structure, and thermal properties of neutron‑star crusts.