A future X-ray glimpse of SN 1987A could test neutron-star superfluidity

A hidden star, not a new detection

Indirect observations suggest that SN 1987A contains a neutron star, the compact remnant left by the supernova. But detecting heat directly from its surface remains a different challenge from finding emission associated with its surroundings.

The paper discusses ALMA and James Webb Space Telescope observations that could be connected to the central object. Some emission may instead come from a pulsar wind nebula: material energized by a rotating neutron star. These possibilities matter because the proposed diagnostic requires the star’s thermal emission, not simply evidence of an energetic central source.

Akira Dohi and colleagues ask what a future thermal X-ray detection could reveal. Their study is a modeling investigation presented as an arXiv preprint, not a report of such a detection.

Why the crust matters

A young neutron star need not have a thermally equilibrated interior. Its crust can remain hotter than its core while heat flows between them. The time needed to settle depends partly on how much heat the material stores and how efficiently it transports energy.

In the inner crust, neutrons may form pairs and enter a superfluid state, a collective quantum state of matter. In the models studied here, stronger pairing suppresses the neutrons’ heat capacity—the energy needed to change their temperature—and generally shortens thermal relaxation.

If pairing is weak, the crust may still be hotter than the core when the star becomes observable. Its surface brightness could then carry information about crustal physics. If relaxation has already occurred, the measurement would instead be more closely connected to cooling governed by the core.

How the researchers modeled the problem

The team used the NSCool cooling code, considering stars with masses roughly 1.2–1.6 times the Sun’s. They compared two equations of state: descriptions of how dense matter’s pressure relates to its physical conditions. They also tested six prescriptions for neutron pairing in the crust, plus an unpaired case, and two amounts of light material in the outer envelope.

These choices affect both internal cooling and how interior heat translates into surface radiation. Other pairing properties in the core were held fixed for the main comparisons.

The researchers then compared predicted thermal luminosities with the detection limits of a hypothetical observatory having Lynx-like capabilities. Their benchmark combines approximately 0.5-arcsecond imaging with a large X-ray collecting area, using a 200,000-second exposure and a three-sigma detection criterion.

Expanding supernova debris absorbs X-rays from the star. The team approximated how this obstruction declines with time and checked the sensitivity at 2047 using synthetic spectra based on three-dimensional remnant simulations.

What the calculations predict

Without rapid cooling, the benchmark generally permits detection around 60 years after the explosion—roughly 2047. Illustrative models with one equation of state become detectable at about 58 or 63 years, depending on the light-element envelope.

This is not a promised observing date. The calculations adopt an illustrative operating window of 2045–2075, and require an instrument capable of reaching the assumed sensitivity.

Rapid neutrino cooling usually makes detection within that window less likely. Neutrinos carry energy away from the interior; the direct Urca process is a particularly efficient channel that operates only under suitable dense-matter conditions.

There are exceptions. Just above the mass threshold for direct Urca cooling, only a small core region may support the process. Some modeled stars then remain detectable for a short interval before fading below the benchmark limit.

The central scientific opportunity is conditional: weak crustal pairing could delay thermal equilibration until after detection, allowing future brightness measurements to test inner-crust models.

Why the telescope and assumptions matter

Collecting more photons is not enough. The surrounding bright ring is only about 1.6 arcseconds across, so sharp imaging helps separate the central source from remnant emission. The authors emphasize that Lynx is an unfunded concept without a scheduled launch. Other observatories cannot inherit its predicted detection dates merely by scaling collecting areas; their responses and contamination require separate simulations.

The physics also remains uncertain. Only two equations of state and a restricted mass range were explored. The approximation for declining absorption becomes less reliable at later ages, although the 2047 check gave a more favorable sensitivity than the approximation. Strong magnetic-field heating was not included. An additional calculation suggests proton superconductivity could shift detection timing by about a decade in a particular slow-cooling comparison, not as a universal error estimate.

A future detection could therefore help discriminate among cooling models rather than uniquely identify superfluid properties. A nondetection would constrain those models only if the observations were sensitive enough to reach their predicted thermal brightness.