Astrophysics

A glimpse inside a magnetar

Hundreds of neutron stars have exhibited 'glitches' in their spin-down rates — an indication of ultra-dense superfluids in their interiors. Now one highly magnetized star has shown a surprising glitch in the 'wrong' direction. See Letter p.591

A neutron star resembles a giant atomic nucleus, with 1–2 times the Sun's mass packed into a ball about 20 kilometres across. Its gravity is so strong that a projectile would need to be launched at about half the speed of light to escape from its surface. Extreme density, pressure, temperature, magnetism and relativistic gravity make these objects fascinating but challenging to study. Surprising observations of spin-down irregularities in one intensely magnetized neutron star, reported by Archibald et al.1 on page 591 of this issue, offer clues about exotic processes occurring deep inside these objects.

The basic structure of a neutron star is generally agreed on. It has a crust about 1 km thick, in which nuclei are arranged in a crystal lattice immersed in a 'sea' of electrons. Near the surface, the nuclei are plain iron, but the pressure and density increase rapidly with depth, so that the nuclei become increasingly bloated and neutron-rich. At moderate depth, neutrons 'drip' out of the nuclei, forming a neutral liquid between the lattice nuclei. At the base of the crust, the bloated nuclei merge. Below this lies pure nuclear fluid, more than 200 trillion times denser than liquid water.

Unlike atomic nuclei, which contain almost equal numbers of protons and neutrons, the nuclear fluid inside a neutron star has roughly 20 neutrons for every proton, a ratio maintained by neutrino-emitting processes. There is also one electron or muon ('heavy electron') per proton, ensuring charge neutrality. These particles are forced together by tremendous pressure, but quantum mechanics requires them to occupy different states, so they fill all available energy states up to a high energy, the Fermi energy. Deep below the crust, as the pressure rises, neutron and proton Fermi energies get so high that exotic, strongly interacting particles such as hyperons and mesons might join the mix. The actual fluid composition at very high densities is uncertain. It is possible that the innermost, central core consists of a 'soup' of quarks, the elementary particles that make up protons and neutrons.

Fortunately, observations of neutron stars yield insight into their interiors. The most thoroughly studied neutron stars are radio pulsars, which emit radio blips as they rotate. Timing the blips reveals that these stars steadily spin down. This is due to their intrinsic magnetism: as radio pulsars spin, they blow out magnetic waves and winds of fast charged particles, which carry away angular momentum. Interestingly, this steady spin-down is occasionally punctuated by 'glitches' — incidents in which the spin rate abruptly 'jumps up' by a small fractional amount. Hundreds of radio pulsars have been accurately timed, and many hundreds of glitches observed2,3. The glitches are attributed to imperfect coupling of superfluids within the stars: as a neutron star spins down, superfluid components tend to spin faster than the rest of the star. A glitch occurs when the superfluid occasionally shifts closer to co-rotation.

Although young neutron stars are hot, even by astrophysical standards, with interior temperatures commonly on the order of 108 kelvin, their strongly interacting neutrons can pair up and rearrange into quantum superfluids: phenomena found in low-temperature physics laboratories4. In the inner crust, below the level of 'neutron drip', the pairing neutrons have zero relative angular momentum, analogous to Cooper electron pairs in a superconductor. In the much denser fluid beneath the crust, the short-range repulsive part of the neutron–neutron interaction does not favour this; instead, pairs have one quantum unit of orbital angular momentum, as in laboratory superfluid helium-3. Protons beneath the crust also pair into a liquid superconducting state if the local magnetic field is not prohibitively intense. Deeper in the star, any mesons present will probably form Bose condensates, another kind of superfluid. Even quark soup will pair up into an exotic state known as colour superconductivity5. Thus, at almost every depth within a neutron star, interpenetrating superfluids exist that flow without viscous drag, and that could conceivably participate in glitches.

The most popular model for radio-pulsar glitches holds the inner-crust neutron superfluid responsible4,6. One property of a rotating superfluid is that all of its vorticity — all circulation tendency in the flow — is concentrated in quantum vortex lines: multitudes of microscopic nodes, or holes, in the superfluid that thread through the fluid parallel to the rotation axis. As the superfluid in a star spins down, these quantum vortices migrate outward towards the equator and ultimately annihilate near the stellar surface. But vortices in the deep crust can become 'pinned', or stuck, to the crust lattice nuclei. This keeps the inner-crust superfluid rotating faster than the rest of the star, until the vortices come loose in a catastrophic unpinning event, observed as a pulsar glitch.

So far, so good. But in their study, Archibald and colleagues report an 'anti-glitch'. Instead of an abrupt spin-up, the star abruptly spun down. (Such phenomena have been seen before, but only at a much smaller level7.) Despite searches with sensitive radio and X-ray telescopes, no surrounding afterglow was detected, arguing against a sudden particle outflow that could have carried off the anti-glitch's angular momentum. The most likely inference seems to be that some superfluid component within the star was rotating more slowly than the crust before the anti-glitch and/or was torqued-up by a sudden internal rearrangement.

This is surprising, because the star in question, like all other solitary, magnetic neutron stars (including all radio pulsars), is spinning down monotonically, apart from occasional, ordinary glitches7. It is expected that internal superfluids can lag behind the general spin-down and act as faster-rotating 'flywheels'. But how could a stellar superfluid come to rotate more slowly than the crust and/or get abruptly spun up?

The observed star is not a radio pulsar. It is a magnetar, an extremely magnetized neutron star with observable emissions powered by magnetic-field decay8. Magnetars are thought to be born spinning fast, with initial rotation periods of the order of several milliseconds. Their intense magnetism probably includes strongly wound-up interior-field components as a relic of this initial spin. The interior evolution of these objects is dominated by diffusing, changing magnetic fields. These exert stresses capable of moving material around, especially within radially concentric shells inside the stably stratified star — a possibility that may hold the key to the anti-glitch puzzle.

Box 1 describes an idealized model for how an anti-glitch could arise in the inner-crust superfluid9. A more promising alternative explanation involves the core. In moderately magnetized zones of the outer core, type II proton superconductivity requires that all magnetic flux is concentrated in quantum flux tubes — microscopic nodes in the superconductor that are threaded by magnetic-field lines. These tubes resist passing through vortices in the interpenetrating neutron superfluid where vortices and tubes intersect10. Thus, many flux tubes together can exert compelling forces on vortices. Elsewhere in the core, or where type I proton superconductivity occurs, macroscopic flux structures with pronounced edges will also exert forces on vortices. As the star's magnetism evolves, any gradual tendency of these flux systems to drive vortices away from the rotation axis will spin down the core neutron superfluid, potentially setting up conditions for an anti-glitch. A sudden 'breakthrough', or any instability by which vortices shift inward, would quickly spin up the superfluid and, as a result of angular-momentum conservation, spin down the rest of the star.

Perhaps of relevance is the fact that recent studies11,12 have raised doubts about whether the inner-crust neutron superfluid is massive enough to account for ordinary radio-pulsar glitches. Therefore, some astrophysicists are already considering sub-crust superfluids as reservoirs for glitch angular momentum. However this issue is resolved, the anti-glitch will probably provide insight into the interiors of neutron stars and help to illuminate the strange life histories of magnetars.

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Correspondence to Robert C. Duncan.

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Duncan, R. A glimpse inside a magnetar. Nature 497, 574–576 (2013). https://doi.org/10.1038/497574a

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