A spinning top whirls so fast it's about to fly apart. Add to it a super-strong magnet — one that, if placed on the Moon, would wipe data from every credit card on Earth. That's a magnetar: an ultra-dense remnant of an exploded star, a lump of matter where a teaspoon weighs as much as a mountain. When such a top is born, its spin can power a flash that briefly outshines an entire galaxy — a hydrogen-poor supernova with record-breaking brightness.
The rotational energy escapes in two ways: through ripples in spacetime (gravitational waves) and through magnetic radiation that heats the supernova's material to a blinding glow. Which path dominates depends on the tilt angle of the magnetic axis — the deviation of the top's axis from vertical. If the angle is small, the magnetar mostly emits gravitational waves and stays dark to ordinary telescopes. If the angle grows, magnetic radiation strengthens, and the supernova flares brighter.
The researchers found that this "glue"—the bulk viscosity of ultra-dense nuclear matter — changes over time and depends on density. A particular surprise came from short-range forces between nucleons (protons and neutrons): they weaken viscosity, like oil added to thick honey. As a result, the magnetic axis grows more slowly, gravitational-wave emission weakens, but the supernova's thermal glow can become several times brighter. For a magnetar with typical mass and rapid spin, this is the difference between a dim flicker and a dazzling fireworks display.
The authors also showed that the "stiffness" of symmetry energy, a characteristic of nuclear forces, strongly influences the threshold for a special cooling process (the direct Urca process), which sharply increases viscosity. A stiffer energy accelerates the growth of the angle and gravitational-wave emission but quenches visible brightness. Thus, tiny details of particle behavior inside a stellar remnant determine whether astronomers will see a new record-breaking explosion in the sky.
The magnetar top keeps spinning, but now it's clear: its fate, to be an invisible source of gravitational ripples or a blinding beacon, is decided by the viscosity of invisible glue in its depths.
Bulk viscosity in a magnetar can change by billions of times within seconds, as if water in a glass momentarily became thicker than resin, then turned liquid again.
In Robert Forward's novel "Dragon's Egg," life is depicted on the surface of a neutron star, where monstrous gravity and magnetic fields create unimaginable forms of matter, just like in the depths of a magnetar.