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The crust of a neutron star is not just a solid shell. Under pressures of billions of tons per square centimeter, atoms lose their electrons, and the remaining nuclei arrange into a rigid crystal lattice, reminiscent of a honeycomb, but made of matter a trillion times denser than steel. When this lattice cracks under accumulated stress, a magnetar flare is born — a brief but dazzling burst of X-rays.

A team of scientists built a computer model of the glow of hot plasma in such fields and ran real flare data through it. The model tested different plasma compositions — from light nuclei like helium to heavy ones like krypton. The spectra (the decomposition of light by energy) matched best when the plasma was dominated by nuclei with an effective charge of about 37. This number is not the charge of a specific element but an averaged characteristic of a mixture rich in iron, nickel, and heavier elements.

Such a composition could not have come from nowhere. Light nuclei falling onto the star from space would not withstand the monstrous pressure and would quickly transform into a denser packing — into those very heavy nuclei. This means the flare brings out material from the crust itself, where it has been stored for millions of years. It's like determining from volcanic ash what minerals make up the magma tens of kilometers deep.

Now astrophysicists have a spectral key to the composition of neutron star interiors. Previously, they could only rely on theoretical calculations, but now they can test these theories against real flares. And when the next magnetar flares up in our Galaxy, its light will tell us not only about the cataclysm but also about what the toughest armor in the Universe is made of.