Radio waves from a gamma-ray burst (the death of a massive star, briefly outshining its galaxy) arrive at us polarized, oscillating in a single plane, like a compass needle frozen in a magnetic field. Astronomers have read the indications of this cosmic needle for the first time.

Using spectroscopy, they traced how polarization increased with frequency: 0.7% at 11 GHz, 3.2% at 25 GHz. But even at its peak, it's far from perfect — the magnetic field in the ejected jet turned out not uniform, but made of many chaotic domains, like thousands of microscopic compasses pointing every which way.

A staggering detail: on the way to Earth, the polarization rotated by –8300 rad/m² — tens of thousands of times stronger than in our Galaxy. This Faraday rotation reveals a dense veil of ionized hydrogen around the explosion site, like a magnetic lens. Now, by tracking the dance of this needle, scientists will be able to peer into the very heart of cosmic cataclysms.