Radio waves from a gamma-ray burst — the cataclysmic death of a massive star whose flash briefly outshines an entire galaxy — reach Earth polarized. This means their electric field oscillates predominantly in a single plane, like a compass needle frozen in a magnetic field. Astronomers have for the first time managed to read this cosmic needle's indications, using spectroscopy — the decomposition of radiation by frequency — to analyze the polarization.

The researchers traced how the degree of polarization increased with radio frequency: from 0.7% at 11 GHz to 3.2% at 25 GHz. Yet even at its peak, the polarization is far from complete. This suggests that the magnetic field in the jet of material ejected by the explosion is not uniform but consists of many chaotically oriented domains — like thousands of microscopic compasses pointing every which way. In such an environment, waves passing through different regions acquire different orientations, and the net polarization ends up only partial.

A stunning detail emerged when measuring Faraday rotation — the effect by which the plane of polarization rotates as it passes through magnetized plasma. On its way to Earth, the polarization rotated by –8300 rad/m², tens of thousands of times larger than typical values for our Galaxy. This colossal rotation reveals a dense veil of ionized hydrogen around the explosion site. Like a magnetic lens, this medium not only rotates the plane of oscillation but also lets us peer into the immediate vicinity of the catastrophe. Now, by tracking the "dance" of the polarized needle at different frequencies, scientists gain a tool to probe the very heart of cosmic explosions.