A radio telescope picks up a signal from the center of the Milky Way. The source MAXI J1744-294 flares in X-rays and radio waves, but where it actually resides is a mystery. It could be near the black hole, or it could just lie along the same line of sight, thousands of light-years away.

For four consecutive nights, astronomers tracked how the radio waves from the source changed their polarization — the direction of oscillation, like a rope shaken up-and-down or left-and-right. Passing through a magnetized gas cloud, the waves get "twisted," and the twist angle reveals the strength and direction of the magnetic field. It turned out that the twist for MAXI J1744-294 is exactly the same as for a known magnetar (a neutron star with a monstrous magnetic field) that orbits the central black hole. This means both objects are looking through the same cloud — which is only possible if MAXI J1744-294 truly lives at the very center, in the stellar "metropolis" around the black hole.

On one of the evenings, the telescope caught a second, fainter polarized signal, as if someone briefly switched on a second spotlight. Its twist was ten times smaller. If this signal is produced by electrons decelerating in a magnetic field, then the field there is monstrous: 15–30 gauss, like the strongest magnets in laboratories. It seems the source ejected a short jet of plasma, like a whale spouting a fountain, but of hot gas.

Now it's clear: MAXI J1744-294 is a local in the center of the Galaxy, not a random passerby. And the mysterious magnetic field around the black hole is its own, not carried in by winds from distant clouds. Radio waves acted like a compass, pointing to the exact address in the heart of the Milky Way.