On the night of August 18, 2025, the LIGO, Virgo, and KAGRA detectors felt a tremor in spacetime — as if two neutron stars had collided somewhere far away. Almost immediately, telescopes found a point of light in the same patch of sky. At first, it was called a kilonova — that's how a cloud of superheated debris glows after a neutron-star merger. But the light spectrum told a different story: this was a Type IIb supernova. Such a flash occurs when a massive star sheds its outer layers and explodes, leaving behind a compact object.
The team of astronomers decided to listen to this supernova in the radio band. Radio waves pass through dust and gas, so they can reveal what is hidden from ordinary telescopes. They used three large radio telescopes: the Karl G. Jansky Very Large Array, the upgraded Giant Metrewave Radio Telescope, and MeerKAT. Weeks and months after the explosion, they caught a faint but confident signal at frequencies from 6 to 10 gigahertz. This signal resembles the noise produced when the blast's shock wave slams into surrounding material — like thunder from lightning, but in the radio band.
Such a radio voice is usually seen in supernovae with a compact progenitor and fast ejecta. But the data also allow a rarer scenario: a jet of matter aimed slightly away from us reached its peak brightness 50–100 days after the gravitational wave. If so, a 'central engine' could be working inside the supernova — a rapidly spinning neutron star or even a black hole that shoots narrow beams of particles. This scenario is called a 'superkilonova': a hybrid of a supernova and a kilonova that could explain where neutron stars with masses below the Sun's come from.
For now, the link between the gravitational wave and the supernova is not definitively confirmed — a more careful analysis of the detector data is needed. But if the link is real, this is the first time we have seen a radio signal from such a hybrid. Then astronomers will begin systematically monitoring supernovae that coincide with suspicious gravitational waves. Perhaps this is how we will learn to catch the birth of the most unusual objects in the universe.
The radio signal from supernova 2025ulz was so faint that its power is comparable to the energy emitted by a mobile phone if it were at the distance of Pluto.
In Carl Sagan's novel 'Contact,' a radio signal from space becomes the first proof of extraterrestrial life — here, radio waves help reveal the nature of a cosmic explosion.