📡 Radio silence broken: a signal detected in deep space

Neutron stars thought to be silent actually emit faint radio signals. An unprecedented observation shows that these stars, called compact central objects, are not completely silent. This result could radically change what we know about the population of pulsars in the Milky Way.

When a massive star explodes as a supernova, its core collapses to form a neutron star. If its magnetic field is strong enough, it emits a beam of radio waves that sweeps across the sky, creating the impression of a regular beat: that is a pulsar. Yet about a dozen neutron stars at the center of supernova remnants remain silent in the radio domain, earning them the name compact central objects, or CCOs. Until now, it was thought that their magnetic field was too weak to produce a detectable jet.


A team led by Zhang Lei from the National Astronomical Observatory of the Chinese Academy of Sciences pointed the MeerKAT radio telescope in South Africa toward a particular CCO named 1E 1207.4-5209. Against all expectations, they captured a very faint radio pulse, repeating every 424 milliseconds. This value exactly matches the known rotation period of the object.

Nicknamed the "blue-eyed pulsar" by astronomer Li Di, it combines a bright X-ray emission with this faint radio glow, evoking a blue eye. The supernova that created it exploded over 4,100 years ago.

In 2015, X-ray observations noted a rotation "glitch," a sudden acceleration likely due to internal movements of matter. According to Lei's team, this glitch either strengthened the pulsar's magnetic field or changed its orientation, enough to trigger or reveal radio waves that were previously too faint to be detected.

After a glitch, a neutron star's rotation gradually slows back to its initial speed. Therefore, the radio emission from the blue-eyed pulsar is expected to fade again. Continuous monitoring will verify this. If so, it would mean that there is a vast population of very faint pulsars in the Galaxy, overlooked if their supernova remnant is not or is no longer detectable. Moreover, some pulsars considered old might actually be young but weak in radio emission.

This discovery could also explain why some supernova remnants seem to lack a pulsar. The most famous case is SN 1987A in the Large Magellanic Cloud. Although indirect evidence indicates the presence of a neutron star at its center, no radio emission has yet been detected.