A massive star was spotted at the moment when the shock wave from its explosion reached its outer layers. The signal lasted only a brief moment and emitted mostly X-rays. A few days later, astronomers confirmed that they were witnessing the birth of the supernova SN 2026gzf.
The event began on March 21, 2026, with the detection of a source called EP260321a by the Einstein Probe satellite. It lies in a galaxy whose light takes about 500 million years to reach us. Its signal displayed the characteristics expected when the shock wave from a stellar explosion suddenly becomes visible.
Illustration of a supernova
This moment is known as "shock breakout." After the core of a massive star collapses, a wave travels outward through its layers. When it reaches the surface or the gas very close to the star, a burst of ultraviolet and X-ray radiation can escape before the supernova becomes bright in visible light.
These signals are difficult to observe because they are brief. In this case, Einstein Probe enabled astronomers to identify the beginning of the explosion almost immediately. Telescopes on the ground and in space then followed the object at different wavelengths.
The supernova belongs to the broad-lined type Ic category. Behind this designation lies a massive star that had lost its hydrogen- and helium-rich outer layers before exploding. The high speeds of the ejected material greatly broaden the visible features in its light spectrum.
This kind of supernova is sometimes accompanied by a gamma-ray burst, an extremely energetic emission associated with a jet of material launched at a speed close to that of light. SN 2026gzf resembles the explosions associated with these bursts in several respects. Yet no gamma-ray burst or X-ray afterglow corresponding to a powerful jet was detected here.
The initial flash was itself unusually faint for this category. The authors identify it as the faintest shock breakout ever associated with a broad-lined Type Ic supernova.
One possible explanation is that a jet began to form but failed to break through the star. Its energy would then have powered a slower and less powerful outflow.
New detections made within the first few minutes will help determine whether this type of explosion without a visible jet is common, and better establish what happens inside a star just before its collapse.