First Candidate Superkilonova AT2025ulz: A Star Exploded Twice

A Landmark Multi-Messenger Discovery in Cosmic Explosions

Astronomers are studying AT2025ulz, a celestial explosion that appears to combine characteristics of both a supernova and a kilonova—a rare double explosion event long hypothesized but never conclusively observed. This hybrid phenomenon, often called a “superkilonova,” may represent a new class of cosmic transient, blending explosive stellar death and neutron star merger physics in one extraordinary event.

What Is AT2025ulz and Why It Is Unique

First detected in August 2025 following a gravitational-wave alert from the LIGO–Virgo–KAGRA collaboration, AT2025ulz initially resembled a kilonova—a compact-object merger whose light is dominated by heavy elements like gold. However, subsequent observations revealed spectral and brightness changes inconsistent with standard kilonova behavior and more typical of a core-collapse supernova observed days later.

The hybrid nature suggests an unprecedented explosion:

1. A massive star exploded, generating a supernova-like display.
2. The core may have formed two neutron stars that spiraled in and merged soon after, producing kilonova features and potentially illuminated by gravitational waves.
3. This rare sequential process has been informally dubbed a superkilonova—a candidate first seen in AT2025ulz.

Multi-Messenger Signals and Scientific Significance

Unlike conventional supernovae, which explode once as a massive star collapses, and standard kilonovae, which are the aftermath of neutron star mergers, a superkilonova would combine both processes in a single event. This explains the unusual observational pattern: early light and color evolution matching kilonova expectations followed by later evolution with hydrogen and helium signatures typical of supernovae.

While the classification as a superkilonova is not universally confirmed, it offers a powerful new lens on how compact objects and extreme stars may interact.

Why This Discovery Matters for Astrophysics

The AT2025ulz event challenges existing stellar explosion models and opens up questions such as:

• How can a massive star’s collapse produce two neutron stars that merge so quickly?
• What does this imply about neutron star formation and masses that deviate from standard limits? (The event’s gravitational-wave data suggests at least one merger partner may have been lighter than a classic neutron star, a puzzling hint for stellar physics.)
• Could there be many more such hybrid explosions hidden in survey data?

Multi-messenger observations—gravitational waves paired with electromagnetic emission—are essential for identifying rare cosmic events like this one and shaping future astrophysical theory.

Global Follow-Up and Future Research

Since its detection, observatories worldwide—from optical survey telescopes to X-ray and radio facilities—have tracked AT2025ulz’s evolving light curve and spectrum. Continued study in 2026 aims to confirm the superkilonova interpretation, compare it with classic kilonovae such as GW170817, and refine models of explosive stellar deaths in the universe.