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6d ago

Caltech Team Reports Candidate 'Superkilonova' From August Gravitational-Wave Alert

A new analysis outlines a scenario in which a supernova births two sub-solar neutron stars that immediately merge, with the association still unproven.

This artist's concepts shows a hypothesized event known as a superkilonova. A massive star explodes in a supernova (left), which generates elements like carbon and iron. In the aftermath, two neutron stars are born (middle), at least one of which is believed to be less massive than our Sun. The neutron stars spiral together, sending gravitational waves rippling through the cosmos, before merging in a dramatic kilonova (right). Kilonovae seed the universe with the heaviest elements, such as gold at platinum, which glow with red light. Credit: Caltech/K. Miller and R. Hurt (IPAC)
artist's impression of a superkilonova

Overview

  • LIGO and Virgo recorded the low-confidence signal S250818k on August 18, 2025, and internal analysis indicates at least one merging object was less massive than a typical neutron star.
  • Within hours, the Zwicky Transient Facility identified AT2025ulz about 1.3 billion light-years away in the gravitational-wave localization region.
  • The transient showed kilonova-like red emission and heavy-element signatures for roughly 72 hours before brightening, turning bluer, and exhibiting hydrogen and helium lines consistent with a Type IIb supernova.
  • The Astrophysical Journal Letters paper reports a combined mass near 0.87 solar masses for the merger and infers a 99 percent probability that one component was below one solar mass.
  • The authors present the event as a candidate superkilonova, acknowledge they cannot rule out a chance coincidence, and point to future detections with upcoming facilities to test the hypothesis.