Astronomers may have witnessed the first known “superkilonova” – an unprecedented cosmic event combining elements of both a supernova and a neutron star merger. The discovery, detailed in the December 20th Astrophysical Journal Letters, centers on a distant stellar explosion that appears to have occurred in two distinct phases. This matters because it challenges existing models of how dense stellar remnants behave, potentially revealing new pathways for the creation of heavy elements in the universe.
The Unusual Signal
The event began with ripples in spacetime detected by the Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo detector in Italy. These signals indicated the merging of two neutron stars approximately 1.8 billion light-years away. What set this event apart was that at least one of the merging neutron stars appeared to have less mass than the sun.
This is significant because established stellar physics predicts neutron stars – the ultra-dense remnants of supernovae – should be at least 1.4 times the mass of our sun. Every previously observed neutron star has been more massive. This anomaly initially puzzled researchers, suggesting something unusual was happening.
From Kilonova to Supernova?
Follow-up observations at the Palomar Observatory revealed a reddish glow originating from the same direction as the gravitational wave signal. The initial data resembled a kilonova, an event already known to produce heavy elements like gold and platinum through rapid neutron capture.
However, unlike typical kilonovae, this object began to brighten again, exhibiting characteristics more commonly associated with supernovae – specifically, the presence of hydrogen. This led astronomers to propose a radical hypothesis: the observed event may be a kilonova within a supernova, or a “superkilonova.”
The Proposed Mechanism
The leading theory suggests that a star first exploded as a supernova, leaving behind a rapidly spinning neutron star. This neutron star then fragmented, either splitting into smaller stars or forming a rotating disk that coalesced into multiple neutron stars – a process akin to planet formation. The subsequent collision of these smaller neutron stars would have produced the observed kilonova signature.
Remaining Uncertainty
Not all researchers are convinced. One key concern is whether the gravitational wave signal was genuine, or simply noise from terrestrial sources. LIGO is conducting further analysis to rule out this possibility. Additionally, verifying that the light and gravitational wave signals truly originate from the same event remains a challenge.
As astronomer Cole Miller points out, “Is the current evidence such that you’re going to sell your house to buy tickets for [the superkilonova theory]? No.” However, the potential implications are strong enough to warrant further investigation.
The Search Continues
Confirming this event requires additional observations. Finding similar events, especially closer to Earth, would significantly strengthen the hypothesis. But these events are exceptionally rare; only two kilonovae have ever been observed with both electromagnetic and gravitational waves.
Despite the uncertainty, the current discovery underscores the universe’s capacity for surprise. The rarity of superkilonovae suggests that these events are not typical, but their existence opens new avenues for understanding extreme astrophysical phenomena.
