The Quest to Watch a Supernova in Real Time

A kilometer below Japanese ground lies a massive cylindrical tank, its steel walls lined with more than 10,000 photomultiplier tubes that await the arrival of neutrinos. Could recent upgrades to the Super-Kamiokande neutrino detector improve our ability to spot and assess supernova explosions in real time?

Memo from a Dying Star

In 1987, 25 tiny messengers arrived at Earth following a tremendous explosion 168,000 light-years away. This signal marked the first time that these near-massless messengers — neutrinos — had ever been directly observed from a Type II supernova, the core collapse of a massive star.

The benefit of observing these particles is clear: because neutrinos so rarely interact with matter, they arrive at Earth carrying untouched information about the death of the star that produced them. And because neutrinos escape the collapsing star more readily than photons do, they arrive before the visible light from the supernova. This means that if scientists can detect and localize a supernova neutrino burst, they can notify observatories about the imminent optical signal from the supernova. Combining the neutrino and electromagnetic observations could then provide valuable insight into long-standing questions, like the mechanism of the star’s explosion.

The catch? We only expect a handful of nearby (i.e., in our galaxy) supernova explosions every century — and what’s more, observing neutrinos is no easy feat! In the nearly four decades since those 25 messengers heralded the supernova SN 1987A, we haven’t detected any other neutrinos linked to supernovae. But that doesn’t mean we haven’t been preparing.

A Salty Upgrade for Super-K

The Super-Kamiokande (Super-K) neutrino detector has recently undergone an upgrade: scientists have dissolved a gadolinium salt in the 55,000 tons of ultrapure water that fills its underground tank. The addition of this rare-earth metal improves the detector’s ability to differentiate between neutrinos and antineutrinos, thereby enabling scientists to more accurately localize the supernova that produced an incoming burst of neutrinos.

In a recent article led by Yuri Kashiwagi (Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo), Super-K scientists have analyzed how this upgraded observatory — and a corresponding real-time alert system developed to notify optical astronomers of the explosion and tell them where to point their telescopes — will respond to a hypothetical supernova within our galaxy.

Passing the Message On

Using multiple different supernova models, Kashiwagi and collaborators simulated the neutrino burst that would be produced by a supernova exploding roughly 33,000 light-years away. Through further simulations, the team then explored how successfully Super-K would detect neutrinos from the burst, how well the supernova’s location could be identified from Super-K’s detections, and how quickly this information could be broadcast to astronomical observatories via the alerting system.

The authors found that the simulated supernova’s location could be rapidly identified to within 3–7° on the sky, and that information could be sent to optical observatories within minutes. In many cases, this response would be sufficient for wide-field telescopes like the upcoming Vera Rubin Observatory to catch the rise of the optical signal from a supernova — and the neutrino data captured by Super-K could even help distinguish between different supernova models.

While there’s still more to learn, it seems likely that Super-K is well prepared for future nearby supernova detections. Now all that’s left is to wait for the next explosion!

Citation

“Performance of SK-Gd’s Upgraded Real-time Supernova Monitoring System,” Y. Kashiwagi et al 2024 ApJ 970 93. doi:10.3847/1538-4357/ad4d8e