Can black holes detonate stars? Rare supernova holds the clues

Astronomers recorded a star merging with a black hole as it burst into a supernova. Thanks to AI-assisted technology, it was possible to study this extraordinary event as it unfolded.

In July 2023, the Zwicky Transient Facility (ZTF) flagged an unusual cosmic flash while scanning the skies for anomalies. The explosion is now known as SN 2023zkd. 

Further observations of its characteristics confirmed that this was no ordinary supernova.

The discovery was made by a collaborative group conducting a survey known as the Young Supernova Experiment (YSE). The team consists of scientists from the Center for Astrophysics at Harvard & Smithsonian and MIT.

A rarity in supernova history

The supernova SN 2023zkd was likely the result of a heavyweight star locked in a tight orbit with a lurking black hole companion.

Over time, the black hole’s gravity pulled out material, like gas and dust, from the star. This reduced the star’s angular momentum, bringing the star ever closer to the black hole.

In close proximity to the black hole, the star was subjected to extreme gravitational stress. Over time, it partially merged with its companion in what astronomers call an instability-induced merger – a process that can prematurely trigger a star’s explosion.

Study lead author Alexander Gagliano is a researcher at the NSF Institute for Artificial Intelligence and Fundamental Interactions.

“This discovery shows how important it is to study how massive stars interact with companions as they approach the end of their lives,” Gagliano said in a news release.

AI-detected star-black hole merger

AI tools are trained on thousands of light curves – graphs that show how a star’s brightness changes over time – to detect subtle signs of an outburst. One of these tools is the Lightcurve Anomaly Identification and Similarity Search (LAISS).

As the scientists were tracking SN 2023zkd’s decline, LAISS detected unusual color changes. Surprisingly, it eventually displayed two additional peaks of brightness following its detection. 

The first peak came when the explosion’s shockwave hit the thin gas the star had shed earlier. The second, delayed peak arose as the ejecta slammed into a denser, disk-shaped cloud of material around the equator of the star.

“Our machine learning system flagged SN 2023zkd months before its most unusual behavior, which gave us ample time to secure the critical observations needed to unravel this extraordinary explosion,” said Gagliano.

Single star or a binary supernova?

At first, astronomers considered single-star explanations because some characteristics were a match. The first possible explanation was a luminous blue variable (LBV), which is a massive, unstable star known for dramatic outbursts. 

SN 2023zkd exhibited multi-year brightening that resembled LBV behavior. But it was far too bright. Its steady, persistent brightness is not compatible with the short bursts typical of LBVs.

Similar to the LBV theory, other single-star theories also crumbled under the weight of the evidence. 

The binary merger theory – the idea that the star collided with a black hole – explained the presence of the equatorial disk of debris, as well as the persistent brightness. 

Opening the secrets of the universe

SN 2023zkd offers a preview of what’s to come as large observatories and AI work together. This study relied on data from telescopes located in various parts of the world, including the United States, Australia, and Japan. 

“We think this might be part of a whole class of hidden explosions that AI will help us discover,” said V. Ashley Villar, an assistant professor of astronomy at Harvard University.

Future campaigns will track AI alerts using telescopes across the spectrum. Astronomers will map stellar gas in optical, infrared, X-ray, and radio wavelengths to piece together the chain of events that cause a star to explode.

“We’re now entering an era where we can automatically catch these rare events as they happen, not just after the fact,” noted Gagliano. “That means we can finally start connecting the dots between how a star lives and how it dies, and that’s incredibly exciting.”

The study was published in the August issue of The Astrophysical Journal.

Image Credit: Melissa Weiss/CfA

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