Astronomers have spotted a supernova that’s unlike any they’ve seen before, and they think it’s a star that exploded while being consumed by a black hole. This rare event, named SN 2023zkd, was first seen in July 2023, about 730 million light-years away in a low-mass host galaxy with little star formation.
Classified as a Type IIn supernova, it stands out because of narrow hydrogen lines in its spectrum, along with strong helium features that make it helium-rich. These traits point to the explosion happening inside a dense cloud of gas the star lost earlier.
A special AI tool, called LAISS, designed to find strange space events, flagged it for immediate attention. This allowed astronomers to get crucial data from the beginning of the explosion, giving them a full picture of what happened.
Scientists from Harvard, MIT, and other universities believe a massive star, starting with at least 30 solar masses and partially stripped of its outer layers, was in a tight orbit with a black hole. As the star got closer, the black hole’s immense gravity began to pull off gas and dust, forming a swirling disk around it.
Before the black hole could fully swallow the star, the intense pressure and gravitational stress caused the star to detonate. The blast ejected about 10 solar masses of material at high speeds, with an energy around 2 × 10^51 ergs.
The light curve showed two peaks: the first reached an absolute magnitude of about -18.7 in the r-band, then it faded, only to brighten again to -18.4 magnitude roughly 240 days later. This second flash came from the shock wave hitting denser material the star had ejected earlier.
Spectra revealed asymmetric lines from hydrogen and helium, with velocities ranging from 400 km/s in slow-moving equatorial hydrogen to 1,000-2,000 km/s in faster polar helium. These suggest an uneven distribution of material around the system.
Looking at old data, scientists found that the system had been glowing more brightly for four years before the big explosion, with a persistent brightness around -15 magnitude in the the r-band. This precursor split into two parts: a long steady phase and a ramp-up in the final year.
The total circumstellar material involved was about 5-6 solar masses, lost in bursts 3-4 years and 1-2 years before the explosion. Such high mass-loss rates, up to 1 solar mass per year, are hard to explain with a single star.
An alternative view is a tidal disruption where the black hole rips the star apart without a true supernova, but the long precursor and spectral details favor the merger scenario. “This is strong evidence for black holes triggering these special explosions,” said Alexander Gagliano, the lead author of the study. It points to a process where orbital instability leads to a common envelope phase, ending in a blast.
V. Ashley Villar, a co-author, believes this could be a new kind of supernova that scientists haven’t recognized before. Most massive stars live in pairs, so binary ends like this might be more common than thought.
This discovery shows how binary systems can produce odd supernovae and perhaps lead to pairs of black holes that merge later, detectable by gravity waves.
As new, powerful telescopes like the Vera C. Rubin Observatory become fully operational, they will scan the entire sky for transient events. Scientists believe that AI tools will be key in sifting through the vast amounts of data to find similar, hidden classes of supernovae.
The findings were published in the Astrophysical Journal.
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