In 2023, astronomers witnessed one of the most unusual cosmic explosions ever recorded.
The event occurred about 750 million light-years away and was first detected on July 7 by the Zwicky Transient Facility. Initially, it appeared to be a typical supernova — the explosive death of a star — and was designated SN 2023zkd.
A Strange Twist Six Months Later
Six months later, however, a search for cosmic oddities revealed something unexpected: the supernova brightened again after its initial flare.
A new analysis proposes a striking explanation — this peculiar pattern may have been caused by a giant star attempting to engulf a black hole.
“Our data suggest the explosion was triggered by a catastrophic encounter with a black hole companion, providing the strongest evidence yet that such close interactions can actually ignite a star,” said Alexander Gagliano, astronomer at the NSF Institute for Artificial Intelligence and Fundamental Interactions.
How Supernovae Usually Behave
Supernovae can form in various ways, often involving the death of massive stars or runaway thermonuclear reactions in white dwarfs. They’re relatively common, with hundreds detected each year, and typically follow a familiar pattern: a sharp burst of light followed by a gradual fade over weeks and months.
When first observed, SN 2023zkd matched this pattern. But in January 2024, an anomaly-detection tool flagged it for a second look. Archived observations showed that, 240 days after the explosion, the object unexpectedly brightened again — almost to its original peak.
Clues from Years of Archival Data
To investigate, researchers examined years of data from the same patch of sky, using machine learning to uncover signals that might have been overlooked. They found that for over four years before the explosion, the star had been gradually brightening with strange fluctuations — behavior not typical of a star on the brink of destruction.
The best-fitting scenario involves a massive, dying star and a compact companion — likely a black hole — orbiting each other closely. Over time, the star shed large amounts of mass, creating a glowing cloud. As the two objects spiraled inward, the star tried to draw the black hole into its core. But the black hole’s extreme gravitational pull destabilized the star, ultimately triggering a supernova.
The initial brightening came from the supernova’s shockwave hitting low-density gas around the system. The second peak resulted from a slower collision with the dense shell of material ejected in the star’s final years. The unusual pre-explosion fluctuations align with the stresses of having a black hole as a close companion.
Why This Could Happen
While it may sound improbable, a black hole’s gravity is equivalent to that of a star of similar mass when at the same distance. The difference is that a black hole is far more compact, allowing objects to approach much closer — even within the size of a normal star — where its gravitational influence grows dramatically.
If the star had more mass than the black hole, it could have pulled the black hole inward before being torn apart. Alternatively, the black hole might have consumed the star entirely before an explosion could occur. Both paths would produce similar collisions with surrounding material.
Either way, the outcome would be the formation of a larger black hole.
We’re now entering an era where we can automatically catch these rare events in real time,” Gagliano noted. “That means we can start linking the life cycles of stars directly to their explosive deaths — and that’s incredibly exciting.
Read the original article on: Science Alert
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