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At first, the gleaming object looked like it could be a supernova, a cataclysmic explosion that marks the end of a star. Stars, as immutable as they may seem to us, blazing brightly in the night sky, can die. The process is ferocious and quick: The star, after millions and millions of years of existence, runs out of the fuel that produces its brilliant light. Its core shrinks, heats up, and then collapses, sending stellar material hurtling across space. The resulting glow briefly outshines whole galaxies before fading away.
Telescopes have observed supernovae before. But this one?
“It was very clear that it was not a normal supernova,” says Raffaella Margutti, an astrophysicist at Northwestern University, one of the many scientists who followed the mystery object, known as AT2018cow.
The alleged supernova was dozens of times as bright as any supernova on record. It reached its peak luminosity in two days and faded after 16 days. This, on the vast timescales of the universe, is extremely fast—faster than astronomers had ever seen.
According to current understanding in astronomy, this scenario should not be possible. The brightest supernovae usually come from the deaths of the biggest stars. These are slow affairs: The more material a dying star expels, the longer it takes for the afterglow of the explosion to reach its peak radiance. How could the mystery explosion flare so brightly and fade so quickly?
“It’s weird to see something that’s bright and evolving fast,” explains Iair Arcavi, an astronomer at Las Cumbres Observatory, a global network of ground-based telescopes. “It’s kind of a contradiction.”
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When astronomers took a closer look at telescope data, they saw an unusual excess of X-rays. “Our first reaction, when we got that data, was maybe we made some mistake, because we’ve never seen that,” Margutti said. It meant that, deep in the core of the explosion, there was a source of X-rays so persistent that it would shine through the material surrounding it.
That gave the astronomers a clue about what they could be looking at. Almost all massive stars are thought to produce new astrophysical objects when they die—either a black hole or a neutron star. Black holes are invisible maws that gobble any material—gas, dust, entire stars—that comes near. Neutron stars are fast-spinning cores of tightly packed neutrons, the stripped-down versions of their progenitors. Both are extremely dense objects capable of violently whisking surrounding material. Margutti and her colleagues suspect that, in this case, those effects were enough to produce an unusually radiant light show.
The telescope, they’ve concluded, had captured the creation of a brand-new astrophysical object, either a black hole or a neutron star, for the first time.