Massive stars know how to go out in style. Some of the brightest, longest-lived flares likely develop as supermassive black holes shred megastars, researchers report.
Such events are a new class of cosmic explosions. They’re called extreme nuclear transients, or ENTs. Transients are events that change the brightness of the night sky. ENTs release more energy than any other transients known.
In fact, they emit about the same energy in a year as our sun will in 10 billion years — its whole lifetime, says Jason Hinkle. He’s an astronomer at the University of Hawaii at Mānoa in Honolulu.
Some sky-brightening transients come from messy meetups between stars and black holes. These usually involve a star less than twice the mass of our sun that veers too close to a black hole. That black hole’s gravity will be stronger than the gravity holding the star together.
Its powerful pull then overwhelms the star, causing it to stretch around the black hole. This is called a tidal disruption event, or TDE. During a TDE, the spewed star matter — some of which gets swallowed by the black hole — heats up. This produces X-rays, ultraviolet radiation and other wavelengths that usually light up the sky for days to months.
But Hinkle and his colleagues were curious about more extreme events.
They searched telescope data for especially radiant cosmic glows that lasted more than a year. And they turned up three.
Each took more than 100 days to reach peak light production. Dimming to half that level took at least 150 days more. Their maximum brightnesses were 30 to 1,000 times that of supernovas — exploding stars.
ENTs prove “very bright and very long-lived,” Hinkle says. That combo, he reports, doesn’t exist in other types of transients.
Supermassive star-shredders
These super-showy flares emitted two to 10 times the energy of those from the most energetic supernovas known.
That ruled out exploding stars as their source. Instead, the researchers determined that these light shows were likely supercharged star-gutting TDEs. Those TDEs involved stars with three to 10 times the mass of the sun. And the black holes ripping them apart had around 250 million times the sun’s mass!
Hinkle’s team shared its findings June 6 in Science Advances.
The newly observed events show that hot gas and dust appears to linger near their supermassive black holes. That hints that these black holes are actively shaping their host galaxies.
Previously studied TDEs had been in dormant galaxies. So researchers have only explored links between the star-shredding events and awake black holes with computer models, says Taeho Ryu. He’s an astrophysicist at the Max Planck Institute for Astrophysics in Garching, Germany. He did not take part in the new work.
The new study provides a first look at star-shredding events in still-forming galaxies. Based on computer models, Ryu notes, TDEs may happen more often with galaxy-shaping black holes than with quiet black holes.
By studying these cosmic explosions, he says, researchers can learn more about TDEs and active black holes at the same time.
These extreme cosmic explosions are quite rare. Only one ENT is estimated to occur for every 10,000 typical TDEs. Still, Hinkle says, they’re by far the best way to study these “very massive, otherwise quiet, supermassive black holes.”