Almost 2 billion light-years away, a supermassive black hole has been slowly swallowing a star. That's a pretty spectacular thing to witness, and astronomers have had ample time to take a peek – this interstellar light show has been visible for over 10 years, making it the most drawn-out death of a star ever observed.
The phenomenon is known as a tidal disruption event (TDE), and it occurs when an object, like a star, passes too close to a black hole and becomes trapped in its gravitational pull. The star is torn apart in the process, blasting some of its matter outwards while the rest is sucked into the black hole. As that material heats up, it creates a flare of X-rays that can be detected by instruments here on Earth and in orbit.
This particular event is occurring at a black hole called XJ1500+0154, at the center of a small galaxy about 1.8 billion light-years away. Its X-ray burst was first spotted on July 23, 2005 by the European Space Agency's XMM-Newton orbital telescope, but it hadn't been seen just three months earlier, when NASA's Chandra X-ray Observatory scanned the area on April 2. Since then, it was observed several times by Chandra, XMM-Newton and the Swift satellite, with the brightness of its X-ray signal increasing by up to 100 times, peaking on June 5, 2008.
"We have witnessed a star's spectacular and prolonged demise," says Dacheng Lin, lead researcher on the study. "Dozens of tidal disruption events have been detected since the 1990s, but none that remained bright for nearly as long as this one."
The length of the event isn't the only extraordinary thing about it, though. It's also shone consistently brighter than models say it should be capable of. The Eddington Limit is a loose upper cap on the luminosity an object should be able to achieve, calculated as a balance between the radiation emitted outward and the gravity pulling matter in. But the X-ray signature of XJ1500+0154 regularly surpassed that supposed limit. That means it's either the most dense star we've seen torn apart, or it's the first time we've managed to catch the whole show as it happens.
"For most of the time we've been looking at this object, it has been growing rapidly," says James Guillochon, co-author of the study. "This tells us something unusual – like a star twice as heavy as our Sun – is being fed into the black hole."
The realm of Super-Eddington luminosities may help researchers explain the mystery of how supermassive black holes grew so dense so fast, in the universe's ancient past.
"This event shows that black holes really can grow at extraordinarily high rates," says Stefanie Komossa, co-author of the study. "This may help understand how precocious black holes came to be."
XJ1500+0154 is still visible today, but the researchers expect that its brightness will fade away over the next few years, as the black hole runs out of star to consume.
The research was published in the journal Nature Astronomy.
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