Above: An animation showing how the ASASSN-15lh most likely happened.

During the ten months that followed the outburst, the astronomers watched as ASASSN-15lh went through a series of phases consistent with a tidal disruption. The object lit up in ultraviolet light and experienced a temperature increase—features that are not consistent with a supernova event. What’s more, the location of the outburst in a red, massive and passive galaxy is not the usual home for a superluminous supernova explosion, which typically happens in blue, star-forming dwarf galaxies.

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The researchers based these conclusions on observations made from the Very Large Telescope at ESO’s Paranal Observatory, the New Technology Telescope at ESO’s La Silla Observatory, and the Hubble Space Telescope.

Above: This simulation shows a star getting torn apart by the gravitational tides of a supermassive black hole. The star gets “spaghettified,” and after several orbits creates an accretion disc. The view on the right is from the side, and at left from face on. (Credit: ESO, ESA/Hubble, N. Stone, K. Hayasaki) 

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That said, this particular tidal disruption—if that’s what it really is—is quite unique, requiring a special set of circumstances to make it happen.

“The tidal disruption event we propose cannot be explained with a non-spinning supermassive black hole,” explained Nicholas Stone from Columbia University. “We argue that ASASSN-15lh was a tidal disruption event arising from a very particular kind of black hole.”

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The supermassive black hole at the core of this galaxy features a mass about 100 million times that of our Sun. Normally, a black hole of this size wouldn’t be able to disrupt stars outside of its event horizon—that fateful boundary within which nothing is able to escape. But if this black hole happens to be a so-called Kerr black hole—a black hole that features a rapid spin—this limit no longer applies, and the black hole is able to suck up any star that ventures within a certain radius.

The researchers aren’t fully confident that the outburst came from a tidal disruption, but the clues certainly fit that scenario. Further observations will be needed to make a definitive case.

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[Nature Astronomy]