No Rare Supernova, But A Black Hole Ripping A Star To Shreds

What scientists initially believed was a rare supernova is now thought to be even more dramatic, a star ripped apart by a vast black hole. Researchers studying results from the All-Sky Automated Survey for Supernovae (ASAS-SN) thought they'd spotted something unusual enough back in January, when they identified what at the time was thought to be a superluminous supernova. Though it was 3.8 billion light years away from Earth, and thus invisible to the human eye, it was nonetheless twice as bright as any previously recorded supernova.

Dubbed ASASSN-15lh, the unusual output was enough to cause waves among astronomers. There are few recorded superluminous supernovas on the books; also known as hypernovas, they're still fairly mysterious in terms of their cause, too.

Now, however, there are doubts that ASASSN-15lh even was such an event. In a study published in Nature, researchers have suggested that, far from being a superluminous supernova, the star actually met its end in a much more dramatic way. Indeed, they argue it was torn apart by a huge and voracious black hole.

Known as a tidal disruption event, it would require both a sizable black hole and very specific positioning in order to take place. Although a black hole was already believed to be at the center of the galaxy in which ASASSN-15lh is placed, the star would need to be sufficiently offset from it that it would not be consumed whole but instead ripped into pieces. That would also require that the black hole itself be spinning rapidly, much in the way that a kitchen blender's blades break apart food.

Nonetheless, while it would demand a "perfect storm" of criteria, the study's authors say several pieces of evidence count in their theory's favor:

"Our observations are more consistent with a tidal disruption event than a superluminous supernova because of the temperature evolution, the presence of highly ionized CNO gas in the line of sight and our improved localization of the transient in the nucleus of a passive galaxy, where the presence of massive stars is highly unlikely" Leloudas, G. et al. "The superluminous transient ASASSN-15lh as a tidal disruption event from a Kerr black hole"

According to the European Space Agency, the star's fate would be a progressive one rather than a sudden end. Its orbit would continually get closer and closer to the black hole, getting "spaghettified", the ESA says, and in the process forming an accretion disc around the supermassive black hole itself. The bright flash would be the result of the final destruction close to the black hole's event horizon.

Sightings of tidal disruptions are so rare that only a handful are on record to compare to the observed fate of ASASSN-15lh. "The rapid spin and high black hole mass can explain the high luminosity of this event," they suggest, though concede that there are still questions ensuing from the data which leaves alternative explanations open.

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