The brightest-ever supernova isn’t a supernova at all, study finds

An object previously believed to have been the brightest supernova ever discovered has actually turned out to be a supermassive black hole that is consuming a star that wandered too close to it, according to a new study published earlier this week in the journal Nature Astronomy.

Originally discovered in May 2015 by the  All-Sky Automated Survey for SuperNovae (ASAS-SN), a network of telescopes in Hawaii and Chile that keeps watch for rapidly changing objects in the sky, the event was first thought to be a superluminous supernova, explained Science.

While a superluminous supernova occurs when a massive star collapses under its own gravity at the end of its life, ejecting a bright but short-lived ball of hot gas and dust before fading away, it turns out that the event known as ASASSN-15lh turned out to be something else entirely – a rare phenomenon called a tidal disruption event (TDE) – the authors of the new study reported.

In a TDE, a star wanders too close to the event horizon of a supermassive black hole and ends up being pulled apart by that black hole’s tidal forces, experiencing spaghettification (the horizontal compression and vertical stretching of an object), giving off a final flash, then fading away.

Event would be just the 10th suspected TDE ever discovered

In the case of ASASSN-15lh, scientists monitoring the object discovered unusual behavior that tipped them off that it might not be a supernova after all, according to Ars Technica. Over a ten month span following its discovery, they found that its changing temperature over time, the fact that it was located in a distant galaxy, and other signs pointed towards spaghettification.

Based on the authors’ calculations, they determined (based on its proportional size to the galaxy in which it resides) that the star was being torn apart by a black hole more than 100 million times more massive than the Sun, and that the flash of light was created by shocks in the star’s material created by collisions involving said material, as well as heat caused by friction.

TDEs themselves are rare events: according to Science, only 10 are currently suspected to have occurred. But as lead author Giorgos Leloudas of the Weizmann Institute of Science in Rehovot, Israel, explained, the changes out output from ASASSN-15lh all appear to suggest a TDE.

In theory, a black hole as massive as this one should consume a nearby star whole, tearing it to pieces only after it passed through the event horizon. However, Leloudas’ team believes that the gravitational field around this particular black hole is different because it is rotating, allowing the TDE to be visible. If verified, this would be the first confirmed spinning black hole at the center of a quiescent galaxy, the journal noted.

However, as Leloudas told Ars Technica, “Even with all the collected data we cannot say with 100 percent certainty that the ASASSN-15lh event was a tidal disruption event.” However, the Weisman Institute astronomer added, “it is by far the most likely explanation” for ASASSN-15lh.

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