The brightest supernova, or an overindulgent black hole?

Last year, a very unusual transient made headlines around the world as ‘the most luminous supernova ever discovered’. ASASSN-15lh was certainly incredibly bright — about double the previous record holder! However, it exhibited a number of weird properties that made it unlike any other superluminous supernova: a much higher temperature; a spectrum lacking most of the usual absorption lines; a huge rebrightening in the UV; and, perhaps most significantly, it occurred right in the centre of a massive, red galaxy that did not seem to be forming any stars. Hanging out at the centre of the galaxy is a clue that a supermassive black hole could be involved. I’ve been helping out some of my PESSTO collaborators with a new study led by Giorgos Leloudas (to be published in the first issue of Nature Astronomy) that shows that in fact most of the observed properties can be explained not as a supernova, but as a star that passed too close to the galaxy’s central black hole and got eaten alive — a so-called Tidal Disruption Event. It’s a messy meal, and the heat released as the stellar debris piles up into a viscous disk and then falls into the black hole can drive a luminous flare. In the case of ASASSN-15lh, it turns out the black hole also needs to be spinning rapidly. It will be fun to see if future data can definitively prove either the supernova or black hole interpretation of this amazing event!

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