Supernova or imposter?

A supposed supernova has recently been exposed as a potential imposter, having suffered many violent eruptions over the 21 years preceding its final large outburst. It could be one of an entirely new class of stellar object.

The spiral galaxy NGC 2770 and its two supernovae as observed at the Asiago Observatory. The image was obtained on 12 January 2008 and shows the then fading SN 2007uy and the newly discovered SN 2008D.

The supernova was seen in the galaxy NGC 2770, a spiral galaxy 88 million lightyears away. Credit: ESO

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A supposed supernova has recently been exposed as a potential imposter, having suffered many violent eruptions over the 21 years preceding its final large outburst.

It could be one of an entirely new class of stellar object.

In 2015, a star in the spiral galaxy NGC 2770 appeared to brighten very rapidly.

At first it was assumed that a star had gone supernova, which would have resulted in its destruction, but there are signs that the star remains intact.

The star was first seen brightening in April 2015, but slowly dimmed until 16 May, when it flared up again.

Rather than tailing off, this second outburst increased in energy until it reached a peak on 24 May.

Later study of archival data of NCG 2770 dating back to 1994 revealed that the star had had frequent minor eruptions for decades beforehand.

“We know already several cases of these luminous blue variables that follow the same patterns: smaller, more or less continuous eruptions over several decades and an outburst between 40 and 80 days before the main explosion.

In fact, the evolution of SN 2015bh turns out to be basically a carbon copy of SN 2009ip, a famous example of a supernova impostor that took place in 2012 whose fate is still highly debated,” says Christina Thöne, from the Institute for Astrophysics of Andalucia.

The team continued to observe the star for 200 days after the May event but were unable to confirm if the star had exploded.

It could be that some kind of stellar object remained intact, and SN 2015bh was not a supernova.

It’s thought that instead, the event set the star down a new track in its stellar evolution.

“SN 2015bh is not an isolated case and there are possibly many more similar objects out there that have gone unnoticed,” says Thöne.

“But it seems we have encountered a new type of stellar event.

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Now we need to uncover the mechanism driving those events and find out why the observed cases show such a very similar behaviour.”