Supernova imposter detected in 2009 finally explodes
Astronomers have revealed that a massive star, which they have watched repeatedly mimicking a supernova since 2009, has finally exploded for real.
The result is of special interest because it provides new critical information on the final death throes of massive stars in the years leading up to their explosion.
The object is SN 2009ip, a luminous extragalactic transient first detected in 2009 in the spiral galaxy NGC 7259, which lies 67 million light-years away in the constellation Piscis Austrinus.
Soon after discovery in 2009, the outburst of SN 2009ip was actually recognized to be a non-terminal event, one in which the culprit star survived.
Investigation into existing data archives revealed that the stellar progenitor of this outburst had been serendipitously detected in multiple images of host galaxy taken throughout the decade prior to discovery.
Detailed analysis of those images determined that the star was blue and hot, emitted about 1 million times as much energy as the Sun, contained 50-80 Suns worth of mass, and varied wildly in brightness. Such objects are known as luminous blue variables (LBVs), stars that have nearly exhausted their hydrogen fuel, and, for reasons, which are poorly understood, undergo brief recurrent episodes of explosive mass loss.
LBV outbursts are so luminous that they can be seen at extragalactic distances.
Since their radiation characteristics mimic true supernovae, they are sometimes referred to as supernova “impostors”. SN 2009ip was one such impostor.
Its initial outburst lasted for 1-2 weeks, and ejected an envelope of hydrogen-rich mass at a speed of roughly 600 km/s, as determined by measurements of the object’s spectrum.
One year after the initial outburst, a second, similar event was detected, and also faded quite rapidly. Then, after nearly 2 years of subsequent quiescence, a third outburst was detected on July 24 2012.
But this time, the situation was different. Instead of fading within days of the outburst, the source maintained its luminosity for much longer.
Moreover, after about two months, the object began to exhibit evidence for very fast material in its spectrum.
Instead exhibiting expansion velocities of 600 km/s, like the impostor eruptions in 2009 and 2010, the bulk velocity of the hydrogen-rich material during the 2012 event had reached values closer to 10,000 km/s.
The spectral velocity measurements were made using Steward Observatory’s Bok telescope.
The findings will be published in the Monthly Notices of the Royal Astronomical Society.