Astronomers have observed what appears to be a new sort of supernova. Perhaps a dozen similar events have been seen before, but we've never collected enough data to do more than guess their nature. While there are still plenty of unknowns about the latest, much shorter, stellar brightening witnessed by the Kepler Space Telescope, the team that discovered it think it involves an exploding star inside a cloud of dust.
Core collapse supernovas occur when the radiation produced can no longer withstand the gravitational forces inwards, causing an implosion that rebounds in an enormous shockwave. Astronomically speaking their lives are exceptionally short, lasting just a few months, a tiny amount of time compared to the millions or billions of years the star that caused the event survived before. However, astronomers have caught glimpses of a few sudden brightenings of stars in other galaxies that appeared to last just weeks, earning the name Fast Evolving Luminous Transients (FELTs).
Like supernovas, FELTS are so bright they can be detected in distant galaxies. Unfortunately, FELTs have only ever been noticed in sky surveys long after the light had dimmed, preventing detailed study. At most, Dr Brad Tucker of the Australian National University told IFLScience, we have had five data points, usually even fewer.
In Nature Astronomy Tucker describes the case of KSN 2015K, which exploded and reached peak brightness in 2.2 days, an eighth the time of a type Ia supernova. It then lost half its brightness in another 6.8 days, where supernovas usually take months. Tucker and fellow authors conclude KSN 2015K represents a supernova hidden inside a cocoon of gas and dust.
Where most supernovas' light comes from the decay of radioactive elements formed in the explosion, particularly nickel-56, Tucker and co-authors attribute what they have seen to the supernova's shockwave heating up the surrounding material to the point where it radiates intensely. As the cocoon cooled, the light faded, and this occurred far more rapidly than nickel-56's radioactive decay.
The big mystery remains why the star released the surrounding material in the first place. This isn't the first time we have seen a star release huge amounts of material at the end of its life, however. The monstrously heavy Eta Carinae has been pouring similar materials into space since the mid 19th century so that it is now dimmed by the material that shrouds it.
Tucker told IFLScience KSN 2015K and Eta Carinae have some similarities, but also major differences. For one thing, Eta Carinae has put out several pulses of material over almost two centuries, and has yet to explode, although it is thought the big event can't be too far away. The material released by KSN 2015K, on the other hand, appears to all have been emitted in the year prior to eruption, judging by its proximity to the star.
Like ordinary core-collapse supernovae, the event should have left a neutron star or black hole behind, but Tucker told IFLScience the shorter span of the event, with a similar peak brightness to other supernovae, indicates there may be more mass left behind, “So there may be an unusually heavy neutron star. We know these exist, and this might explain where they come from.”
The unprecedented detail exists because the Kepler Space Telescope measures the light from the stars it studies every 30 minutes, tracking KSN 2015K's rise and fall in hundreds of details. While designed to detect the dips in brightness of stars in our galaxy as planets pass in front of them, sometimes Kepler picks up something interesting in the background. KSN 2015K was found through the good fortune that the young spiral galaxy in which the explosion occurred is nearly behind one of Kepler's target stars, but 1.3 billion light-years further away.
Kepler collects data for periods of months before processing and downloading it for astronomers to study, so by the time the February 2016 event was noticed it was too late to give it extra attention with Earth-bound telescopes.
We don't, at this stage, know that all FELTs have the same cause. Indeed, KSN 2015K appears extreme even in its class, with an even shorter peak than most. However, the shrouded supernova theory was already one of the competing ways astronomers were trying to explain FELTs, and now looks the clear favorite.
