The strange supernova that just kept exploding may not be quite so weird, after all.
Most supernovae brighten once as they explode and then fade, but one that was first spotted three years ago has brightened five times so far and has only just started to dim over the past few months. Researchers thought it might be a new kind of explosion, but new observations show it might be a regular supernova whose light is lent extra power by a dense disc of debris.
at University of California, Santa Barbara, and his colleagues first saw supernova iPTF4hls in September 2014 and noticed that while its light spectrum was fairly normal, it was far brighter and longer-lasting than expected.
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To figure out what was causing this supernova鈥檚 weirdness, and at the University of Arizona in Tucson examined its light with the Multiple Mirror Telescope in Arizona. They saw evidence of a disc of material around the exploding star that was moving much slower than the ejecta from the explosion itself.
This doughnut of debris could have been causing the explosion to repeatedly brighten and dim again. As the ejected material rammed into the cloud of gas and dust, some of the energy from its motion could have been converted into light and heat, powering the supernova鈥檚 extreme and persistent brightness.
Another possible mystery
They did not see signatures of this ring of dense material at first, probably because it was covered by the edge of the explosion. But, as the shell of gas and plasma from the supernova expands and cools over time, it becomes transparent and we can catch hints of what鈥檚 going on inside.
鈥淭he disc or torus of material is basically acting as another engine to keep the supernova going, but it鈥檚 hidden during the main explosion,鈥� says Andrews. 鈥淣ot until you open the hood and see the steam come out do you realize what鈥檚 actually going on.鈥�
Arcavi says that while this scenario could account for the supernova鈥檚 varied and long-lasting brightness, there are other aspects that remain baffling. For example, a group of Chinese researchers recently detected a source of gamma radiation in the area around iPTF4hls that did not exist before the explosion.
鈥淲e鈥檝e never seen gamma rays coming from a supernova in this way, so it鈥檚 kind of like we鈥檝e moved forward one step but also added another possible mystery,鈥� Arcavi says.
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Read more: Cold case: The unsolved mystery of what lit Kepler鈥檚 supernova
