Return of superstar supernova raises doubt about its identity

Brightest explosion’s resurgence suggests black hole mischief

IT’S BAAAACK An alleged supernova — the brightest known — detected in 2015 is back for another round, researchers report. Left and right panels show before and after images from the initial detection.

THE DARK ENERGY SURVEY, B. SHAPPEE AND THE ASAS-SN TEAM

By Christopher Crockett

May 24, 2016 at 2:57 pm

A celebrity supernova appears to have come back for an encore — with hints that it might not be a supernova after all.

Last year, astronomers reported that a burst of light from about 3 billion light-years away was the brightest exploding star seen to date. After about 80 days of fading away, the cosmic flash (dubbed ASASSN-15lh) started to get bright again, researchers report online May 12 at arXiv.org. Another 80 days later, it was blasting out as much ultraviolet light as some other supernovas.

At its peak, ASASSN-15lh shone as bright as about 550 billion suns and was more than twice as luminous as the previous record holder (SN Online: 1/16/2016). Peter Brown, an astronomer at Texas A&M University in College Station, and colleagues noticed the rebound while monitoring ASASSN-15lh with the Swift and Hubble space telescopes.

Radioactive decay of nickel or a run-in between stellar detritus and gas surrounding the star might be responsible for the brightening. But if so, there should be signs of nickel, hydrogen and other elements absorbing some of the light, which the researchers don’t see.

Rather than a supernova, ASASSN-15lh might be a star being torn apart by a supermassive black hole. The flash came from near the center of its host galaxy, where such behemoths typically live. The resurgence could be caused by a second wave of stellar guts spiraling down on the black hole, though the lack of hydrogen and helium is puzzling.