Brightest Supernova Ever
Hailed as the “most luminous supernova,” the object dubbed ASASSN-15lh is still mystifying astronomers. It appeared to be the death of an extreme type of star, exploding with twice the brightness of any other known supernova, then threw researchers for a loop when it rebrighted. A new study by Giorgios Leloudas in Israel and colleagues using the Hubble Space Telescope refines ASASSN-15lh’s location, placing it within 1,000 light-years of its host galaxy’s center—and possibly in the center itself. Paired with spectroscopic observations, the result suggest the burst might instead be the death of a star ripped apart by the galaxy]s supermassive black hole. Other researchers dispute the spectral lines’ meaning and the mystery remains.
A number of explanations have been tossed around to explain these objects, including magnetars (neutron stars with extremely powerful magnetic fields) or extremely massive stars. Suffice it to say, superluminous supernovae aren’t well understood, but they do have typical qualities and this new object appeared to match them. Using the superluminous scenario, some astromers predicted that the object would display magnesium lines in its spectrum, and subsequent measurements from the SALT telescope in South Africa showed the lines as predicted.
Then ASASSN-15lh threw researchers for another loop — as it was in the process of fading away into obscurity, it suddenly got brighter. For reasons still unknown, superluminous supernovae sometimes do this, so this rebrightening still fit into the supernova narrative.
More details at https://is.gd/asassntdeass
 Monica Young, “’Brightest Supernova Ever,’ a Shredded Star?” in Sky & Telescope (133, 4, April 2017).