Obscure origins
As Dong had noted in a report posted on the arXiv physics preprint server last July2, the blast belongs to a class of superluminous supernovae that are low in hydrogen. These kinds of explosions are thought to be fuelled by magnetars: compact, rapidly rotating, highly magnetized cores that are left behind after a supernova explosion casts off a star’s outer layers. The magnetar’s powerful magnetic field launches a wind that heats gas flying from the supernova.
Wayne Rosing
Two of the telescopes used by the All Sky Automated Survey for SuperNovae (ASAS-SN) that discovered the brightest-ever supernova, from Cerro Tololo, Chile.
But ASASSN-15lh is hotter and more luminous than other hydrogen-poor supernovae — so much so that a magnetar would need to be spinning at its maximum possible rate and convert its spin energy into heat at 100% efficiency, says Dong. Because such changes are so extreme, Dong doubts that the magnetar model would work.
And whereas other hydrogen-poor supernovae reside in dim dwarf galaxies, ASASSN-15lh seems to originate near the centre of a massive galaxy that is brighter than the Milky Way — although Dong’s team says that it can’t rule out the possibility that it exploded in a dwarf galaxy that seemed to align with the larger one.
There’s a slim chance that ASASSN-15lh is not a supernova at all. Instead, it might be the fireworks associated with a star that has been gravitationally torn apart by a supermassive black hole, says supernova observer Edo Berger at Harvard University in Cambridge, Massachusetts, who was not part of the study.