Brightest supernova ever discovered? Or a big mistake?
In 2015, astronomers detected what they believed to be the most luminous supernova ever recorded, which shone 20 times brighter than the light output of the entire Milky Way. However, fresh research suggests that the event, which has since been named ASASSN–15lh, might not have been a supernova at all, but instead the light signature of a vast, fast-spinning black hole devouring a Sun-like star.
For the new study, an international team of astronomers made use of a number of famous ground and space-based observatories including the Hubble Space Telescope, and the European Southern Observatory's Very Large Telescope, located at the Paranal Observatory, Chile.
The telescope data revealed that in the ten months following the supposed supernova event, ASASSN-15lh experienced a significant increase in both temperature and the emission of ultraviolet light. According to the researchers, these characteristics make it unlikely that the 2015 event took the form of a superluminous supernova.
Instead the team suggest that the light signature detected in 2015 was created when a Sun-like star orbiting in close proximity to the supermassive black hole located at the centre of ASASSN -15lh's host galaxy was torn apart, or "spaghettified" by the gravity of the black hole.
For the black hole to rip the star apart as it orbited beyond its event horizon, it must have been spinning incredibly fast. The extreme gravitational influence of such a fast spinning black hole, known as a Kerr black hole, could have created a tidal disruption event capable of destroying a star orbiting beyond its event horizon. Only 10 tidal disruption events have been observed to date, making it a rare occurrence.
The researchers believe that the 2015 light signature was created when the remnants of a star destroyed by a tidal disruption event collided as they fell towards the event horizon of the supermassive black hole, which has at least 100 times the mass of our own sun. This process would have led to the release of an enormous amount of energy, a portion of which was released in the form of ASASSN-15lh.
The nature of the galaxy in which ASASSN-15lh was discovered, and the close proximity of the event to the supermassive black hole's event horizon further supports the tidal disruption theory. Superluminous supernovae are typically discovered in galaxies that are alight with the activity of star creation. However, ASASSN-15lh was found in a passive, red galaxy, which exhibits a relatively low-level of star formation.
"We've only been studying the optical flares of tidal disruptions for the last few years," said co-author Iair Arcavi, principal investigator of the Las Cumbres Observatory program used to observe ASASSN-15lh. "ASASSN-15lh is similar in some ways to the other events we've been seeing but is different in ways we didn't expect. It turns out that these events — and the black holes that make them — are more diverse than we had previously imagined."
Scroll down to see a simulation of the supermassive black hole thought to reside at the centre of ASASSN-15lh's host galaxy destroying a Sun-like star via spaghettification.
Source: ESO / Video: ESO, ESA/Hubble, N. Stone, K. Hayasaki