In 2015, the All Sky Automated Survey for SuperNovae Automated Search for Supernovae Survey (ASAS-SN) detected an event, dubbed ASASSN-15lh, which was considered the supernova. or brightest stellar explosion ever.
The study was published in Science in January this year, and the event classified as a superluminous supernova, the explosion of an extremely massive star at the end of its life. It was twice as bright as the previous record holder and, at its peak, it was 20 times brighter than the total light of the entire Milky Way.
Now, an international team, led by Giorgos Leloudas of the Weizmann Institute of Sciences (Israel) and the Center for Dark Cosmology (Denmark), has carried out more observations of the distant galaxy in which the explosion took place (located about 4,000 million light years from Earth) and has proposed a new explanation for this extraordinary event.
"After the event we observed the source for 10 months and have concluded that the explanation does not fit with an extraordinarily bright supernova. Our results indicate that the event was probably caused by a supermassive black hole rotating at high speed at a high speed. as it destroys a low-mass star, "explains Leloudas.
In this scenario, the extreme gravitational forces of a supermassive black hole, located in the center of the host galaxy, have tore apart a star similar to our Sun that was too close (an event called a “tidal disruption event” that even now it has only been observed about ten times).
In the process, the star was "spaghetti" and collisions between the debris and the heat generated by the accretion triggered an explosion of light. This gave the event the appearance of a very bright supernova explosion, even though the star would not have become a supernova on its own since it did not have enough mass.
The team bases its new findings on observations carried out with a selection of telescopes, both on the ground and in space. Among them is the VLT (Very Large Telescope) at ESO's Paranal Observatory; the NTT (New Technology Telescope) telescope at ESO's La Silla Observatory; and the NASA / ESA Hubble Space Telescope. The NTT observations were made as part of ESO's Public ESO Spectroscopic Survey of Transient Objects (ESO public spectroscopic survey of objects in transit).
"There are several independent aspects to the observations that suggest that this event was, in effect, a tidal disturbance and not a superluminous supernova," explains co-author Morgan Fraser, from the University of Cambridge (UK) and now at the University College of Dublin (Ireland).
A tidal disruption event?
In particular, the data revealed that the event went through three distinct phases during the 10-month follow-up observations. The total set of data is more like what is expected of a tidal interruption than a superluminous supernova.
In addition, a rebound of brightness has been observed in ultraviolet light, as well as an increase in temperature, which reduces the probability that it is a supernova. On the other hand, the event took place in a place - a red, massive and passive galaxy - that is not the usual one for these superluminous supernova explosion events, which usually occur in star-forming dwarf galaxies.
Although the team states that it is highly unlikely to be a supernova event, they accept that a classic tidal disruption event is not an adequate explanation either. One of the team members, Nicholas Stone, from Columbia University (USA), explains: "The tidal interruption event that we propose cannot be explained with a supermassive black hole that does not rotate. We argue that ASASSN-15lh was a tidal disruption event derived from a very particular type of black hole. "
The mass of the host galaxy implies that the supermassive black hole at its center has a mass of at least 100 million times that of the Sun. A black hole of this mass would normally be unable to interfere with stars beyond its event horizon, the limit beyond which nothing is able to escape its gravitational pull. However, if the black hole is of a particular rapidly rotating type - a so-called Kerr black hole - the situation changes and this limit does not apply.
"Even with all the data collected we cannot say with 100% certainty that the ASASSN-15lh event was a tidal interruption event," concludes Leloudas, "but it is by far the most likely explanation."