What is a ‘fast blue optical transient’?
If you observe a patch of sky repeatedly, you will see objects that suddenly appear, brighten and then disappear. We call these objects ‘transients’. For example, stellar explosions, or supernovae, are one type of transient. In recent years, as astronomical surveys have been observing the sky more frequently, astronomers have identified a new class of transient that appears and disappears in just a few days. As these transients are fast and blue at optical wavelengths, we call them fast blue optical transients (FBOTs).
It is not yet clear what FBOTs are. They are too bright and fast to be ‘normal’ supernovae. One leading model is that they are explosions of stars that have shed a lot of material into space before they exploded. The FBOT is then the brightening and cooling of this material as the supernova blast wave runs through it – in other words, this material changes the supernova’s properties and how it looks to us. However, we now know of at least three FBOTs – AT 2018cow, CSS161010 and the Koala – where the FBOT is powered by a black hole or neutron star. This would form in the core of the stellar explosion and then launch a powerful jet.
In this case the FBOT could be the light from this process as seen through the surrounding material. In the upcoming years as we observe more FBOTs across the electromagnetic spectrum, we will be able to conclusively distinguish between these and other models. Dr Deanne Coppejans is a postdoctoral associate at the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, Illinois