AT 2018cow at ~5 years: additional evidence for a tidal disruption origin

The luminous fast blue optical transient (LFBOT) AT 2018cow is the prototype of its class with an extensive set of multiwavelength observations. Despite a rich data set there is, still, no consensus about the physical nature and origin of this event. AT 2018cow remained UV bright 2–4 yr after the explosion, which points at an additional energy injection source, most likely from an accretion disc. We present additional late time ultraviolet (UV) data obtained with the Hubble Space Telescope, to show there is no significant fading in the optical since the last epoch and only marginal fading in the UV. The new UV data points match the predictions of previously published accretion disc models, where the disc is assumed to form from the tidal disruption of a low-mass star by an intermediate-mass black hole. This consistency provides evidence that AT 2018cow could indeed be a tidal disruption event. The marginal decay is in contrast with the predictions of light curves produced by interacting supernovae. The difference between expectations for disc emission and interacting supernovae will further increase with time, making data at even later times a route to robustly rule out interaction between supernova ejecta and circumstellar material.