A hot and luminous source at the site of the fast transient AT2018cow at 2-3 years after its explosion

Abstract

We report the discovery of a luminous late-time source at the position of the fast blue optical transient (FBOT) AT2018cow on images taken by the Hubble Space Telescope (HST) at 714 d and 1136 d after its explosion. This source is detected at both UV and optical wavelengths and has prominent H$\alpha$ emission. It has a very stable brightness between the two epochs and a very blue spectral energy distribution (SED) consistent with $f_{\lambda }$ $\propto$ ${\lambda }^{-4.1\pm 0.1}$, i.e. the Rayleigh-Jeans tail of a hot blackbody with a very high temperature of log($T$/K) $>$ 4.6 and luminosity of log($L$/$L_{\odot }$) $>$ 7.0. This late-time source is unlikely to be an unrelated object in chance alignment, or due to a light echo of AT2018cow. Other possible scenarios also have some difficulties in explaining this late-time source, including companion star(s), star cluster, the survived progenitor star, interaction with circumstellar medium (CSM), magnetar, or delayed accretion in a tidal disruption event (TDE). Long-term and multi-wavelength monitoring will help to resolve its nature and finally reveal the origin of the "Cow".

5 pages, 4 figures, accepted for publication in MNRAS Letters

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• Late-time Hubble Space Telescope Observations of AT 2018cow. II. Evolution of a UV bright Underlying Source 2--4 Yr Post-discovery
• A possible 250-second X-ray quasi-periodicity in the fast blue optical transient AT2018cow
• On the nature of Fast Blue Optical Transients
• Neutrino Emission from Luminous Fast Blue Optical Transients
• Multi-wavelength observations of the Luminous Fast Blue Optical Transient AT2023fhn
• A Snapshot Survey of Nearby Supernovae with the Hubble Space Telescope
• Late-time Hubble Space Telescope Observations of AT 2018cow. I. Further Constraints on the Fading Prompt Emission and Thermal Properties 50--60 Days Post-discovery
• Terminating a common envelope jets supernova impostor event with a super-Eddington blue supergiant
• The spectra of a radiative reprocessing outflow model for fast blue optical transients
• Luminous Fast Blue Optical Transients as "Failed" Gravitational-wave Sources: Helium Core$-$Black Hole Mergers Following Delayed Dynamical Instability
• A study on late time UV-emission in core collapse supernovae and the implications for the peculiar transient AT2018cow
• Luminous Fast Blue Optical Transients and Type Ibn/Icn SNe from Wolf-Rayet/Black Hole Mergers