Implications of the UV/optical Plateau of AT2018cow

Abstract

AT2018cow, the prototypical luminous fast blue optical transient (LFBOT), shows a slowly fading plateau in the UV/optical light lasting at least 5 years after the explosion. The plateau SED is blackbody-like with temperature $\gtrsim$ 2e4 K and an emission radius ~40 Rsun, which has been attributed to a geometrically thin accretion disk around a compact object. Viscously powered disk models, however, require an unusually massive black hole $\gtrsim$ 200 Msun (for viscosity parameter $\alpha$ > 0.01). Moreover, accretion onto such a massive black hole produces X-ray emission that is likely in tension with the XMM-Newton constraint, unless the X-ray is highly anisotropic or absorbed. We propose instead that the UV/optical emission arises from the photosphere of a super-Eddington wind launched from the inner disk, together with irradiation and reprocessing by the geometrically thin outer disk. Fitting the model to four-band HST photometry at t ~ 4 yr, we find that all accretors from 1.4 Msun neutron star to ~100 Msun black hole produce similarly good fits. This degeneracy arises because the wind photospheric color temperature depends only weakly on the accretor mass. We also find that, at NIR-MIR wavelengths (1 to 10 um), wind free-free emission differs by an order of magnitude across the allowed mass range, providing a potential discriminant accessible to JWST. Another potential signature to differentiate the accretor mass and disk composition is the recombination line emission (HeII1640 or H$\alpha$) from the disk wind. The inferred disk parameters from our model are consistent with a micro-tidal disruption event, either tidal disruption or merger with a companion star, in which an initially geometrically thick disk transitions to a thin phase within the first year and subsequently the thin outer disk evolves on the observed ~10 yr viscous timescale.