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The bright transient AT2018cow has been unlike any other known type oftransient. Its high brightness, rapid rise and decay and initially nearlyfeatureless spectrum are unprecedented and difficult to explain using modelsfor similar burst sources. We present evidence for faint gamma-ray emissioncontinuing for at least 8 days, and featureless spectra in the ultravioletbands -- both unusual for eruptive sources. The X-ray variability of the sourcehas a burst-like character. The UV-optical spectrum does not show any CNO linebut is well described by a blackbody. We demonstrate that a model invoking thetidal disruption of a 0.1 - 0.4 Msun Helium White Dwarf (WD) by a 100,000 toone million solar mass Black Hole (BH) located in the outskirts of galaxyZ~137-068 could provide an explanation for most of the characteristics shown inthe multi-wavelength observations. A blackbody-like emission is emitted from anopaque photosphere, formed by the debris of the WD disruption. Broad featuresshowing up in the optical/infrared spectra in the early stage are probablyvelocity broadened lines produced in a transient high-velocity outward movingcocoon. The asymmetric optical/infrared lines that appeared at a later stageare emission from an atmospheric layer when it detached from thermalequilibrium with the photosphere, which undergoes more rapid cooling. Thephotosphere shrinks when its temperature drops, and the subsequent infall ofthe atmosphere produced asymmetric line profiles. Additionally, a non-thermaljet might be present, emitting X-rays in the 10-150 keV band.

Swift spectra of AT2018cow: A White Dwarf Tidal Disruption Event?