An Embedded X-Ray Source Shines through the Aspherical AT2018cow: Revealing the Inner Workings of the Most Luminous Fast-evolving Optical Transients

We present the first extensive radio to gamma-ray observations of a fast-rising blue optical transient, AT 2018cow, over its first similar to 100 days. AT 2018cow rose over a few days to a peak luminosity L-pk similar to 4 x 10(44) erg s(-1), exceeding that of superluminous supernovae (SNe), before declining as L proportional to t(-2). Initial spectra at delta t less than or similar to 15 days were mostly featureless and indicated large expansion velocities v similar to 0.1c and temperatures reaching T similar to 3 x 10(4) K. Later spectra revealed a persistent optically thick photosphere and the emergence of H and He emission features with v similar to 4000 km s(-1) with no evidence for ejecta cooling. Our broadband monitoring revealed a hard X-ray spectral component at E >= 10 keV, in addition to luminous and highly variable soft X-rays, with properties unprecedented among astronomical transients. An abrupt change in the X-ray decay rate and variability appears to accompany the change in optical spectral properties. AT 2018cow showed bright radio emission consistent with the interaction of a blast wave with v(sh) similar to 0.1c with a dense environment (M similar to 10(-3)-10(-4) M-circle dot yr(-1) for v(w) = 1000 km s(-1)). While these properties exclude Ni-56-powered transients, our multiwavelength analysis instead indicates that AT 2018cow harbored a "central engine," either a compact object (magnetar or black hole) or an embedded internal shock produced by interaction with a compact, dense circumstellar medium. The engine released similar to 10(50)-10(51.5) erg over similar to 10(3)-10(5) s and resides within low-mass fast-moving material with equatorial-polar density asymmetry (M-ej,M- fast less than or similar to 0.3 M-circle dot). Successful SNe from low-mass H-rich stars (like electron-capture SNe) or failed explosions from blue supergiants satisfy these constraints. Intermediate-mass black holes are disfavored by the large environmental density probed by the radio observations.