UV to X-Ray Comptonization Delay in Mrk 493

The broadband X-ray emission from type 1 active galactic nuclei, dominated by a power-law continuum, is thought to arise from repeated inverse Compton scattering of seed optical/UV photons by energetic electrons in a hot corona. The seed optical/UV photons are assumed to arise from an accretion disk, but direct observational evidence has remained elusive. Here we report the discovery of variations in the UV emission preceding the variations in the X-ray emission based on similar to 100 ks XMM-Newton observations of the narrow-line Seyfert 1 galaxy Mrk. 493. We find that the UV emission leads by similar to 5 ks relative to the X-ray emission. The UV lead is consistent with the time taken by the UV photons to travel from the location of their origin in the accretion disk to the hot corona, and the time required for repeated inverse Compton scattering converting the UV photons into X-ray photons. Our findings provide the first direct observational evidence for the accretion disk being responsible for the seed photons for thermal Comptonization in the hot corona, and for constraining the size of the corona to similar to 20r(g).