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The broadband X-ray emission from type 1 active galactic nuclei, dominated bya powerlaw continuum, is thought to arise from repeated inverse Comptonscattering of seed optical/UV photons by energetic electrons in a hot corona.The seed optical/UV photons are assumed to arise from an accretion disc but adirect observational evidence has remained elusive. Here we report thediscovery of variations in the UV emission preceding the variations in theX-ray emission based on $\sim100\,\mathrm{ks}$ XMM-Newton observations of thenarrow-line Seyfert 1 galaxy Mrk 493. We find the UV emission to lead by$\sim5\,\mathrm{ks}$ relative to the X-ray emission. The UV lead is consistentwith the time taken by the UV photons to travel from the location of theirorigin in the accretion disc to the hot corona and the time required forrepeated inverse Compton scattering converting the UV photons into X-rayphotons. Our findings provide first direct observational evidence for theaccretion disc to be responsible for the seed photons for thermalComptonization in the hot corona, and constrain the size of the corona to be$\sim20r_{g}$.

UV to X-ray Comptonization delay in Mrk 493