# UV to X-ray Comptonization delay in Mrk 493

## TL;DR

This study presents the first direct observational evidence that UV photons from the accretion disc in Mrk 493 lead X-ray variations, confirming the disc as the seed photon source for Comptonization in the corona and estimating the corona's size.

## Contribution

It provides the first direct evidence linking UV emission from the accretion disc to X-ray emission via Comptonization in an active galactic nucleus.

## Key findings

- UV emission leads X-ray emission by ~5 ks
- Coronal size estimated at ~20 gravitational radii
- Supports disc as seed photon source for X-ray production

## Abstract

The broadband X-ray emission from type 1 active galactic nuclei, dominated by a powerlaw 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 disc but a 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 $\sim100\,\mathrm{ks}$ XMM-Newton observations of the narrow-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 consistent with the time taken by the UV photons to travel from the location of their origin in the accretion disc to the hot corona and the time required for repeated inverse Compton scattering converting the UV photons into X-ray photons. Our findings provide first direct observational evidence for the accretion disc to be responsible for the seed photons for thermal Comptonization in the hot corona, and constrain the size of the corona to be $\sim20r_{g}$.