On the UV/optical variation in NGC 5548: new evidence against the reprocessing diagram

TL;DR

This study reveals that the color variability in NGC 5548 depends on timescale, challenging the reprocessing model's ability to explain UV/optical variations in AGNs, and highlights the importance of timescale analysis in understanding AGN physics.

Contribution

It provides the first evidence of timescale-dependent color variability in an individual AGN, questioning the validity of the reprocessing diagram for UV/optical variations.

Findings

Color variation is more prominent at shorter timescales.

Reprocessing models cannot reproduce the observed timescale dependency.

Timescale dependency is a powerful probe of AGN UV/optical physics.

Abstract

The reprocessing scenario is widely adopted in literature to explain the observed tight inter-band correlation and short lags in the UV/optical variations of active galactic nuclei (AGNs). In this work we look into the color variability of the famous Seyfert galaxy NGC 5548 with high quality Swift multi-band UV/optical light curves. We find the color variation of NGC 5548 is clearly timescale-dependent, in a way that it is more prominent at shorter timescales. This is similar to that previously detected in quasar samples, but for the first time in an individual AGN. We show that while a reprocessing model with strict assumptions on the driving source and the disk size can apparently match the observed light curves and inter-band lags, it fails to reproduce the observed timescale dependency in the color variation. Such discrepancy raises a severe challenge to, and can hardly be reconciled under the widely accepted reprocessing diagram. It also demonstrates that the timescale dependency of the color variation is uniquely powerful in probing the physics behind AGN UV/optical variations.