Multiwavelength power-spectrum analysis of NGC 5548

TL;DR

This study analyzes the power spectra of NGC 5548's UV, optical, and X-ray emissions to understand their variability and relation, suggesting a complex accretion disc structure with a warm corona affecting UV/optical signals.

Contribution

It provides a comprehensive power spectrum analysis of NGC 5548 across multiple wavelengths, exploring the disc-corona interaction and variability characteristics.

Findings

Power spectrum results align with a standard disc illuminated by X-rays at low accretion rates.

Inner disc may be covered by a warm corona, affecting UV/optical emission detection.

UV emission of NGC 5548 is strongly non-stationary.

Abstract

NGC 5548 was recently monitored intensively from NIR to X-rays as part of the STORM campaign. Its disc emission was found to lag behind the observed X-rays, while the measured time lag was increasing with wavelength. These results are consistent with the assumption that short term variability in AGN emission is driven by the X-ray illumination of the accretion disc. In this work, we studied the power spectrum of UV/optical and X-ray emission of NGC 5548, using the data of the STORM campaign as well as previous Swift data, in order to investigate the relation between the UV/optical and X-ray variability and to examine its consistency with the above picture. We demonstrate that even the power spectrum results are compatible with a standard disc being illuminated by X-rays, with low accretion rates, but the details are not entirely consistent with the results from the modelling of the "{\tau} vs {\lambda}" relation. The differences indicate that the inner disc might be covered by a "warm corona" which does not allow the detection of UV/optical emission from the inner disc. Finally, we found strong evidence that the UV emission of NGC 5548 is not stationary.