# A novel approach to study the variability of NGC 5548

## TL;DR

This study applies multifractal analysis to the light curves of NGC 5548, revealing complex nonlinear interactions between the continuum and emission lines, which enhances understanding of the active galactic nucleus's inner regions.

## Contribution

It introduces the use of Multifractal Detrended Moving Average analysis to detect multifractality and nonlinearity in AGN light curves, highlighting differences in multifractality degrees.

## Key findings

- Detected multifractal signatures in both light curves.
- Found the Hβ line exhibits twice the multifractality of the continuum.
- Identified anti-persistent long-range correlations as the source of multifractality.

## Abstract

Understanding the properties of the continuum radiation and broad emission lines of active galactic nuclei provide significant information not only to model the radiation mechanism and constrain the geometry and kinematics of the broad-line region (BLR) but also to probe the central engine of the sources. Here we investigate the multifractal behaviour of the H$\beta$ emission line and the 5100 {\AA} continuum flux light curves of NGC 5548. The aim is to search for multi-scaling signatures in the light curves and check if there is a possible nonlinear relationship between them. To this end, we use a multifractality analysis technique called Multifractal Detrended Moving Average (MFDMA) analysis. We detect multifractal (nonlinear) signatures in the full monitoring and densely sampled period of the H$\beta$ line and 5100 {\AA} continuum light curves of NGC 5548, possibly indicating the presence of complex and nonlinear interaction in the 5100 {\AA} continuum and H$\beta$ emission line regions. Moreover, the degree of multifractality of H$\beta$ line is found to be about twice that of the 5100 {\AA} continuum. The nonlinearity of both emissions could be generated when the broad-line region reprocesses the radiation from the central compact source. Finally, we found that anti-persistent long-range temporal correlation is the main source of the multifractality detected in both light curves.