A novel approach to study the variability of NGC 5548
  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.
