Time Series Analysis of Active Galactic Nuclei: The case of Arp 102B, 3C
  390.3, NGC 5548 and NGC 4051
  We used the Z-transformed Discrete Correlation Function (ZDCF) and the
Stochastic Process Estimation for AGN Reverberation (SPEAR) methods for the
time series analysis of the continuum and the H${\alpha}$ and H${\beta}$ line
fluxes of a sample of well known type 1 active galactic nuclei (AGNs): Arp
102B, 3C 390.3, NGC 5548, and NGC 4051, where the first two objects are showing
double-peaked emission line profiles. The aim of this work is to compare the
time lag measurements from these two methods, and check if there is a
connection with other emission line properties. We found that the obtained time
lags from H$\beta$ are larger than those derived from the H$\alpha$ analysis
for Arp 102B, 3C 390.3 and NGC 5548. This may indicate that the H$\beta$ line
originates at larger radii in these objects. Moreover, we found that the ZDCF
and SPEAR time lags are highly correlated ($r \sim0.87$), and that the error
ranges of both ZDCF and SPEAR time lags are correlated with the FWHM of used
emission lines ($r\sim 0.7$). This increases the uncertainty of the black hole
mass estimates using the virial theorem for AGNs with broader lines.
