A Degeneracy in DRW Modelling of AGN Light Curves
  Individual light curves of active galactic nuclei (AGNs) are nowadays
successfully modelled with the damped random walk (DRW) stochastic process,
characterized by the power exponential covariance matrix of the signal, with
the power $\beta=1$. By Monte Carlo simulation means, we generate mock AGN
light curves described by non-DRW stochastic processes ($0.5\leq\beta\leq 1.5$
and $\beta\neq1$) and show they can be successfully and well-modelled as a
single DRW process, obtaining comparable goodness of fits. A good DRW fit, in
fact, may not mean that DRW is the true underlying process leading to
variability and it cannot be used as a proof for it. When comparing the input
(non-DRW) and measured (DRW) process parameters, the recovered time scale
(amplitude) increases (decreases) with the increasing input $\beta$. In
practice, this means that the recovered DRW parameters may lead to biased (or
even non-existing) correlations of the variability and physical parameters of
AGNs if the true AGN variability is caused by non-DRW stochastic processes. The
proper way of identifying the processes leading to variability are
model-independent structure functions and/or power spectral densities and then
using such information on the covariance matrix of the signal in light curve
modelling.
