The Nonlinear Long - term X - ray Variability of Mrk 421

In this work, I present the results of a thorough nonlinear time series analysis study concerning the long-term variability behaviour of Mrk 421. I use all the archival data obtained by the proportional counter array (PCA) and the all-sky monitor (ASM) aboard the Rossi X-ray timing explorer (RXTE), since 1996. Linearity and stationarity tests reveal that the long-term timing properties of the system originate from a strictly nonlinear and bistable system respectively. Moreover, from the dimensionality analysis I conclude that the underlying variability process of the system consists of numerous physical parameters, each one contributing to the observed X-ray variations equivalently in a stochastic way.