Quasiperiodic and chaotic behaviours in time evolution of pulsar spin

In this article, dynamical behaviour of pulsar spin-down is analysed with the presence of time-dependent external torques. The model incorporates nonlinear superfluidity of the core of pulsar. The spin-down rate of the crust becomes perturbed whenever a glitch occurs. An abrupt increase appears in the pulsar spin-down rate for a short period of time. Although there are several mechanisms proposed to understand those glitches, the exact mechanism is still unknown. The fluctuations in pulsar spin-down cannot be predicted either from time series analysis or governing equations for rotational dynamics. Long-term irregularities in pulsar spin-down give clues about some spiking activity or intermittency in the governing dynamics, which immediately enforce one to think about chaotic dynamics that may this trigger glitch behaviour. With this motivation behind, we modified an existing model known as vortex creep model to figure out the stochasticity in the dynamics of the system. The new model exhibits very interesting dynamical features. First of all, it is able to demonstrate glitch-like behaviour for suitable parameters. With the applied modification in the equations of motion, the system is showing different dynamical regimes from periodicity to quasiperiodicity and also chaotic dynamics become obvious for special parameter settings. The rich dynamical feature of the system is shown with the aid of nonlinear tools such as phase portraits, bifurcation diagrams, Lyapunov exponents and Poincaré sections.