Exploiting pulsar timing at low radio frequencies: applications to Pulsar Timing Arrays, Interstellar medium and HII regions

Pulsar timing at low radio frequencies offers a powerful way to investigate chromatic propagation effects in the ionised interstellar medium and to refine the modelling of noise processes in Pulsar Timing Array (PTA) analyses. A central element of this thesis is the combination of observations from LOFAR and NenuFAR with higher-frequency data from the European PTA (EPTA). By extending the frequency coverage of common pulsars, it becomes possible to place tighter constraints on chromatic delays, dispersion measure (DM) variations and scattering, and to update the noise models currently adopted in PTA studies. The improved observational framework naturally opens the way to a physical interpretation of the measured DM variability. Using structure functions and complementary observational data, I investigated the role of plasma turbulence along the lines of sight of LOFAR pulsars. Particular attention is given to discrete ionised regions, such as HII regions, whose presence can shape both the amplitude and the spectral behaviour of DM fluctuations. This approach links the observed chromatic noise to the properties and location of intervening scattering screens. To assess how well such variability can be modelled in timing analyses, I developed simulations of pulsar time series that include realistic chromatic noise. These are used to evaluate the performance and limitations of the recovery techniques currently employed by PTA collaborations. The comparison highlights how standard modelling assumptions can introduce biases when interpreting DM variations and scattering-induced delays. Overall, the thesis shows how low-frequency pulsar timing can both enhance gravitational-wave detection efforts and provide new insight into the structure and turbulence of the ionised interstellar medium.