A Study of Birefringent Scintillation Towards the Millisecond Pulsar J0437-4715
Pulsars are highly magnetized rapidly rotating neutron stars that emit streams of energetic charged particles along their magnetic axes. They are observable in the radio spectrum when their emission sweeps across the line of sight of radio telescopes. Pulsars are useful tools for testing theories of relativistic gravity and as probes of the magnetoionic plasma contained within the galactic interstellar medium (ISM). The latter motivation is given focus in this thesis with the aim of contributing towards current understanding of the nature of diffuse astrophysical plasmas on small scales.
This thesis presents a study of birefringent multipath propagation effects along the line of sight to the nearby millisecond pulsar PSR J0437-4715 with the objective of probing for small scale variations of interstellar magnetic fields within the Local Bubble of the ISM. We analyze more than 60 hours of calibrated data observed from MeerKAT radio observatory to observe the phenomena of differential scintillation of orthogonal senses of circularly polarized emission from the pulsar, owing to birefringence in the thin scattering screens of magnetoionic plasma that causes scintillation. The derived limits on the magnitude of differential scintillation are used to constrain the amplitude of spatial variations of magnetic fields on scales of less than 1 x 10¹¹ cm within the scattering region.
Our approach is mainly twofold; we first create dynamic spectra of left and right circular polarization intensities using the psrflux program of the PSRCHIVE pulsar data analysis software. We then search for manifestations of significant differential phase effects caused by birefringent scintillation by computing the difference between the dynamic spectra, the secondary spectrum, and the secondary cross spectrum. Through our analysis, we have found a signal where phase varies slowly at low Doppler shifts and low spatial frequencies from the imaginary part of the secondary cross spectrum, however, the stochastic nature of the signal along the parabola lead us to believe that the signal phase is dominated by the jitter noise intrinsic to the pulsar. We conclude by calculating 3σ upper limit constraints on the amplitude of magnetic field fluctuations from the variance of differential phase measured from the normalization of phase from the secondary cross spectrum. Our constraints provide insights on the sensitivities of the MeerKAT L-band receiver towards detecting magnetic field fluctuations on small turbulent scales from birefringent scintillation.