Modelling The Multi-Wavelength Emission And Polarisation Signatures Of The Novel White-Dwarf Pulsar System Ar Sco
The serendipitous reclassification of AR Scorpii (AR Sco) from a delta Scuti variable star to a white dwarf binary system has initiated an in-depth exploration of this novel system. These observations led to discovering pulsar-like emission features from the source, earning it the title of ‘first white dwarf pulsar’. Over the last decade, there have been many multi-wavelength and polarimetric observations on this source yet the proposed models remain unable to concurrently predict the emission and polarisation features. The main aim of this work was to develop a general emission code to concurrently model the emission maps, light curves, and spectra at various orbital phases for AR Sco. As a precursor to developing this code, I followed up on our previous work by fitting our geometric rotating vector model to the orbital-resolved polarisation position data. I found the magnetic inclination angle and observer angle to vary by ∼ 10◦ and ∼ 30◦, respectively, over the binary orbit. Further periodicity analysis of the spin- and beat-coupled linear fluxes showed that the spin-coupled emission dominates between orbital phases 0.1 − 0.6 and the beat-coupled emission dominates between orbital phases 0.6 − 1.1. These results emphasise the need to probe various injection scenarios proposed for AR Sco. For the development of the emission code, I solved the general equations of motion with included classical radiation reaction forces (RRF) by implementing the Dormand-Prince 8(7) numerical integrator with adaptive time-step methods to exploit the higher accuracy of the scheme. This yielded improved accuracy and computational time vs. the commonly used Vay symplectic integrator, particularly for the high B-fields, E⊥-fields, and RRF needed for pulsar and pulsar-like magnetospheres. In my investigation of the RRF’s effect on the particle pitch angle, I reaffirmed the notion that the RRF does not significantly affect the pitch angle in the super-relativistic regime as well as the E × B-drift maintaining a large pitch angle deviation of the trajectory with respect to the local B-field. Additionally, I demonstrated the novel result of the particles entering and conforming to the radiation reaction limit regime of Aristotelian Electrodynamics (AE) for uniform electromagnetic fields. This served as an excellent validation for my RRF calculations. As calibration for the radiation calculations, emission maps, and spectra I compared my model output with results from the code of the pulsar emission model of Harding and collaborators for a pulsar with 10% the B-field strength of Vela and how my results converged to theirs. I also showed the novel results of the particles conforming to the AE regime but for more realistic force-free fields. This once again validates my results but also validates the AE assumptions since it is notably a solution to the equilibrium electrodynamics. Furthermore, I assessed two existing synchro-curvature radiation calculation methods, assessing whether they are appropriate for use in the high E⊥-fields present in pulsar magnetospheres. These results highlight the importance of following the particle along its E × B drifted trajectory (AE trajectory) and calculating the applicable radiation along this curve. One is thus interested in the angle of the particle as it gyrates around this curve instead of the traditional pitch angle. Having established confidence in my particle dynamics and emission calculations, I next showed my exploratory modelling of the magnetic mirror scenario proposed for AR Sco. I demonstrate I could fit the observational spectral energy distribution including the recent NICER pulsed X-ray spectrum well, constraining the white dwarf B-field to BS = (2.5 − 3.0) × 108 G and the electron power-law index to p ∼ 2.9, falling within the observational estimate of between 0.8 − 1.4 for the Optical/UV photon spectral index. Finally, I showed the effect the B-field, E⊥-field, initial pitch angle, and initial parti