Pulsar magnetospheres and intra - pulse variability by plasmoid formatio

Abstract

Pulsars are remnants of massive stars that have endured supernova explosions. They are rapidly rotating neutron stars with extraordinary magnetic fields. These celestial objects serve as natural laboratories for studying physics under extreme conditions that includes physics of dense matter, strong-field gravity, and relativistic plasma processes. Their magnetospheres are dominated by strong magnetic fields and populated by an electron-positron plasma. It exhibits complex dynamics that give rise to a broad spectrum of electromagnetic emissions which spans from radio waves to gamma-rays. Among the various phenomena observed, pulse-to-pulse variability offers critical insights into the underlying plasma processes and emission mechanisms. It also includes intra-pulse structures such as subpulses. The precise origins of this variability remain incompletely understood despite extensive research. It is posing a significant challenge in pulsar astrophysics. What is investigated in this thesis is the role of plasmoid formation induced by magnetic reconnection in pulsar magnetospheres and its impact on intra-pulse variability. Magnetic reconnection in the current sheet beyond the light-cylinder radius ($\rlc$) is a fundamental plasma process. It can lead to the formation of plasmoids—coherent structures consisting of plasma and magnetic fields. The intermittent nature of reconnection and the hierarchical merging of plasmoids can introduce significant fluctuations in the emitted radiation. It is potentially accounting for the observed pulse-to-pulse variability and subpulse phenomena. To examine this hypothesis, we employ two-dimensional particle-in-cell (PIC) simulations using the \texttt{ZELTRON} code, modeling an orthogonal pulsar magnetosphere restricted to the equatorial plane. The simulations begin with a split magnetic monopole configuration and evolve into a quasi-steady-state force-free magnetosphere filled with electron-positron plasma. Plasma is continuously injected from the neutron star surface, ensuring a sustained supply of charged particles. The wind current sheet forms as two thin Archimedean spirals extending beyond $\rlc$, where magnetic reconnection triggers the tearing instability. This instability leads to the fragmentation of the current sheet into a dynamic chain of plasmoids, which evolve through processes of growth, merging, and outward propagation. Our simulations expose that plasmoids are predominantly formed near the light cylinder and subsequently grow and merge hierarchically as they move out at relativistic speeds. The size distribution of plasmoids ranges from small-scale structures on the order of $0.02 \rlc$ to large entities comparable to the neutron star radius ($\sim 0.3 \rlc$). Statistical analysis indicates that the plasmoid size distribution follows an inverse relationship with their width. This is consistent with theoretical predictions for hierarchical merging in relativistic reconnection processes. This distribution suggests that a multitude of small plasmoids coexist with fewer large ones, contributing differently to the emission characteristics. We compute synthetic synchrotron emission profiles by tracking particles and electromagnetic fields within the simulation. Our computation focuses on the incoherent high-energy emission originating from the current sheet. The results demonstrate that plasmoid formation leads to significant intra-pulse variability. This variability is manifested as bright subpulses superimposed on the main pulse profile. The subpulses predominantly appear on the leading edge of each pulse and exhibit a broad range of fluxes and durations. A key finding is the proportionality between subpulse flux and width in pulsar phase. This correlation indicates that the larger plasmoids produce brighter and broader subpulses. This relationship arises because larger plasmoids contain more energetic particles and occupy a greater angular extent. It enhances their contribution to the observed emission. The statistical properties of the subpulses align with the plasmoid size distribution with the number of subpulses following a power-law dependence on their flux and width. This correlation suggests that the observed intra-pulse variability is directly linked to the dynamics of plasmoid formation and evolution within the magnetosphere. The power-law behavior indicates that there is a significant probability of observing exceptionally bright and wide subpulses. This corresponds to the fact that the largest plasmoids formed through merging events while most subpulses are weak and narrow. Advanced analysis shows that the intermittent nature of magnetic reconnection leads to episodes where the current sheet thickens near the light cylinder. It temporarily suppresses plasmoid formation. These quiet periods result in reduced emission and are followed by recovery phases. The intense subpulses reappear in the recovery phases and this corresponds to the delayed fragmentation of the stretched current sheet. This behavior contributes to the overall variability observed in the emission profiles. It may explain observed phenomena such as nulling and mode changes in some pulsars. Our study offers several testable predictions.