Years have passed since the first detection of pulsed very-high-energy (VHE; E \textgreater 100 GeV) gamma-rays from the Crab pulsar with VERITAS, yet much is still unresolved in relation to the nature of pulsar emission mechanisms (see \cite{Venter:2018iax}) and how they interact with the surrounding medium.
No completely satisfactory model has been produced that can accurately describe all aspects of the pulsed gamma-ray emission observed from the Crab and other pulsars. Understanding the properties of VHE emission detected in observations made by many different experiments still poses a significant challenge to theoreticians, and hence experimentalists, working in the field. The crux of the issue remains; is the Crab pulsar unique\footnote{Although not as `canonical' as the Crab pulsar, there are confirmed pulsed VHE gamma-rays from one or two other pulsars (see section \ref{Section:MotivationforInvestigating MSPs}), as of writing.}, or do other pulsars also exhibit the same behavior in the VHE regime, and, in either case, what are the underlying mechanisms? To try and answer this question, while also learning more about the pulsar population and the physics of VHE gamma-ray production, this work will present the results of a search for pulsed emission in the VHE band from six Millisecond Pulsars (MSPs) in the archival VERITAS data-set, the first such survey of MSPs, and the most sensitive VHE measurements ever made for the targets. I test to see if significant pulsed emission is detected, report the observed VHE pulsed flux and gamma-ray conversion efficiency of these MSPs, to determine if there is an appearance of a VHE flux element at these energies, for the sources studied here. As the analyses result in non-detections, in every case, upper limits are placed on the aforementioned quantities. The upper limits are compared with a modern, comprehensive pulsar model energy spectrum and are found to be compatible with the proposed theoretical scenario, although we are limited by a lack of target-specific predictions. In addition, PSR J0030+0451 is proposed as a promising candidate for future study with CTA; as the limits placed here indicate that, with similar exposure and assuming a non-detection, CTA would likely produce flux limits that challenge the scenario of F=kCrabE˙/d2 for the MSP population.
Pulsars are also sources of non-pulsed gamma-rays. However, at the time of writing, there has been no decisive detection of the TeV emission expected by current models from any pulsar tail that is also seen in the X-ray or radio bands. An observational campaign has been carried out by VERITAS to hunt for VHE gamma-ray emission from the candidate tail regions associated with three nearby pulsars (PSR~B0355+54, PSR~J0357+3205 and PSR~J1740+1000) that move supersonically and exhibit significant X-ray tails. The results of this analysis provide quantification of the TeV flux and luminosity, from the tail regions of the targets, for comparison with other pulsar wind nebulae observations and the predictions of modern pulsar tail models. The results of this search also provide guidance for the selection of additional candidates, and quantifying the properties of pulsar tails, for new pulsars tails that may be observed in the VHE regime.
In order to analyze data from IACTs, such as VERITAS, detailed and extensive simulation works are necessary to understand the gamma-ray-induced EASs and the detector response. I will detail the work I undertook to produce the most modern and comprehensive simulation set for VERITAS to date.
In addition to the aforementioned research, that aims to further our understanding of pulsars in the VHE domain, in this document, I will describe my contributions to the building of the Cherenkov Telescope Array (CTA), the most sensitive IACT instrument ever constructed to observe the gamma-ray sky. As the timescales of such huge projects are so long, it is natural for researchers to work with an existing instrument (in this case, VERITAS) and help run and improve the experiment, along with analyzing data products, while also contributing to the building of future instruments, that build on the previous observatory's endeavors. The research, herein, will be the most up to date analysis of the target sources, and so provides the most modern insights into the nature of these objects, but also serves as an excellent guide for source-selection, and even the models to be tested, for future works. For example, the improved sensitivity that CTA will achieve, over the current generation IACTs, will allow even deeper investigation of the pulsars studied here. Directly quantifying the standards that need to be met for the next generation of IACTs is a hugely important task and works, such as this one, aid in achieving this goal and also help bridge the gap between the generations of IACTs. This is an integral part of the evolution of the field and this thesis ties together the current era with the future research in the CTA era.
I will also include details on my contribution to a novel study of Lorentz-Invariance Violation and, hence, what we can learn about possible quantum substructure of spacetime through VHE gamma-ray observations, via collaboration with the other major IACT groups.Ph.D
