Studying Millisecond Pulsars and Pulsar Tails in the Very-High-Energy Gamma-ray Regime with VERITAS

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 $F = k_{Crab}\ \sqrt{\dot{E}} / d^2$ 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

First combined studies on Lorentz Invariance Violation from observations of astrophysical sources

International audienceImaging Atmospheric Cherenkov Telescopes study the highest energy (up to tens of TeV) photon emission coming from nearby and distant astrophysical sources, thus providing valuable results from searches for Lorentz Invariance Violation (LIV) effects. Highly variable, energetic and distant sources such as Pulsars and AGNs are the best targets for the Time-of-Flight LIV studies. However, the limited number of observations of AGN flares or of high-energy pulsed emission greatly restricts the potential of such studies, especially any potential LIV effects as a function of redshift. To address these issues, an inter-experiment working group has been established by the three major collaborations taking data with Imaging Atmospheric Cherenkov Telescopes (H.E.S.S., MAGIC and VERITAS) with the aim to increase sensitivity to any effects of LIV, together with an improved control of systematic uncertainties, by sharing data samples and developing joint analysis methods. This will allow an increase in the number of available sources and to perform a sensitive search for redshift dependencies. This presentation reviews the first combined maximum likelihood method analyses using simulations of published source observations done in the past with H.E.S.S., MAGIC and VERITAS. The results from analyses based on combined maximum likelihood methods, the strategies to deal with data from different types of sources and instruments, as well as future plans will be presented

VTSCat: The VERITAS Catalog of Gamma-Ray Observations

We present a catalog of results of gamma-ray observations made by VERITAS, published from 2008 to 2020. VERITAS is a ground based imaging atmospheric Cherenkov telescope observatory located at the Fred Lawrence Whipple Observatory (FLWO) in southern Arizona, sensitive to gamma-ray photons with energies in the range of $\sim$ 100 GeV - 30 TeV. Its observation targets include galactic sources such as binary star systems, pulsar wind nebulae, and supernova remnants, extragalactic sources like active galactic nuclei, star forming galaxies, and gamma-ray bursts, and some unidentified objects. The catalog includes in digital form all of the high-level science results published in 112 papers using VERITAS data and currently contains data on 57 sources. The catalog has been made accessible via GitHub and at NASA's HEASARC

Design and performance of the prototype Schwarzschild-Couder telescope camera

International audienceThe prototype Schwarzschild-Couder Telescope (pSCT) is a candidate for a medium-sized telescope in the Cherenkov Telescope Array. The pSCT is based on a dual-mirror optics design that reduces the plate scale and allows for the use of silicon photomultipliers as photodetectors. The prototype pSCT camera currently has only the central sector instrumented with 25 camera modules (1600 pixels), providing a 2.68-deg field of view (FoV). The camera electronics are based on custom TARGET (TeV array readout with GSa/s sampling and event trigger) application-specific integrated circuits. Field programmable gate arrays sample incoming signals at a gigasample per second. A single backplane provides camera-wide triggers. An upgrade of the pSCT camera that will fully populate the focal plane is in progress. This will increase the number of pixels to 11,328, the number of backplanes to 9, and the FoV to 8.04 deg. Here, we give a detailed description of the pSCT camera, including the basic concept, mechanical design, detectors, electronics, current status, and first light

Combined dark matter searches towards dwarf spheroidal galaxies with Fermi-LAT, HAWC, H.E.S.S., MAGIC, and VERITAS

Cosmological and astrophysical observations suggest that 85\% of the total matter of the Universe is made of Dark Matter (DM). However, its nature remains one of the most challenging and fundamental open questions of particle physics. Assuming particle DM, this exotic form of matter cannot consist of Standard Model (SM) particles. Many models have been developed to attempt unraveling the nature of DM such as Weakly Interacting Massive Particles (WIMPs), the most favored particle candidates. WIMP annihilations and decay could produce SM particles which in turn hadronize and decay to give SM secondaries such as high energy $\gamma$ rays. In the framework of indirect DM search, observations of promising targets are used to search for signatures of DM annihilation. Among these, the dwarf spheroidal galaxies (dSphs) are commonly favored owing to their expected high DM content and negligible astrophysical background. In this work, we present the very first combination of 20 dSph observations, performed by the Fermi-LAT, HAWC, H.E.S.S., MAGIC, and VERITAS collaborations in order to maximize the sensitivity of DM searches and improve the current results. We use a joint maximum likelihood approach combining each experiment's individual analysis to derive more constraining upper limits on the WIMP DM self-annihilation cross-section as a function of DM particle mass. We present new DM constraints over the widest mass range ever reported, extending from 5 GeV to 100 TeV thanks to the combination of these five different $\gamma$-ray instruments