Astrophysical events that evolve on short timescales (from milliseconds to years) are widely referred as transient events. In many cases, transient events are explosions or mergers of astrophysical objects that emit particles of all energies. This thesis focuses on very-high-energy (VHE; 100 GeV to 100 TeV) gamma rays, observed by the VERITAS telescope, to understand two types of transients, superluminous supernovae (SLSNe) and classical novae. In the first part, the background physics and technical approach of an imaging atmospheric Cherenkov technique deployed by VERITAS is reviewed in depth, including the analysis pipeline of VERITAS data, from camera data reduction to high level analysis output. In addition to supporting the ongoing work at VERITAS, the second part of this thesis describes the extensive effort to develop, commission and align the optical system of the prototype Schwarzschild-Couder telescope, also located at the VERITAS observatory. This new telescope provides an wider field of view and higher angular resolution compared with the conventional IACT design of current telescopes, and will join the next generation of VHE telescopes in the Cherenkov Telescope Array (CTA) project currently in development. The observation and analysis of two SLSNe is performed, on SN 2015bn and SN 2017egm, with both VERITAS and Fermi-LAT telescopes. The upper limit is reported on both events and is compared to a simple and a self-consistent model for parameter estimation. In addition, a population of sources were analyzed to estimate the future detection potential of new SLSNe with several gamma-ray observatories. Finally, the observation and analysis of several novae were performed. These sources were observed with VERITAS and Fermi-LAT. The observation of Nova Herculis 2021 is noteworthy in that the overlap of both observations may constrain the cutoff energy of the nova spectral model, providing a estimate for the maximum energy of the accelerated particles in the nova’s shock region.

Thesis (Ph. D.)--University of Washington, 2004. / Vita. Includes bibliographical references (leaves 92-96).

Blazars, active galactic nuclei with relativistic jets pointed almost directly at Earth, are powerful and highly variable sources of nonthermal electromagnetic radiation, including very-high-energy gamma rays. We can detect these gamma rays with arrays of imaging atmospheric Cherenkov telescopes (IACTs), including the Very Energetic Radiation Imaging Telescope Array System (VERITAS) and the upcoming Cherenkov Telescope Array (CTA). After reviewing the science of blazars and the methods used by IACTs, we investigate how gamma-ray variability can provide insight into blazars' physical properties while also complicating efforts to understand these sources as a population. We first present a study of three flaring blazars observed with VERITAS and analyze these sources' spectral and variability characteristics, taking into account data at other wavebands, including that of the Large Area Telescope aboard the Fermi space telescope (Fermi-LAT). Next, after laying out how observing biases and intrinsic variability can confound blazar population studies with IACTs, we propose methods to account for these effects, and use simulated data to report expectations for a blazar luminosity function measurement with VERITAS. Sophisticated new instruments and data analysis methods can further expand the frontier of gamma-ray blazar science. To that end, we design a camera software system to enable safer and more efficient operations of a next-generation IACT being developed for CTA, the prototype Schwarzschild-Couder Telescope (pSCT). Finally, we develop methods to apply deep neural networks to the analysis of IACT data and employ these methods to reject background events detected by simulated arrays of IACTs.

The modern fields of 𝜸-ray astrophysics, multi-messenger astronomy, and particle astrophysics, have a shared history and future. This thesis will focus on the interconnectedness between them, and the role that measurements of very high-energy (VHE; 100 GeV < 𝐸 < 100 TeV) 𝜸 rays play in contributing to them. In particular, one confirmed and one putative source class for VHE 𝜸 rays will be explored: active galactic nuclei (AGN) and short gamma-ray bursts (sGRBs) (associated binary neutron star (BNS) merger progenitors), characterized by their variable and transient emission behavior respectively. The particle acceleration and radiative processes which can result in this energetic electromagnetic (EM) emission in these sources is described. Subsequently, the detection principles of VHE 𝜸-ray photons is detailed, highlighting the unique detection technique of imaging atmospheric Cherenkov telescopes (IACTs) to use the Earth’s atmosphere as an effective high-energy particle calorimeter. Advancements in the field of these detectors will be discussed, from the first-generation of detectors up to its current generation, which includes the VERITAS 𝜸-ray observatory, the principal instrument used to conduct the analyses in this thesis. Additionally, a prototype for the novel two-mirror Schwarzschild-Couder telescope (SCT) design will be introduced, which is proposed for inclusion in the future Cherenkov Telescope Array Observatory (CTAO), set to be the next-generation observatory for VHE 𝜸 rays. Finally, work from three projects, each demonstrating the contributions VHE ? rays can make to the initially listed fields, will be discussed. First, a multi-messenger search for VHE 𝜸-ray emission in VERITAS associated with sub-threshold gravitational wave (GW) signals. No VHE ?-ray emission is detected; the upper limit on the integral flux for all VERITAS observations is reported and compared to a sGRB model. Next, a search for the imprint of beyond-the-standard-model axion-like particles (ALPs) on the flaring VHE spectrum of the radio galaxy (RG) AGN NGC 1275, embedded in the cool-core Perseus cluster. No detection is made; limited exclusions on the mass and coupling strength to electromagnetism are reported. Finally, a VERITAS detection of a VHE ?-ray flare from the blazar AGN B2 1811+31 will be presented. Preliminary work from the broadband analysis of the source are shown, and the future of the project is discussed.