A Case Study of Very-High-Energy γ-ray Emission from Galactic Supernova Remnant with VERITAS: γ Cygni

Abstract

Very-high-energy (VHE; E & 100 GeV) g-ray astronomy provides a powerful probeof non-thermal particle acceleration in extreme astrophysical environments. Supernova remnants (SNRs) are widely believed to be the primary sources of Galactic cosmic rays up to at least the “knee” of the cosmic-ray spectrum, yet the mechanisms responsible for accelerating particles to such energies and producing the observed g-ray emission remain actively debated. Observations spanning the GeV–TeV energy range are particularly valuable for distinguishing between leptonic and hadronic emission scenarios and for probing the interaction between expanding shock fronts and the surrounding interstellar medium. This dissertation presents a detailed study of the Galactic supernova remnant G78.2+2.1 (γ Cygni) using observations from the Very Energetic Radiation Imaging Telescope Array System (VERITAS). γ Cygni is a particularly compelling target due to its complex multiwavelength morphology, evidence for interaction with the surrounding interstellar medium, and the potential contributions from both SNR shell and pulsar-driven emission. These characteristics make it an ideal laboratory for disentangling leptonic and hadronic emission processes and for probing the interplay between shock acceleration, particle transport, and environmental effects in a middle-aged supernova remnant. The goal of this work is to investigate the morphology and broadband spectral properties of the VHE g-ray emission from this complex region and to place new constraints on the particle acceleration processes responsible for the observed radiation. In conducting this analysis, particular attention is given to a comparison between traditional background estimation techniques and an independent Singular Templates of Imaging Cherenkov Shower distribution (STOICS) method designed to improve analyses of extended g-ray sources. Independent analyses using multiple reconstruction pipelines show consistent morphology and spectral results, demonstrating the robustness of the analysis. Spatial analyses of VERITAS observations reveal statistically significant VHE g-ray emission associated with the γ Cygni region, including a prominent hotspot coincident with the source MAGIC J2019+408 and extended emission across the western portion of the SNR shell. Energy-dependent morphology studies suggest a broadening of the emission region at higher energies. The VERITAS spectrum is combined with Fermi-LAT, MAGIC, and HAWC measurements to construct a broadband spectral energy distribution spanning the GeV–TeV regime. Modeling of the broadband emission using hadronic scenarios, including broken power-law proton spectra, provides constraints on the underlying particle population responsible for the observed g rays.