The Very Energetic Radiation Imaging Telescope Array System observations of the starburst galaxy M82

This work describes the Very Energetic Radiation Imaging Telescope Array Systems (VERITAS) observations of the starburst galaxy M82 by documenting the analysis of 231 quality-selected hours of observational data taken between 2008 and 2014. The prototypical starburst galaxy, M82's high supernova (SN) rate and dense central accumulation of molecular gas make it a promising candidate for studying cosmic ray (CR) acceleration and propagation with the detection of diffuse very high energy (VHE; approximately 100 GeV-100 TeV) gamma-ray emission. This diffuse emission is predicted to result from proton-proton interactions within the galaxy's core that produce VHE gamma -rays through neutral pion decay. This work confirms the results of the initial VERITAS publication covering 137 hours of M82 observations between January 2008 and April 2009, yielding a total of 103.5 excess gamma-ray-like events (0.007 gamma/min, 5.7 pre-trial statistical significance) from a deeper exposure of 231 hours of observation. The spectral properties found are in agreement with the original detection within errors (Gamma=2.85 +/- 0.39). These results are consistent with paradigms that describe the production of CRs via the conversion of mechanical energy generated in supernovae (SNe). These findings will improve current diffuse emission models by better constraining galaxy parameters and by providing insight into CR proton loss processes and timescales, with further understanding to be gained with the introduction of the Cherenkov Telescope Array (CTA).