Particle Astrophysics with the VERITAS Detector

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This application is for continued support for the McGill group in the VERITAS gamma-ray astrophysics project. VERITAS (the Very Energetic Imaging Telescope Array System) is an array of four twelve-meter telescopes that studies astrophysical gamma rays with energies between 100 GeV and 30 TeV, using the Imaging Atmospheric Cherenkov Technique; it is one of three major detectors in operation world-wide. The telescopes detect the Cherenkov light from the charged particle 'showers' that arise due to interactions of the gamma rays in the upper atmosphere; the Cherenkov light propagates to the ground (even though the charged particle showers are absorbed much higher in the atmosphere) and is imaged with cameras composed of fast phototubes. The gamma rays that VERITAS studies - at energies above 100 GeV - are produced by even-more-energetic charged particles in some of the most extreme environments in the Universe, including supernova remnants and active galactic nuclei (galaxies harbouring a super-massive black hole whose spin axis is approximately pointed along our line-of-sight). Since its debut in 2007, VERITAS has detected more than 60 astrophysical sources (approximately half of those being new discoveries), including both of these source classes as well as several others. In addition, there are other possible, more exotic sources of high-energy gamma rays, including annihilation of dark-matter particles and evaporation of primordial black holes. The study of the more-classical astrophysical sources as well as the search for more speculative signals can provide information about particle physics at energies well above those available at today's accelerators. VERITAS also works closely with multi-wavelength and multi-messenger partners to carry out coordinated observations of particularly interesting sources. We regularly collaborate with groups working in the radio, optical, and X-ray regimes. Closer to our own energy range are the NASA Fermi Gamma-ray Space Telescope (in the MeV to 10's of GeV regime) and the ground-based HAWC all-sky detector (in the > 10 TeV regime). Fermi sees the entire sky multiple times per day (albeit at low sensitivity due to its small size) and HAWC sees the entire northern hemisphere each day - they can both be used to trigger the smaller-field-of-view but more sensitive VERITAS instrument. In addition, and very germane to our particle physics interests, are the IceCube South Pole neutrino detector and the LIGO gravitational wave detector; VERITAS has a program of coordinated observations with both of these instruments. VERITAS has been in operation since 2007, and the McGill group has been a part of the collaboration since the mid 2000's. The instrument is very well-understood and stable, although we continue to upgrade it with new capabilities. We anticipate that it will operate for several more years with continued high-quality scientific output.