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AB In 2019, the Very Energetic Radiation Imaging Telescope Array System (VERITAS) was augmented with high-speed optical electronics in order to allow for Stellar Intensity Interferometry (SII) observational capabilities. This research shows how VERITAS-SII (VSII), which measures correlations of starlight intensity fluctuations across spatially separated telescopes, can enable the characterization of binary stellar systems. We first use VSII data collected on the binary star Spica to develop a dynamic analysis technique. We then simulate the squared visibility curve given a particular orientation of Spica's components. Because of Spica's 4.0145-day period, the binary separation and orientation, and therefore the simulated squared visibility, should vary greatly from night to night. These variations are consistent with measured variations in the observed squared visibility curves. The initial results indicate that VSII observations potentially demonstrate good sensitivity to the evolution of the Spica binary system. With further development, it may be possible to fit a multi-dimensional image to the system, opening the door to model-dependent VSII imaging. 

The VERITAS Imaging Air Cherenkov Telescope array (IACT) was augmented in 2019 with high-speed focal plane electronics to create a new Stellar Intensity Interferometry (SII) observational capability (VERITAS-SII, or VSII). VSII operates during bright moon periods, providing high angular resolution observations ( < 1 mas) in the B photometric band using idle telescope time. VSII has already demonstrated the ability to measure the diameters of two B stars at 416 nm (Bet CMa and Eps Ori) with < 5% accuracy using relatively short (5 hours) exposures.1 The VSII instrumentation was recently improved to increase instrumental sensitivity and observational efficiency. This paper describes the upgraded VSII instrumentation and documents the ongoing improvements in VSII sensitivity. The report describes VSII’s progress in extending SII measurements to dimmer magnitude stars and improving the VSII angular diameter measurement resolution to better than 1%. 

High-energy cosmic rays that hit the Earth can be used to study large-scale atmospheric perturbations. After a first interaction in the upper parts of the atmosphere, cosmic rays produce a shower of particles that sample it down to the detector level. The HAWC (High-Altitude Water Cherenkov) gamma-ray observatory in Central Mexico at 4,100 m elevation detects air shower particles continuously with 300 water Cherenkov detectors with an active area of 12,500 m2. On January 15th, 2022, HAWC detected the passage of the pressure wave created by the explosion of the Hunga volcano in the Tonga islands, 9,000 km away, as an anomaly in the measured rate of shower particles. The HAWC measurements are used to determine the propagation speed of four pressure wave passages, and correlate the variations of the shower particle rates with the barometric pressure changes. The profile of the shower particle rate and atmospheric pressure variations for the first transit of the pressure wave at HAWC is compared to the pressure measurements at the Tonga island, near the volcanic explosion. By using the cosmic-ray propagation in the atmosphere as a probe for the pressure, it is possible to achieve very high time-resolution measurements. Moreover, the high-altitude data from HAWC allows to observe the shape of the pressure anomaly with less perturbations compared to sea level detectors. Given the particular location and the detection method of HAWC, our high-altitude data provides valuable information that contributes to fully characterize this once-in-a-century phenomenon. 

A modern implementation of a stellar intensity interferometry (SII) system on an array of large optical telescopes would be a highly valuable complement to the current generation of optical amplitude interferometers. The SII technique allows for observations at short optical wavelengths (U/B/V bands) with potentially dense (u,v) plane coverage. We describe a complete SII system that is used to measure the spatial coherence of a laboratory source which exhibits signal to noise ratios comparable to actual stellar sources. A novel analysis method, based on the correlation measurements between orthogonal polarization states, was developed to remove unwanted effects of spurious correlations. Our system is currently being tested in night sky observations at the StarBase Observatory (Grantsville, Utah) and will soon be ported to the VERITAS (Amado, AZ) telescopes. The system can readily be integrated with current optical telescopes at minimal cost. The work here serves as a technological pathfinder for implementing SII on the future Cherenkov Telescope Array. 

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 ∼ 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.