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1. Direct measurement of stellar angular diameters by the VERITAS Cherenkov telescopes

   https://doi.org/10.1038/s41550-019-0741-z  

   Benbow, W. ; Bird, R. ; Brill, A. ; Brose, R. ; Chromey, A. J. ; Daniel, M. K. ; Feng, Q. ; Finley, J. P. ; Fortson, L. ; Furniss, A. ; et al ( April 2019 , Nature Astronomy)

   The angular size of a star is a critical factor in determining its basic properties. Direct measurement of stellar angular diameters is difficult: at interstellar distances stars are generally too small to resolve by any individual imaging telescope. This fundamental limitation can be overcome by studying the diffraction pattern in the shadow cast when an asteroid occults a star, but only when the photometric uncertainty is smaller than the noise added by atmospheric scintillation. Atmospheric Cherenkov telescopes used for particle astrophysics observations have not generally been exploited for optical astronomy due to the modest optical quality of the mirror surface.more »However, their large mirror area makes them well suited for such high-time-resolution precision photometry measurements. Here we report two occultations of stars observed by the Very Energetic Radiation Imaging Telescope Array System (VERITAS) Cherenkov telescopes with millisecond sampling, from which we are able to provide a direct measurement of the occulted stars’ angular diameter at the ≤0.1 mas scale. This is a resolution never achieved before with optical measurements and represents an order of magnitude improvement over the equivalent lunar occultation method. We compare the resulting stellar radius with empirically derived estimates from temperature and brightness measurements, confirming the latter can be biased for stars with ambiguous stellar classifications.« less
2. Demonstration of stellar intensity interferometry with the four VERITAS telescopes

   https://doi.org/10.1038/s41550-020-1143-y  

   Abeysekara, A. U. ; Benbow, W. ; Brill, A. ; Buckley, J. H. ; Christiansen, J. L. ; Chromey, A. J. ; Daniel, M. K. ; Davis, J. ; Falcone, A. ; Feng, Q. ; et al ( July 2020 , Nature Astronomy)

   High angular resolution observations at optical wavelengths provide valuable insights into stellar astrophysics, and enable direct measurements of fundamental stellar parameters and the probing of stellar atmospheres, circumstellar disks, the elongation of rapidly rotating stars and the pulsations of Cepheid variable stars. The angular size of most stars is of the order of one milliarcsecond or less, and to spatially resolve stellar disks and features at this scale requires an optical interferometer using an array of telescopes with baselines on the order of hundreds of metres. We report on the implementation of a stellar intensity interferometry system developed for themore »four VERITAS imaging atmospheric Cherenkov telescopes. The system was used to measure the angular diameter of the two sub-milliarcsecond stars β Canis Majoris and ϵ Orionis with a precision of greater than 5%. The system uses an offline approach in which starlight intensity fluctuations that are recorded at each telescope are correlated post observation. The technique can be readily scaled onto tens to hundreds of telescopes, providing a capability that has proven technically challenging to the current generation of optical amplitude interferometry observatories. This work demonstrates the feasibility of performing astrophysical measurements using imaging atmospheric Cherenkov telescope arrays as intensity interferometers and shows the promise for integrating an intensity interferometry system within future observatories such as the Cherenkov Telescope Array.« less
3. Technical and scientific performance of the prototype Schwarzschild-Couder telescope for CTA

   https://doi.org/10.1117/12.2594580  

   Adams, Colin B. ; Ambrosi, Giovanni ; Ambrosio, Michelangelo ; Aramo, Carla ; Batista, Pedro I. ; Benbow, Wystan ; Bertucci, Bruna ; Bissaldi, Elisabetta ; Bitossi, Massimiliano ; Boiano, Alfonso ; et al ( August 2021 , Proceedings Volume 11820, Astronomical Optics: Design, Manufacture, and Test of Space and Ground Systems III;)

   Hallibert, Pascal ; Hull, Tony B. ; Kim, Daewook ; Keller, Fanny (Ed.)

   The Cherenkov Telescope Array (CTA) is the next-generation ground-based observatory for very-high-energy gamma rays. One candidate design for CTA's medium-sized telescopes consists of the Schwarzschild-Couder Telescope (SCT), featuring innovative dual-mirror optics. The SCT project has built and is currently operating a 9.7-m prototype SCT (pSCT) at the Fred Lawrence Whipple Observatory (FLWO); such optical design enables the use of a compact camera with state-of-the art silicon photomultiplier detectors. A partially-equipped camera has recently successfully detected the Crab Nebula with a statistical significance of 8.6 standard deviations. A funded upgrade of the pSCT focal plane sensors and electronics is currently ongoing,more »which will bring the total number of channels from 1600 to 11328 and the telescope field of view from about 2.7° to 8° . In this work, we will describe the technical and scientific performance of the pSCT.« less
4. Gamma-ray counterparts of 2WHSP high-synchrotron-peaked BL Lac objects as possible signatures of ultra-high-energy cosmic ray emission

   https://doi.org/10.1093/mnras/staa1990  

   Toomey, Michael W ; Oikonomou, Foteini ; Murase, Kohta ( September 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present a search for high-energy γ-ray emission from 566 Active Galactic Nuclei at redshift z > 0.2, from the 2WHSP catalogue of high-synchrotron peaked BL Lac objects with 8 yr of Fermi-LAT data. We focus on a redshift range where electromagnetic cascade emission induced by ultra-high-energy cosmic rays can be distinguished from leptonic emission based on the spectral properties of the sources. Our analysis leads to the detection of 160 sources above ≈5σ (TS ≥25) in the 1–300 GeV energy range. By discriminating significant sources based on their γ-ray fluxes, variability properties, and photon index in the Fermi-LAT energy range,more »and modelling the expected hadronic signal in the TeV regime, we select a list of promising sources as potential candidate ultra-high-energy cosmic ray emitters for follow-up observations by Imaging Atmospheric Cherenkov Telescopes.« less
5. Verification of the optical system of the 9.7-m prototype Schwarzschild-Couder Telescope

   https://doi.org/10.1117/12.2568134  

   Adams, Colin ; Alfaro, Ruben ; Ambrosi, Giovanni ; Ambrosio, Michelangelo ; Aramo, Carla ; Benbow, Wystan ; Bertucci, Bruna ; Bissaldi, Elisabetta ; Bitossi, M. ; Boiano, Alfonso ; et al ( August 2020 , SPIE Optical Engineering + Applications, 2020, Online Only)

   Sasián, José ; Youngworth, Richard N. (Ed.)

   For the first time in the history of ground-based x-ray astronomy, the on-axis performance of the dual mirror, aspheric, aplanatic Schwarzschild-Couder optical system has been demonstrated in a 9:7-m aperture imaging atmospheric Cherenkov telescope. The novel design of the prototype Schwarzschild-Couder Telescope (pSCT) is motivated by the need of the next-generation Cherenkov Telescope Array (CTA) observatory to have the ability to perform wide (>=8°) field-of-view observations simultaneously with superior imaging of atmospheric cascades (resolution of 0:067 per pixel or better). The pSCT design, if implemented in the CTA installation, has the potential to improve significantly both the x-ray angular resolutionmore »and the off-axis sensitivity of the observatory, reaching nearly the theoretical limit of the technique and thereby making a major impact on the CTA observatory sky survey programs, follow-up observations of multi-messenger transients with poorly known initial localization, as well as on the spatially resolved spectroscopic studies of extended x-ray sources. This contribution reports on the initial alignment procedures and point-spread-function results for the challenging segmented aspheric primary and secondary mirrors of the pSCT.« less