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1. Alignment of the optical system of the 9.7-m prototype Schwarzchild-Coulder Telescope

   https://doi.org/10.1117/12.2564653  

   Adams, C.~B.~ ; Alfaro, R.~ ; G.~Ambrosi, Ambrosi ; Ambrosio, M.~ ; Aramo, C.~ ; Benbow, W.~ ; Bertucci, B.~ ; Bissaldi, E.~ ; Bitossi, M.~ ; Boiano, A.~ ; et al ( December 2020 , Proceedings Volume 11445, Ground-based and Airborne Telescopes VIII;)

   Marshall, Heather K. ; Spyromilio, Jason ; Usuda, Tomonori (Ed.)

   The novel 9.7m Schwarzschild-Couder Telescope (SCT), utilizing aspheric dual-mirror optical system, has been constructed as a prototype medium size x-ray telescope for the Cherenkov Telescope Array (CTA) observatory. The prototype SCT (pSCT) is designed to achieve simultaneously the wide (≥ 8°) field of view and the superior imaging resolution (0.067 per pixel) to significantly improve scientific capabilities of the observatory in conducting the sky surveys, the follow-up observations of multi-messenger transients with poorly known initial localization and the morphology studies of x-ray sources with angular extent. In this submission, we describe the hardware and software implementations of the telescope optical system as well as the methods specifically developed to align its complex optical system, in which both primary and secondary mirrors are segmented. The pSCT has detected Crab Nebula in June 2020 during ongoing commissioning, which was delayed due to worldwide pandemic and is not yet completed. Verification of pSCT performance is continuing and further improvement of optical alignment is anticipated. 

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2. 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, 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. 

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3. The upgraded camera for the prototype Schwarzschild-Couder Telescope of the Cherenkov Telescope Array

   https://doi.org/10.1117/12.2530431  

   Paoletti, Riccardo ( September 2019 , The upgraded camera for the prototype Schwarzschild-Couder Telescope of the Cherenkov Telescope Array)

   The Schwarzschild-Couder Telescope (SCT) is a mid-size telescope proposed for the Cherenkov Telescope Array. In order to substantially improve the eld of view and image resolution compared to i traditional Davies-Cotton telescopes, innovative solutions are foreseen in the design, like the use of Silicon Photomultipliers (SiPM) as light sensors and waveform digitizers for recording the fast light signals from atmospheric showers. A project is now underway to upgrade the camera by increasing its pixel count to 11; 328 pixels and field of view of 8:0. The camera electronics has been completely redesigned by using new waveform digitizer and trigger ASICs with the final goal of lowering the gamma-ray energy threshold and therefore provide an excellent instrument tailored for extended sources investigations and multi-messenger astronomy. 

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4. Validation of standardized data formats and tools for ground-level particle-based gamma-ray observatories

   https://doi.org/10.1051/0004-6361/202243527  

   Albert, A. ; Alfaro, R. ; Arteaga-Velázquez, J. C. ; Ayala Solares, H. A. ; Babu, R. ; Belmont-Moreno, E. ; Brisbois, C. ; Caballero-Mora, K. S. ; Capistrán, T. ; Carramiñana, A. ; et al ( November 2022 , Astronomy & Astrophysics)

   Context. Ground-based γ-ray astronomy is still a rather young field of research, with strong historical connections to particle physics. This is why most observations are conducted by experiments with proprietary data and analysis software, as is usual in the particle physics field. However, in recent years, this paradigm has been slowly shifting toward the development and use of open-source data formats and tools, driven by upcoming observatories such as the Cherenkov Telescope Array (CTA). In this context, a community-driven, shared data format (the gamma-astro-data-format , or GADF) and analysis tools such as Gammapy and ctools have been developed. So far, these efforts have been led by the Imaging Atmospheric Cherenkov Telescope community, leaving out other types of ground-based γ -ray instruments. Aims. We aim to show that the data from ground particle arrays, such as the High-Altitude Water Cherenkov (HAWC) observatory, are also compatible with the GADF and can thus be fully analyzed using the related tools, in this case, Gammapy. Methods. We reproduced several published HAWC results using Gammapy and data products compliant with GADF standard. We also illustrate the capabilities of the shared format and tools by producing a joint fit of the Crab spectrum including data from six different γ -ray experiments. Results. We find excellent agreement with the reference results, a powerful confirmation of both the published results and the tools involved. Conclusions. The data from particle detector arrays such as the HAWC observatory can be adapted to the GADF and thus analyzed with Gammapy. A common data format and shared analysis tools allow multi-instrument joint analysis and effective data sharing. To emphasize this, a sample of Crab nebula event lists is made public with this paper. Because of the complementary nature of pointing and wide-field instruments, this synergy will be distinctly beneficial for the joint scientific exploitation of future observatories such as the Southern Wide-field Gamma-ray Observatory and CTA. 

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5. Optical spectroscopy of blazars for the Cherenkov Telescope Array

   https://doi.org/10.1051/0004-6361/202040090  

   Goldoni, P. ; Pita, S. ; Boisson, C. ; Max-Moerbeck, W. ; Kasai, E. ; Williams, D. A. ; D’Ammando, F. ; Navarro-Aranguiz, V. ; Backes, M. ; Barres de Almeida, U. ; et al ( June 2021 , Astronomy & Astrophysics)

   null (Ed.)

   Context. Blazars are the most numerous class of high-energy (HE; E ∼ 50 MeV−100 GeV) and very high-energy (VHE; E ∼ 100 GeV−10 TeV) gamma-ray emitters. Currently, a measured spectroscopic redshift is available for only about 50% of gamma-ray BL Lacertae objects (BL Lacs), mainly due to the difficulty in measuring reliable redshifts from their nearly featureless continuum-dominated optical spectra. The knowledge of the redshift is fundamental for understanding the emission from blazars, for population studies and also for indirect studies of the extragalactic background light and searches for Lorentz invariance violation and axion-like particles using blazars. Aims. This paper is the first in a series of papers that aim to measure the redshift of a sample of blazars likely to be detected with the upcoming Cherenkov Telescope Array (CTA), a ground-based gamma-ray observatory. Methods. Monte Carlo simulations were performed to select those hard spectrum gamma-ray blazars detected with the Fermi -LAT telescope still lacking redshift measurements, but likely to be detected by CTA in 30 hours of observing time or less. Optical observing campaigns involving deep imaging and spectroscopic observations were organised to efficiently constrain their redshifts. We performed deep medium- to high-resolution spectroscopy of 19 blazar optical counterparts with the Keck II, SALT, and ESO NTT telescopes. We searched systematically for spectral features and, when possible, we estimated the contribution of the host galaxy to the total flux. Results. We measured eleven firm spectroscopic redshifts with values ranging from 0.1116 to 0.482, one tentative redshift, three redshift lower limits including one at z ≥ 0.449 and another at z ≥ 0.868. Four BL Lacs show featureless spectra. 

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