Observation of the Crab Nebula with the Single-Mirror Small-Size Telescope stereoscopic system at low altitude

Abstract

The Single-Mirror Small-Size Telescope (SST-1M) stereoscopic system is composed of two Imaging Atmospheric Cherenkov Telescopes (IACTs) designed to deliver optimal performance for gamma-ray astronomy in the multi-TeV energy range. It features a 4-m diameter tessellated mirror dish and an innovative SiPM-based camera. Its optical system features a 4-m diameter spherical mirror dish based on the Davies-Cotton design, maintaining a good image quality over a large field of view (FoV), while minimizing optical aberrations. In 2022, two SST-1M telescopes were installed at the Ondřejov Observatory, Czech Republic at an altitude of 510 meters above sea level, collecting data for commissioning and astronomical observations since then. We present the first SST-1M observations of the Crab Nebula, conducted between September 2023 and March 2024 in both mono and stereoscopic modes. During this observation period, 46 hours for the SST-1M-1 and 52 hours for the SST-1M-2 were collected (of which 33 hours were in stereoscopic mode). In this work, we used the Crab Nebula observation to validate the expected performance of the instrument, as evaluated by Monte Carlo (MC) simulations that were carefully tuned to account for instrumental and atmospheric effects. We determined that the energy threshold at the analysis level for the zenith angles below 30° is 1 TeV for mono mode and 1.3 TeV for stereo mode. The energy and angular resolutions were approximately 20% and 0.18° for mono mode and 10% and 0.10° for stereo mode, respectively. We present an off-axis performance assessment of the instrument and a detailed study of the systematic uncertainties. The full simulation results for the telescope and its camera are compared to the data for the first time, enabling a deeper understanding of the SST-1M array performance.