Simulation studies for the first pathfinder of the CATCH space mission

The Chasing All Transients Constellation Hunters (CATCH) space mission is an intelligent constellation consisting of 126 micro-satellites in three types (A, B, and C), designed for X-ray observation with the objective of studying the dynamic universe. Currently, we are actively developing the first Pathfinder (CATCH-1) for the CATCH mission, specifically for type-A satellites. CATCH-1 is equipped with Micro Pore Optics (MPO) and a 4-pixel Silicon Drift Detector (SDD) array. To assess its scientific performance, including the effective area of the optical system, on-orbit background, and telescope sensitivity, we employ the Monte Carlo software Geant4 for simulation in this study. The MPO optics exhibit an effective area of 41 cm \(^2\) ${}^2$ at the focal spot for 1 keV X-rays, while the entire telescope system achieves an effective area of 29 cm \(^2\) ${}^2$ at 1 keV when taking into account the SDD detector’s detection efficiency. The primary contribution to the background is found to be from the Cosmic X-ray Background. Assuming a 625 km orbit with an inclination of \(29^\circ \) ${29}^{\circ }$ , the total background for CATCH-1 is estimated to be \(8.13\times 10^{-2}\) $8.13\times {10}^{-2}$  counts s \(^{-1}\) ${}^{-1}$ in the energy range of 0.5–4 keV. Based on the background within the central detector and assuming a Crab-like source spectrum, the estimated ideal sensitivity could achieve \(1.9\times 10^{-12}\) $1.9\times {10}^{-12}$  erg cm \(^{-2}\) ${}^{-2}$  s \(^{-1}\) ${}^{-1}$ for an exposure of 10 \(^4\) ${}^4$  s in the energy band of 0.5–4 keV. Furthermore, after simulating the background caused by low-energy charged particles near the geomagnetic equator, we have determined that there is no need to install a magnetic deflector.