The Gamma-ray Transients Monitor (GTM) on board Formosat-8B

The Gamma-ray Transients Monitor (GTM) is a secondary payload on board Formosat-8B (FS-8B), a Taiwanese remote-sensing satellite scheduled to launch in 2026. The goal of GTM is to monitor Gamma Ray Bursts (GRBs) and other bright gamma-ray transients in the energy range from 50 keV to 2 MeV. GTM consists of two identical modules located on two opposite sides of FS-8B. Each module has four sensor units facing different directions to cover half of the sky. The two modules will then cover the whole sky, including the direction occulted by the Earth. Each sensor unit is composed of a GAGG scintillator array (50 mm × 50 mm × 8 mm) to be readout by SiPM with 16 pixel-channels. Base on detailed simulations, the detection efficiency is larger than 50% when the GRB fluences (in the energy range from 10 keV to 1 MeV) are larger than 2×10⁻⁶ erg/cm² and 6×10⁻⁷ erg/cm² for long and short GRBs, respectively. GTM is expected to detect about 50 GRBs per year, according to the Fermi/GBM 10-year GRB-fluence distribution and the one-third duty cycle as being the secondary payload. For localization capability, the localization uncertainty at 3σ confidence level of long GRBs with the fluences of 4×10⁻⁵ erg/cm² and 4×10⁻⁶ erg/cm² are about 3 and 30 degrees, respectively.