KOYOH: the x-ray transient monitoring and rapid alert satellite

The KOYOH satellite, launched on December 1, 2023, aboard a SpaceX Falcon 9 rocket from Vandenberg Space Force Base, is a 50-kg class microsatellite designed to advance the understanding of short gamma-ray bursts (GRBs) and their association with gravitational wave (GW) events. The satellite operates in a Sun-Synchronous Orbit (SSO), equipped with the Transient Localization Experiment (T-LEX) and the Gamma-ray Detector (KGD). T-LEX, a wide-field X-ray imaging detector, and KGD, a wide-field gamma-ray detector, are designed to detect and localize GRBs and other high-energy transients. Following its launch and initial deployment, the satellite successfully established communication links and confirmed the operational status of its subsystems. Initial telemetry data verified the deployment of the Solar Array Paddles (SAP) and correct orientation towards the Sun. The satellite's power systems were confirmed to be functioning nominally. Subsequent tests established the functionality of the mission instruments. The KGD exhibited a count rate of about 300-500 counts/sec at low latitudes, while T-LEX showed a count rate of approximately 200-300 counts/sec in total. These initial results confirm the successful detection of X-ray and gamma-ray photons by both instruments in orbit. This paper details the design, launch, initial operations, and early mission results of the KOYOH satellite, highlighting its contributions to multi-messenger astronomy.