Concept of the MONSTER onboard the HiZ-GUNDAM satellite

The High-z Gamma-ray Bursts for Unraveling the Dark Ages Mission (HiZ-GUNDAM) is dedicated to time-domain and multi-messenger astronomy, focusing on the observation of high-energy astronomical transient events, such as gamma-ray bursts (GRBs). The mission concept of HiZ-GUNDAM has been selected as a candidate for the competitively chosen medium-class mission of the Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency. Conceptual studies of the satellite and its onboard instruments are currently ongoing, with a target launch in the 2030s. HiZ-GUNDAM is designed to deliver rapid alerts of high-redshift GRBs through the use of its wide-field X-ray monitor, the Exploration of Ancient GRBs with Lobster Eye (EAGLE), and a co-mounted 30-cm Multiband Optical and Near-infrared Simultaneous Telescope for Efficient Response (MONSTER), which performs immediate photometric follow-up observations. Upon detecting a transient object, the HiZ-GUNDAM satellite is going to autonomously adjust its attitude toward detected transient events, initiate follow-up observations using MONSTER, and transmit alert information, detailing the object’s position, apparent magnitude, and photometric redshift within 1 h of detection. The MONSTER instrument achieves simultaneous five-band observations in the 0.5 to 2.5 μm range using a beam splitter and a Kösters prism. The incoming light is divided into visible (0.5 to 0.9 μm) and near-infrared components by the beam splitter, with the visible light directed to a visible light detector. The near-infrared light is further divided into four bands (0.9 to 1.3 μm, 1.3 to 1.7 μm, 1.7 to 2.1 μm, and 2.1 to 2.5 μm) by the Kösters prism and detected by a near-infrared detector. To ensure optimal performance, the telescope, beam splitter, Kösters prism, and visible-light detector are cooled to temperatures below 200 K, whereas the near-infrared detector is further cooled to below 120 K through radiative cooling. In addition, all mirrors and structural components of the telescope are constructed from the same aluminum alloy to minimize alignment errors during the cooling process. We provide an overview of the current status of the conceptual study of MONSTER onboard the HiZ-GUNDAM satellite.