Development of a microcalorimeter array for the Diffuse Intergalactic Oxygen Surveyor (DIOS) mission

We are developing a superconducting transition-edge sensor (TES) microcalorimeter array for the Diffuse Intergalactic Oxygen Surveyor (DIOS) mission. DIOS is a relatively small Japanese X-ray mission which will study large-scale distribution of the warm-hot intergalactic medium (WHIM) using O_VII and O_VIII emission lines. The satellite weighs about 400 kg equipped with a four-reflection X-ray telescope (FXT) and a TES microcalorimeter array (XSA). The design goal of the observing system is an effective area larger than 100 cm² at the oxygen line energy, a field of view about 50 arcmin square, and an energy resolution about 2 eV in the energy range of 0.3-1 keV. The TES microcalorimeter array provides the large field of view and good energy resolution at the same time. We plan to install an array comprising 16 × 16 pixels with an overall size of 1 cm square, which is cooled with a cryogen-free cooler. Pixels are readout by multiplexing signals using a multi-input SQUID amplifier, with each input connected to a TES microcalorimeter which is AC biased with a different frequency. We report the design and present status of the XSA system development.