Development of an HV-CMOS active pixel sensor "AstroPix" for all-sky medium-energy gamma-ray telescopes

All-sky medium-energy gamma-ray observations are essential to deepen our understanding of physics in extreme astronomical objects such as gamma-ray bursts and active galactic nuclei. Those observations are also highly needed to further develop multi-messenger astronomy. Next-generation all-sky MeV gamma-ray telescopes must have a large area detector and keep high sensitivities even in the energies in which Compton scattering is dominant. Having both a silicon tracker (scatterer) and a calorimeter (absorber) as the main detector is a promising configuration for such a space mission. In order to fulfill the requirements such as a large sensitive area, low noise, high energy/positional resolution, and low power, we have been developing an HV-CMOS active pixel sensor, AstroPix. In this contribution, we report performance evaluations of AstroPix such as I-V and noise, energy calibration/resolution/threshold, and depletion depth measurements. Current achievements as a sensor for next-generation all-sky MeV gamma-ray telescopes and future development will be discussed.