First results from the radiation belt monitors onboard the 6U CubeSat X-Ray Observatory NinjaSat

NinjaSat, a 6U CubeSat-sized (30 cm x 20 cm x 10 cm) X-ray observation satellite, was launched into a low Earth orbit at an altitude of 550 km in November 2023. NinjaSat is equipped with two 1U CubeSat-sized (10 cm x 10 cm x 10 cm) gas X-ray detectors (GMC) as primary detectors for astronomical observations of bright X-ray sources such as neutron stars and black holes. Initial operations of NinjaSat have been underway since January 2024.

NinjaSat is also equipped with two radiation belt monitors (RBM) as sub-detectors to protect the GMCs from discharges in the gas cells potentially caused by excessive amount of incident charged particles. NinjaSat RBM uses a 9 mm x 9 mm Si-PIN photodiode to detect any increase in the count rate of either protons or electrons by exploiting the difference in the sensor's response to protons and electrons, and sends alert signals to GMCs which help ramping down high voltage applied to gas cells in the region of high charged particle rates. NinjaSat RBM is approximately 6% of the size of a 1U CubeSat in volume and weighs only 70 grams. Because NinjaSat RBM uses inexpensive, commercially available sensors and operates on an internal board independent of the primary detectors, it can be installed on other small satellites which need to monitor surrounding charged particle environment with relatively few development resources.

Since NinjaSat will operate in a sun-synchronous orbit, a global map of charged particles can be obtained. We will present the development of NinjaSat RBM and the first results of charged particle maps in the radiation belt obtained during the initial operational period of NinjaSat.