NinjaSat: Initial Operation Results of the First Japanese 6U CubeSat for Bright X-ray Sources

We report the initial operation results of the first Japanese 6U CubeSat X-ray observatory NinjaSat, which was launched into a sun-synchronous orbit at an altitude of 530 km on November 11, 2023, by the SpaceX Transporter-9 mission. NinjaSat is designed to observe bright X-ray sources in the sky, such as black holes and neutron stars, which are often difficult to observe with modern large X-ray satellites due to instrument limitations. After the payload verification, NinjaSat observed the Crab Nebula on February 9 and correctly detected the 33.8 ms pulsation from the neutron star. With this observation, NinjaSat met the minimum success criteria. NinjaSat observed 10 X-ray sources by June 20 and successfully demonstrated that many X- ray sources can be observed even with a CubeSat, which is limited in terms of resources available for science payloads. Specifically, NinjaSat conducted the follow-up observation of a newly discovered X-ray transient SRGA J144459.2−604207 two days after its discovery, detecting multiple type I X-ray bursts. NinjaSat also observed type II X-ray bursts from a rapid burster MXB 1730−335. To the best of our knowledge, these are the first observations of X-ray bursts with a CubeSat, enabled by the large effective area of NinjaSat. NinjaSat continues observations to achieve full success and extra success.