Gamma-ray burst observation & gravitational wave event follow-up with CALET on the International Space Station

Abstract

The CALorimetric Electron Telescope (CALET) has been observing high-energy cosmic rays and gamma-rays on the International Space Station since October 2015. The CALET gamma-ray burst monitor (CGBM), mounted on CALET to observe prompt emissions of gamma-ray bursts (GRBs) in the hard X-ray and soft gamma-ray band, has been monitoring all-sky with ∼ 60 % duty cycle without any problems since October 2015. As of end May 2021, CGBM has detected 254 GRBs, including 31 short GRBs, thanks to the onboard trigger system. The Calorimeter (CAL), the primary instrument of CALET, has also collected gamma-ray data in the energy range from 1 GeV to 10 TeV while maintaining both instruments in good condition. We continue searching for high- energy gamma-rays from GRBs detected by CGBM, and have found two possible gamma-rays from GRBs. As described above, CALET can detect prompt emissions and high energy gamma- ray emission of GRBs. Therefore, we also have actively participated in the follow-up campaign for electromagnetic counterparts of the gravitational wave events observed by LIGO/Virgo since the operation start of the CALET. Although we have found no candidates of electromagnetic counterparts of the gravitational wave events, we have derived upper limits of the high-energy gamma-ray flux for 26 events in the LIGO/Virgo third observation run.