The first detection of a gravitational wave event by advanced LIGO in 2015 prompted the search for and study of electromagnetic counterparts to gravitational wave events on a worldwide scale.
If an electromagnetic signal is also observed, it would provide additional information that can give us significantly better constraints on the parameters (such as mass, orbit, spin) of the binary system.
The CALorimetric Electron Telescope (CALET) is a wide field-of-view instrument to study high-energy cosmic-rays onboard the International Space Station accumulating scientific data since October 2015.
Gamma-rays in the energy range of 1 GeV to 10 TeV are monitored
continuously with its 30 radiation-length deep calorimeter (CAL),
with a field-of-view of about 2 steradian and an angular resolution
better than 0.4 degree above 10 GeV.
We have already reported an upper limit on the GW151226 event claimed
by advanced LIGO.
In this paper, we describe the refined data analysis of CAL for the GW151226 event and the preliminary results on the third gravitational event, GW170104.
Hard X-ray results from CALET Gamma-ray Burst Monitor will be presented separately.