Observation of X-ray couterparts of GW Sources with MAXI

We present the results and prospects of the search for X-ray counterparts of gravitational wave sources by the Monitor of All-sky X-ray Image (MAXI), an all-sky X-ray monitor onboard the International Space Station (ISS). With the orbital revolution of the ISS, MAXI scans most of the sky in the 2-25 keV band every 92 minutes with a typical transit duration of 60 seconds.Thus MAXI has chance to obtain the early X-ray observation of gravitational wave events without planned pointing based on GW localization alerts.At the epoch of GW170817, MAXI was not operational due to its orbital position with high charged particle background. Even worse, with an unfortunate coincidence of the orbital geometry and phase with respect to the source location in the sky, the source was not observed for the subsequent few orbits. Still MAXI provided the earliest X-ray upper limit for GW180817 albeit not sufficiently constraining.With the more opportunities of GW events in the coming LIGO/Virgo O3, MAXI should be able to provide early X-ray observations, with which we can test various hypothesis on the X-ray emission from the neutron star merger events, and compare the known X-ray signatures of short GRBs such as extended emission and afterglows.