X-ray upper limits of GW150914 with MAXI
We searched for X-ray candidates of the gravitational wave (GW) event GW150914 with Monitor of All-sky X-ray Image (MAXI). MAXI observed the error region of the GW event GW150914 from 4 minutes after the event and covered about 90% of the error region in 25 minutes. No significant time variations on timescales of 1 s to 4 days were found in the GW error region. The $3\sigma$ upper limits for the X-ray emission associated with the GW event in 2--20 keV were 9.5 $\times 10^{-10}$, 2.3 $\times 10^{-10}$, and 0.8 $\times 10^{-10}$ ergs cm$^{-2}$ s$^{-1}$ for the time scale of $\sim$ 1000 s, 1 day, and 10 days, respectively. If GW events are associated with short GRBs like GRB 050709, MAXI will be able to detect X-ray emissions from the source.