Speaker
Description
In the O4a period of gravitational wave observation, only two LIGO observatories were available, and thus, a 90% credible region was typically several percent to several tens of percent of the whole sky. Therefore, we need instruments with a wide field of views. Monitor of All-sky X-ray Image (MAXI) is an X-ray detector aboard the International Space Station (ISS). MAXI is sensitive to X-rays in 2-20 keV and has a field of view of 3$^\circ$ x 160$^\circ$. MAXI can observe 85% of the all-sky in 92 minutes (orbital period of the ISS) and almost 100% in two weeks. The ability to observe X-rays over a wide area is advantageous for searching for X-ray counterparts of gravitational wave sources. Because it is estimated that the electromagnetic emissions generated by gravitational wave events decay rapidly over time, the ability to observe X-rays over a wide area is advantageous for searching for gravitational wave sources. From the data analyses of MAXI, we can obtain the observation time and X-ray fluxes or upper limits for each region of the all-sky. The predicted flux of most events is an upper limit due to MAXI's insufficient sensitivity. However, it could be a severe constraint on the radiation model if the region was observed just after the gravitational wave event. In this study, we analyzed gravitational wave events with FAR < 20 per year ( 6.34 x 10$^{-7}$ Hz ) and have Prob NS > 5%, which may include neutron stars. 13 events met these two criteria. We confirmed that none of the events had bright X-ray sources that MAXI could detect. In these 13 events, the average coverage of LIGO's 90% credible region of gravitational wave in one scan (92 minutes) of MAXI was 80%. Next, we obtained the upper limit of the X-ray flux by analyzing the MAXI data. First, we adjusted the resolution of the probability map of LIGO to the MAXI's point spread function. Then the number of photons in each region was counted in the pixels in the 90% credible region. Finally, the numbers of the photons were divided by the MAXI exposure (effective area x time) to obtain the flux upper limit of each position. This method was applied to 13 events, and we obtained the upper limits. For example, the upper limit of S230615az was the average was 1.3 x 10$^{-10}$ to 5.4 x 10$^{-9}$ erg cm$^{-2}$ s$^{-1}$, the average was 9 x 10$^{-10}$ erg cm$^{-2}$ s^$^{-1}$.
