タイトル：New method for black-hole spin measurement based on flux variation from an infalling gas clouds

アブスト：We propose a new method for black-hole spin measurement by considering a gas cloud falling onto a black hole from the marginally stable orbit. We calculate the gas motion and photon trajectories in the Kerr space-time and carefully examine how it is observed by a distant observer. The flux first gradually increases with time due to the focusing effect around the photon circular orbit and then rapidly decreases due to the gravitational redshift, as the gas ring approaches the event horizon. The flux variation timescale depends on the spin for a fixed black hole mass so that we can, in principle, determine it. To check the validity of our methodology, we examined the GR Radiation-MHD simulation data, finding that the gas rings are actually formed and fall onto the black hole, giving rise to flux variations. We will discuss the observational implication of the flux variation of the infalling gas clouds.