  We propose a new method for black hole spin measurement. In this method, we
consider a gas blob or ring falling onto a black hole from the marginally
stable orbit, keeping its initial orbital angular momentum. We calculate the
gas motion and photon trajectories in the Kerr space-time and, assuming that
the gas blob or ring emits monochromatic radiation, carefully examine how it is
observed by a distant observer. The light curve of the orbiting gas blob is
composed of many peaks because of periodic enhancement of the flux due to the
gravitational lensing and beaming effects. Further, the intensity of each peak
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 blob approaches the event horizon. The light curve of the
gas ring is equivalent to a superposition of those of the blobs with various
initial orbital phases, and so it is continuous and with no peaks. The flux
first gradually increases and then rapidly decays, as in the blob model. The
flux variation timescale depends on the black hole spin and is independent from
the inclination angle, while time averaged frequency shift have dependences of
both effects. We can thus, in principle, determine spin and inclination angle
from observations. The observational implications and future issues are briefly
discussed.
