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Search for quiet stellar-mass black holes by asteroseismology from space

Stars with an initial mass more than ~25 Msun are thought to ultimately become black holes. Then stellar-mass black holes should be ubiquitous but fewer than 20 have been found in our Galaxy to date, all of which have been found through their X-ray emission. In most cases these are soft X-ray transients --- low-mass X-ray binaries whose optical counterparts are late type stars filling their Roche lobes, leading to accretion onto black holes. In one case, the stellar-mass black hole is in a high-mass X-ray binary whose optical counterpart is an early type star. Its strong stellar winds are accreted by the black hole, producing X-ray emission. It follows that X-ray-quiet stellar-mass black holes exist in wide binary systems. The discovery of black holes in the optical through their gravitational interactions would be a major scientific breakthrough. Recent space-based photometry has made it possible to measure phase or frequency modulation of pulsating stars to extremely high precision. Such modulation is caused by orbital motion, and its analysis offers the lower limit for the mass of the companion to the pulsating star. If the companions are non-luminous and if the masses of the companions exceed the mass limit for neutron stars (~3 Msun), the companions should be black holes. We review the methodology, and analyses of some encouraging cases are demonstrated.