Possible power source and time variation of quasars and Seyfert galaxies

Accreting massive black holes in dense stellar systems are investigated to explain enormous energy source and time variation of quasars and Seyfert galaxies. A star pass in the vicinity of a black hole is disrupted by tidal force and forms an accretion disk around the black hole. Particular emphasis is given to the collision between an old accretion disk and a new gas cloud formed by tidal breakup. X-rays and high energy radiations are generated by shock heating of gas at the collision. After the collision the gas, that loses kinetic energy and then falls in the vicinity of the black hole, eventually forms an inner accretion disk. Ultraviolet and soft X-ray are emitted from the inner accretion disk. These collisions also give rise to variations in ultra-violet and X-ray flux. Depending on the mass and radius of stellar system and on the mass of the black hole, various time scales of variations are possible. These time scales are 1 -- 100 years for ultra-violet and 1 hour -- 100 years for X-ray flux, respectively. (author)