Unsteady Disk Accretion via External Radiation Drag

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  • Unsteady Disk Accretion via External Ra

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Time-dependent disk accretion/accretion disks driven by external radiation drag are investigated using self-similar transformations in the gravitational potential by a point-mass M_BH. The external drag force is assumed to be proportional to the fluid velocity v as -betav, where beta is a function of the look-back time, -t. We find self-similar solutions which satisfy the asymptotic behavior, such that the radial infall velocity V_r is -2betar near to the center or early epoch (-t ->infty),and 0 or -{beta+1/(-t)}r far from the center or later epoch (-t -> 0), while the rotation velocity v_phi is almost Keplerian sqrt(GM_BH/r) near to the center and proportional to sqrt(GM_BH/r){(-t)sqrt(GM_BH/r^3)}^eta or sqrt(GM_BH/r){(-t)sqrt(GM_BH/r^3}^{(eta-1)/3eta+3)} far from the center (or later epoch), where eta is a constant defined such that beta(-t)= eta/(-t). This solution approaches the steady state solution derived by Fukue and Umemura(1994 PASJ 46, 87) near to the center of the disk (or early epoch). In this paper, the critical nature and self-similar solutions are shown for several values of eta. These solutions include a critical solution and a transonic solution. The velocity and thermal structures of self-similar accretion disks and the mass-accretion rate are presented. Such accretion disks, in which the angular momenta are removed via external drag proportional to the velocities, are possible when the systems are embedded in radiation fields. The present self-similar solution may be applicable for gas accretion into a point-mass potential which is, for instance, produced by a massive black hole formed during an early epoch.

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