Neutrino Cooled Accretion Disk and Its Stability

We investigate the structure and stability of hypercritical accretion flows around stellar-mass black holes, taking into account neutrino cooling, lepton conservation, and firstly a realistic equation of state in order to properly treat the dissociation of nuclei. We obtain the radial distributions of physical properties, such as density, temperature and electron fraction, for various mass accretion rates 0.1 ∼ 10M⊙s . We find that, depending on mass accretion rates, different physics affect considerably the structure of the disk; most important physics is (1) the photodissociation of nuclei around r ∼ 100rg for relatively low mass accretion rates (Ṁ ∼ 0.01−0.1M⊙ s ), (2) efficient neutrino cooling around r ∼ 10−100rg for moderately high mass accretion rate (Ṁ ∼ 0.2−1.0M⊙s ), and (3) neutrino trapping (r ∼ 3 − 10rg) for very high mass accretion rate (Ṁ & 2.0M⊙s ). We also investigate the stability of hypercritical accretion flows by drawing the thermal equilibrium curves, and find that efficient neutrino cooling makes the accretion flows rather stable against both thermal and viscous modes. Subject headings: accretion, accretion disks—black hole physics—gamma-ray bursts—neutrinos—instability