The vertical composition of neutrino-dominated accretion disks in gamma-ray bursts

We investigate the vertical structure and elements distribution of neutrino-dominated accretion flows around black holes in spherical coordinates with the reasonable nuclear statistical equilibrium. According our calculations, heavy nuclei tend to be produced in a thin region near the disk surface, whose mass fractions are primarily determined by the accretion rate and the vertical distribution of temperature and density. In this thin region, we find that $^{56}\rm Ni$ is dominant for the flow with low accretion rate (e.g., $0.05$$M_{\odot}$ $\rm s^{-1}$) but $^{56}\rm Fe$ is dominant for the high counterpart (e.g., $1 M_{\odot}$ $\rm s^{-1}$). The dominant $^{56}\rm Ni$ in the special region may provide a clue to understand the bumps in the optical light curve of core-collapse supernovae.Comment: 15 pages, 2 figures, accepted for publication in Ap

Spectral Analyses of the Nearest Persistent Ultraluminous X-Ray Source M33 X-8

We provide a detailed analysis of 12 XMM observations of the nearest persistent extragalactic ultraluminous X-ray source (ULX), M33 X-8. No significant spectral evolution is detected between the observations, therefore we combine the individual observations to increase the signal-to-noise ratio for spectral fitting. The combined spectra are best fitted by the self-consistent p-free disk plus power-law component model with p = 0.571_{-0.030}^{+0.032}, kT_{in} = 1.38_{-0.08}^{+0.09} keV, and the flux ratio of the p-free disk component to the power-law component being 0.63:0.37 in the 0.3 -- 10 keV band. The fitting indicates that the black hole in M33 X-8 is of \sim 10 M_{\odot} and accretes at a super-Eddington rate (\sim 1.5 L_{Edd}), and the phase of the accretion disk is close to a slim disk (p = 0.5). We report, for the first time, that an extra power-law component is required in addition to the p-free disk model for ULXs. In super-Eddington cases, the power-law component may possibly result from the optically thin inner region f the disk or a comptonized corona similar to that of a standard thin disk.Comment: 11 pages, 1 table, 2 figures, accepted by PAS

Possible Outflow Formation in the Central Engine of GRBs

We investigate the vertical structure of neutrino-dominated accretion flows in spherical coordinates. In our calculation, the empty funnel along the rotation axis can naturally explain the neutrino annihilable ejection. The outflow is possible due to the positive Bernoulli function, and the luminosity of neutrino annihilation is enhanced by one or two orders of magnitude

Non-axisymmetric instabilities in shocked accretion flows with differential rotation

The linear stability of a shocked isothermal accretion flow onto a black hole is investigated in the inviscid limit. The outer shock solution, which was previously found to be stable with respect to axisymmetric perturbations, is, however, generally unstable to non-axisymmetric ones. Eigenmodes and growth rates are obtained by numerical integration of the linearized equations. The mechanism of this instability is based on the cycle of acoustic waves between their corotation radius and the shock. It is a form of the Papaloizou-Pringle instability, modified by advection and the presence of the shock. As such it can be generalized to non isothermal shocked accretion flows. Blobs and vortices are generated by the shock as a by-product of the instability

A note on the slim accretion disk model

We show that when the gravitational force is correctly calculated in dealing with the vertical hydrostatic equilibrium of black hole accretion disks, the relationship that is valid for geometrically thin disks, i.e., c(s)/Omega(K) H = constant, where c(s) is the sound speed, Omega(K) is the Keplerian angular velocity, and H is the half-thickness of the disk, does not hold for slim disks. More importantly, by adopting the correct vertical gravitational force in studies of thermal equilibrium solutions, we find that there exists a maximal possible accretion rate for each radius in the outer region of optically thick accretion flows, such that only the inner regions of these flows can possibly take the form of slim disks, and strong outflows from the outer region are required to reduce the accretion rate in order for slim disks to be realized

Understanding Simulations of Thin Accretion Disks by Energy Equation

We study the fluctuations of standard thin accretion disks by linear analysis of the time-dependent energy equation together with the vertical hydrostatic equilibrium and the equation of state. We show that some of the simulation results in Hirose et al. (2009b), such as the time delay, the relationship of power spectra, and the correlation between magnetic energy and radiation energy, can be well understood by our analytic results.Comment: 13 pages, 3 figure, accepted for publication in Ap

Non-axisymmetric instabilities in shocked adiabatic accretion flows

We investigate the linear stability of a shocked accretion flow on to a black hole in the adiabatic limit. Our linear analyses and numerical calculations show that, despite the post-shock deceleration, the shock is generally unstable to non-axisymmetric perturbations. The simulation results of Molteni, Toth & Kuznetsov can be well explained by our linear eigenmodes. The mechanism of this instability is confirmed to be based on the cycle of acoustic waves between the corotation radius and the shock. We obtain an analytical formula to calculate the oscillation period from the physical parameters of the flow. We argue that the quasi-periodic oscillation should be a common phenomenon in accretion flows with angular momentum

Bimodal accretion disks: Shakura-Sunyaev disk-advection-dominated accretion flow transitions

We show that, unlike the results presented previously in the literature, the transition from an outer Shakura-Sunyaev disk (SSD) to an advection-dominated accretion flow (ADAF) is possible for large values of the viscosity parameter alpha > 0.5. The transition is triggered by the thermal instability of a radiation pressure-supported SSD. The transition radius is close to the central black hole. We confirm our qualitative prediction by actually constructing global bimodal SSD-ADAF solutions

Advection-Dominated Accretion Disks: Geometrically Slim or Thick?

We revisit the vertical structure of black hole accretion disks in spherical coordinates. By comparing the advective cooling with the viscous heating, we show that advection-dominated disks are geometrically thick, i.e., with the half-opening angle > 2\pi/5, rather than slim as supposed previously in the literature.Comment: 12 pages, 3 figures, accepted by PAS

Maximal possible accretion rates for slim disks

By adopting the correct vertical gravitational force in studies of thermal equilibrium solutions, we find that there exists a maximal possible accretion rate for each radius in the outer region of optically thick accretion flows, such that only the inner regions of these flows can possibly take the form of slim disks